Natural environment
influenced by humans. A geographical area is regarded as a natural environment.
It is difficult to find absolutely natural environments, and it is common that the naturalness varies in a continuum, TOSHIBA Satellite U305-S2804 Laptop Keyboard
from ideally 100% natural in one extreme to 0% natural in the other. More precisely, we can consider the different aspects or components of an environment, and see that their degree of naturalness is not uniform.[2] SAMSUNG X20 Laptop Keyboard
If, for instance, we take an agricultural field, and consider the mineralogic composition and the structure of its soil, we will find that whereas the first is quite similar to that of an undisturbed forest soil, the structure is quite different. SONY 148084721 Laptop Keyboard
Natural environment is often used as a synonym for habitat. For instance, when we say that the natural environment of giraffes is the savanna.
Earth science generally recognizes 4 spheres, the lithosphere, the hydrosphere, theatmosphere, and the biosphere[3] GATEWAY M-6813M Laptop Keyboard
as correspondent to rocks, water, air, and life. Some scientists include, as part of the spheres of the Earth, the cryosphere (corresponding toice) as a distinct portion of the hydrosphere, as well as the pedosphere (corresponding to soil) as an active and intermixed sphere. Earth science (also known as geoscience, the geosciences or the Earth Sciences), TOSHIBA Satellite U305-S2804 Laptop Keyboard
is an all-embracing term for the sciences related to the planet Earth.[4] There are four major disciplines in earth sciences, namely geography,geology, geophysics and geodesy. These major disciplines use physics, chemistry, SONY A1731834A Laptop Keyboard
biology,chronology and mathematics to build a qualitative and quantitative understanding of the principal areas or spheres of the Earth system. The Earth's crust, or lithosphere, is the outermost solid surface of the planet and is chemically and mechanically different from underlying mantle. Lenovo Thinkpad T420 Laptop Keyboard
It has been generated largely by igneous processes in which magma cools and solidifies to form solid rock. Beneath the lithosphere lies the mantle which is heated by thedecay of radioactive elements. HP Mini 210-1109TU Laptop Keyboard
The mantle though solid is in a state of rheic convection. This convection process causes the lithospheric plates to move, albeit slowly. The resulting process is known as plate tectonics.[5][6][7] Volcanoes result primarily from the melting of subducted crust material or of rising mantle at mid-ocean ridges and mantle plumes. HP Pavilion DV3000 Laptop Keyboard
An ocean is a major body of saline water, and a component of the hydrosphere. Approximately 71% of the Earth'ssurface (an area of some 362 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. SONY Vaio PCG-K23 Laptop Keyboard
More than half of this area is over 3,000 meters (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Though generally recognized as several 'TOSHIBA Satellite U305-S2804 Laptop Keyboard
separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean.[8][9] This concept of a global ocean as a continuous body of HP 640892-001 Laptop Keyboard
water with relatively free interchange among its parts is of fundamental importance to oceanography.[10] The major oceanic divisions are defined in part by thecontinents, various archipelagos, and other criteria: these divisions are (in descending order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean and the Arctic Ocean. Compaq Presario CQ35-110TU Laptop Keyboard
A river is a natural watercourse,[11] usually freshwater, flowing toward an ocean, a lake, a sea or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. TOSHIBA Satellite U305-S2804 Laptop Keyboard
Small rivers may also be termed by several other names, including stream, creek and brook. In the United States a river is generally classified as a watercourse more than 60 feet (18 metres) wide. The water in a river is usually in a channel, made up of a stream bed between banks. In larger rivers there is also a wider floodplain shaped by waters over-topping the channel. HP G61-450SG Laptop Keyboard
Flood plains may be very wide in relation to the size of the river channel. Rivers are a part of thehydrological cycle. Water within a river is generally collected from precipitationthrough surface runoff, groundwater recharge, springs, and the release of water stored inglaciers and snowpacks. HP Envy 15-1100 Series Laptop Keyboard
A lake (from Latin lacus) is a terrain feature, a body of water that is localized to the bottom of basin. A body of water is considered a lake when it is inland, is not part of an ocean, is larger and deeper than a pond, and is fed by a river.[13][14] SONY VAIO VGN-CS115J/Q laptop keyboard
Natural lakes on Earth are generally found in mountainous areas, rift zones, and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last Ice Age. TOSHIBA 9j.n7482.201 laptop keyboard
All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.
The atmosphere of the Earth serves as a key factor in sustaining the planetary ecosystem. The thin layer of gases that envelops the Earth is held in place by the planet's gravity. Dry air consists of 78% nitrogen, 21% oxygen, TOSHIBA Tecra M10-S1001 laptop keyboard
1% argon and otherinert gases, such as carbon dioxide. The remaining gases are often referred to as trace gases,[16] among which are thegreenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Compaq Presario CQ35 laptop keyboard
Air also contains a variable amount of water vapor and suspensions of water droplets andice crystals seen as clouds. Many natural substances may be present in tiny amounts in an unfiltered air sample, includingdust, pollen and spores, sea spray, volcanic ash, and meteoroids. DELL NSK-D8001 laptop keyboard
Various industrial pollutants also may be present, such aschlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulphur compounds such as sulphur dioxide[SO2]. TOSHIBA Satellite L755-SP5167CM laptop keyboard
The ozone layer of the Earth's atmosphere plays an important role in depleting the amount of ultraviolet (UV) radiation that reaches the surface. As DNA is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes. HP G61-450SG laptop keyboard
Weather is a set of all the phenomena occurring in a given atmospheric area at a given time.[22] Most weather phenomena occur in the troposphere,[23][24] just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time.[25]When used without qualification, "weather" is understood to be the weather of Earth. FUJITSU Siemens M1010 laptop keyboard
Weather occurs due to density (temperature and moisture) differences between one place and another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. The strong temperature contrast between polar and tropical air gives rise to the jet stream. TOSHIBA MP-06873US-6983 laptop keyboard
Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. SONY VAIO VGN-C2S Series Laptop Keyboard
On the Earth's surface, temperatures usually range ±40 °C (100 °F to −40 °F) annually. Over thousands of years, changes in the Earth's orbit have affected the amount and distribution of solar energy received by the Earth and influence long-term climate SAMSUNG Q330-JS01 Laptop Keyboard
Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes due to differences in compressional heating. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. SONY VAIO VGN-NW20SF Laptop Keyboard
The atmosphere is a chaotic system, and small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout human history, and there is evidence that human activity such as agriculture and industry has inadvertently modified weather patterns. TOSHIBA Satellite L750-ST4N02 Laptop Keyboard
Evidence suggests that life on Earth has existed for about 3.7 billion years.[26] All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. SONY VAIO VGN-N250E/W Laptop Keyboard
Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by organization,metabolism, growth, adaptation, response to stimuli and reproduction.[27] Life may also be said to be simply the characteristic state of organisms. SONY Vaio PCG-K415S laptop keyboard
In biology, the science of living organisms, "life" is the condition which distinguishes active organisms frominorganic matter, including the capacity for growth, functional activity and the continual change preceding death.[28][29] SONY VAIO VGN-CR123E laptop keyboard
A diverse variety of living organisms (life forms) can be found in the biosphere onEarth, and properties common to these organisms—plants, animals, fungi, protists,archaea, and bacteria—are a carbon- and water-based cellular form with complexorganization and heritable genetic information. IBM ThinkPad T40-2373 laptop keyboard
Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. SONY VAIO VGN-FS395VP laptop keyboard
More complex living organisms can communicate through various means.
An ecosystem (also called as environment) is a natural unit consisting of all plants, animals and micro-organisms (bioticfactors) in an area functioning together with all of the non-living physical (abiotic) factors of the environment.[30] HP AEUT5U00010 laptop keyboard
Central to the ecosystem concept is the idea that living organisms are continually engaged in a highly interrelated set of relationships with every other element constituting the environment in which they exist. Eugene Odum, one of the founders of the science of ecology, stated: "Any unit that includes all of the organisms (ie: ASUS X53S laptop keyboard
the "community") in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e.: exchange of materials between living and nonliving parts) within the system is an ecosystem."[31] HP Pavilion DV7t-3100 laptop keyboard
The human ecosystem concept is then grounded in the deconstruction of the human/naturedichotomy, and the emergent premise that all species are ecologically integrated with each other, TOSHIBA 9J.N9282.W01 laptop keyboard
as well as with the abiotic constituents of their biotope.
A greater number or variety of species or biological diversity of an ecosystem may contribute to greater resilience of an ecosystem, because there are more species present at a location to respond to change and thus "absorb" or reduce its effects. DELL Inspiron 1731 laptop keyboard
This reduces the effect before the ecosystem's structure is fundamentally changed to a different state. This is not universally the case and there is no proven relationship between the species diversity of an ecosystem and its ability to provide goods and services on a sustainable level. Packard Bell Easynote TK37 laptop keyboard
The term ecosystem can also pertain to human-made environments, such as human ecosystems and human-influenced ecosystems, and can describe any situation where there is relationship between living organisms and their environment. TOSHIBA 9J.N9282.W01 laptop keyboard
Fewer areas on the surface of the earth today exist free from human contact, although some genuine wilderness areas continue to exist without any forms of human intervention.
Biomes are terminologically similar to the concept of ecosystemsLenovo 3000 G530 4151 laptop keyboard
and are climatically and geographically defined areas of ecologically similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, often referred to as ecosystems. Biomes are defined on the basis of factors such as plant structures (such as trees, shrubs, and grasses), TOSHIBA 9J.N9282.W01 laptop keyboard
leaf types (such as broadleaf and needleleaf), plant spacing (forest, woodland, savanna), and climate. Unlikeecozones, biomes are not defined by genetic, taxonomic, or historical similarities. Biomes are often identified with particular patterns of ecological succession and climax vegetation. ACER Aspire 5562WXMi laptop keyboard
Wilderness is generally defined as a natural environment on Earth that has not been significantly modified by human activity. The WILD Foundation goes into more detail, defining wilderness as: "The most intact, undisturbed wild natural areas left on our planet - those last truly wild places that humans do not control and have not developed with roads, SONY 148084122 laptop keyboard
pipelines or other industrial infrastructure."[33] Wilderness areas and protected parks are considered important for the survival of certain species, ecological studies, conservation, solitude, and recreation. Wilderness is deeply valued for cultural, spiritual, moral, and aesthetic reasons. Some nature writers believe wilderness areas are vital for the human spirit and creativity.[34] ACER Extensa 5635Z laptop keyboard
The word, "wilderness", derives from the notion of wildness; in other words that which is not controllable by humans. The word's etymology is from the Old English wildeornes, which in turn derives from wildeor meaning wild beast (wild + deor = beast, deer).[35] From this point of view, it is the wildness of a place that makes it a wilderness. TOSHIBA Satellite l25-s121 laptop keyboard
The mere presence or activity of people does not disqualify an area from being "wilderness." Many ecosystems that are, or have been, inhabited or influenced by activities of people may still be considered "wild." This way of looking at wilderness includes areas within which natural processes operate without very noticeable human interference. SONY KFRGBA028B laptop keyboard
Wildlife includes all non-domesticated plants, animals and other organisms. Domesticating wild plant and animal species for human benefit has occurred many times all over the planet, and has a major impact on the environment, both positive and negative. Wildlife can be found in all ecosystems. Deserts, rain forests, HP Pavilion DV7t-3300 CTO laptop keyboard
plains, and other areas—including the most developed urban sites—all have distinct forms of wildlife. While the term in popular culture usually refers to animals that are untouched by human factors, most scientists agree that wildlife around the world is impacted by human activities. SONY VAIO VGN-FS215M laptop keyboard
In biology, an organism is any contiguous living system (such as animal, fungus, micro-organism, or plant). In at least some form, all types of organisms are capable of responding to stimuli, reproduction, growth and development, and maintenance ofhomeostasis as a stable whole. HP Pavilion dv6-3134nr laptop keyboard
An organism may be either unicellular (a single cell) or, as in the case of humans, comprise many trillions of cells grouped into specialized tissues and organs. The term multicellular (many cells) describes any organism made up of more than one cell. An organism may be either a prokaryote or a eukaryote. TOSHIBA MP-06873US-6983 laptop keyboard
Prokaryotes are represented by two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound co HP Pavilion dv6-2170ee laptop keyboard
mpartments called organelles (such asmitochondria in animals and plants and plastids in plants and algae, all generally considered to be derived fromendosymbiotic bacteria).[1] Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes. FUJITSU Lifebook S7110 laptop keyboard
In 2002 Thomas Cavalier-Smith proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved from Bacteria, more specifically from Actinobacteria.[2] FUJITSU Siemens M1010 laptop keyboard
See the article: Branching order of bacterial phyla (Cavalier-Smith, 2002).
The word organism may broadly be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. However, many sources propose definitions that exclude viruses and theoretically possible man-made non-organic life forms.[18] Viruses are dependent on the biochemical machinery of a host cell for reproduction. SAMSUNG N128 laptop keyboard
Chambers Online Reference provides a broad definition: "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction".[19]
In multicellular terms, "organism" usually describes the whole hierarchical assemblage of systems (for example circulatory, SAMSUNG NP-N145-JP01 laptop keyboard
digestive, or reproductive) themselves collections of organs; these are, in turn, collections of tissues, which are themselves made of cells. In some plants and the nematode Caenorhabditis elegans, individual cells are totipotent. TOSHIBA Satellite l10 laptop keyboard
A superorganism is an organism consisting of many individuals working together as a single functional or social unit.
Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, SONY 013-000A-8715-C laptop keyboard
growth or metabolism. This controversy is problematic because some cellular organisms are also incapable of independent survival (but not of independent metabolism and procreation) and live as obligatory intracellular parasites. HP Pavilion dv6-3050tx laptop keyboard
Naturally, this rules out autonomous reproduction and they can only be passively replicated by the machinery of the host cell. In this sense they are similar to inanimate matter. While viruses sustain no independent metabolism, and thus are usually not accounted organisms, they do have their own genes and they do evolve by similar mechanisms by which organisms evolve. HP Pavilion dv6-3093ca laptop keyboard
The most common argument in support of viruses as living organisms is their ability to undergo evolution and replicate through self-assembly. Some scientists argue that viruses neither evolve, nor self- reproduce. In fact, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. DELL 0PVDG3 laptop keyboard
If host cells did not exist, viral evolution would be impossible. This is not true for cells. If viruses did not exist, the direction of evolution could be different; however, the ability to evolve would not be affected. HP Mini 210-1170NR Laptop Keyboard
As for the reproduction, viruses totally rely on hosts' machinery to replicate themselves.[20] The discovery of viral megagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong on the tree of life. The presence of these genes suggested that viruses could metabolize in the past. HP Compaq NC8430 Laptop Keyboard
It was found later that the genes coding for energy and protein metabolism have cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts.[20]
To give an example, SAMSUNG R19 Laptop Keyboard
Homo sapiens is the Latin binomial equating to modern humans. All members of the species sapiens are, at least in theory, genetically able to interbreed. Several species may belong to a genus, but the members of different species within a genus are usually unable to interbreed to produce fertile offspring. GATEWAY MT6920 Laptop Keyboard
(The red wolf Canis lupus rufus, however, may be a result of interbreeding between the grey wolf Canis lupus and the coyote Canis latrans.) Homo only has one surviving species (sapiens), Homo erectus, Homo neanderthalensis, LENOVO IdeaPad S10 20015 Laptop Keyboard
etc. having become extinct thousands of years ago; some scientists argue for interbreeding between H. sapiens and H. neanderthalensis with fertile progeny. Several genera belong to the same family and so on up the hierarchy. Eventually, the relevant kingdom (Animalia, HP G62-a43SE Laptop Keyboard
in the case of humans) is placed into one of the three domains depending upon certain genetic and structural characteristics. SONY VAIO VGN-CR22/W Laptop Keyboard
All living organisms known to science are given classification by this system such that the species within a particular family are more closely related and genetically similar than the species within a particular phylum. DELL Vostro 3500 Laptop Keyboard
Since viruses are not living organisms, their classification is a challenging task. At first, viruses were classified according to their hosts: plant viruses, animal viruses, bacteriophages. Later, they were classified by the disease that they cause. For example, respiratory viruses, enterics. Now, viruses are classified based on the nucleic acid content, capsid symmetry and the presence or absence of the envelope. SONY 148096222 Laptop Keyboard
Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival. The same laws that govern non-living chemistry govern the chemical processes of life. lENOVO IdeaPad G550 Laptop Keyboard
It is generally the phenomena of entire organisms that determine their fitness to an environment and therefore the survivability of their DNA-based genes.
Organisms clearly owe their origin, HP 407218-001 Laptop Keyboard
metabolism, and many other internal functions to chemical phenomena, especially the chemistry of large organic molecules. Organisms are complex systems of chemical compounds that, through interaction and environment, play a wide variety of roles. TOSHIBA Satellite l25-s121 laptop keyboard
Organisms are semi-closed chemical systems. Although they are individual units of life (as the definition requires), they are not closed to the environment around them. To operate they constantly take in and release energy. Autotrophs produce usable energy (in the form of organic compounds) using light from the sun or inorganic compounds while heterotrophs take in organic compounds from the environment. SONY VAIO VGN-SZ210P/B laptop keyboard
The primary chemical element in these compounds is carbon. The chemical properties of this element such as its great affinity for bonding with other small atoms, including other carbon atoms, and its small size making it ASUS X53S laptop keyboard
capable of forming multiple bonds, make it ideal as the basis of organic life. It is able to form small three-atom compounds (such as carbon dioxide), as well as large chains of many thousands of atoms that can store data (nucleic acids), hold cells together, and transmit information (protein). HP G61-420CA Laptop Keyboard
The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells, SONY 148738121 Laptop Keyboard
and cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
There are two types of cells, TOSHIBA Satellite L300 Laptop Keyboard
eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multicellular organisms. Prokaryotic cells lack a nuclear membrane so DNA is unbound within the cell, eukaryotic cells have nuclear membranes. HP G42-415DX Laptop Keyboard
All cells, whether prokaryotic or eukaryotic, have a membrane, which envelops the cell, separates its interior from its environment, regulates what moves in and out, and maintains the electric potential of the cell. HP Pavilion G6-1C58DX Laptop Keyboard
Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. HP Mini 110-1212NR Laptop Keyboard
In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool. Evidence for common descent may be found in traits shared between all living organisms. HP HDX18 Laptop Keyboard
In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds have wings, even those that do not fly.
Today, there is debate over whether or not all organisms descended from a common ancestor, or a "last universal ancestor" (LUA), HP 597635-001 Laptop Keyboard
also called the "last universal common ancestor" (LUCA), the hypothetical unicellular organism or single cell that gave rise to all life on Earth 3.5 to 3.8 billion years ago.[22] The universality of genetic coding suggests common ancestry. For example, HP Mini 210-1130LA Laptop Keyboard
every living cell makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins, although exceptions to the basic twenty amino acids have been found. However, throughout history groupings based on appearance or function of species have sometimes been polyphyletic due to convergent evolution. Lenovo ThinkPad T410-2537 Laptop Keyboard
It was once thought that the genetic code was invariably universal (see: Universal genetic code), but many variations have been discovered[23] including various alternative mitochondrial codes.[24] Back in the early 1970s, IBM Lenovo ThinkPad R50 2883 Laptop Keyboard
evolutionary biologists thought that a given piece of DNA specified the same protein subunit in every living thing, and that the genetic code was thus universal. This was interpreted as evidence that every organism had inherited its genetic code from a single common ancestor, aka, an LUCA. In 1979, however, HP Pavilion DV7-3065dx Laptop Keyboard
exceptions to the code were found in mitochondria, the tiny energy factories inside cells. Researchers studying human mitochondrial genes discovered that they used an alternative code, and many slight variants have been discovered since,[23] SONY VGN-FE670G Laptop Keyboard
including various alternative mitochondrial codes,[24] as well as small variants such as Mycoplasma translating the codon UGA as tryptophan. Biologists subsequently found exceptions in bacteria and in the nuclei of algae and single-cell animals. For example, certain proteins may use alternative initiation (start) SAMSUNG N150 Laptop Keyboard
codons not normally used by that species.[25] In certain proteins, non-standard amino acids are substituted for standard stop codons, depending upon associated signal sequences in the messenger RNA: UGA can code for selenocysteine and UAG can code for pyrrolysine. Packard Bell PEW91 laptop keyboard
Selenocysteine is now viewed as the 21st amino acid, and pyrrolysine is viewed as the 22nd. A detailed description of variations in the genetic code can be found at the NCBI web site.
Sexual reproduction (Sex) is widespread among current eukaryotes, and evidence suggests that it is a primordial and fundamental characteristic of eukaryotes.[26] HP Mini 210-1170NR laptop keyboard
Dacks and Rogers[27] proposed, on the basis of a phylogenetic analysis, that sex was present in the common ancestor of all eukaryotes. The finding of a core set of genes necessary for meiosis in the descendants of lineages that diverged early form the eukaryotic evolutionary tree also led Ramesh et al.[28] and Malik et al.[29] TOSHIBA Satellite U305-S2808 laptop keyboard
to suggest that sex was present in a common ancestor of all eukaryotes. Furthermore, this view is supported by evidence that eukaryotes previously regarded as “ancient asexuals”, such as amoeba, were likely sexual in the past, and that most present day asexual amoeboid lineages likely arose recently and independently.[30] HP Compaq NC8430 laptop keyboard
In prokaryotes, natural bacterial transformation involves the transfer of DNA from one bacterium to another and integration of the donor DNA into the recipient chromosome by recombination. Natural bacterial transformation is considered to be a primitive sexual process and occurs in both bacteria and archaea, ACER TravelMate 2403 Series laptop keyboard
although it has been studied mainly in bacteria. Transformation is clearly a bacterial adaptation and not an accidental occurrence, because it depends on numerous gene products that specifically interact with each other to enter a state of natural competence to perform this complex process.[31] SONY VAIO VGN-CR22/W laptop keyboard
Transformation is a common mode of DNA transfer, and over 60 prokaryotic species are known to be naturally competent for transformation.[32] Transformation in prokaryotes has basic similarities to sex in eukaryotes, and it has been proposed that eukaryotic sex evolved from prokaryotic sex.[33] HP AEAT8TPU017 laptop keyboard
Little is known about the earliest developments in life. However, all existing organisms share certain traits, including cellular structure and genetic code. Most scientists interpret this to mean all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, SONY Vaio VGN-AW390J laptop keyboard
but there is no scientific consensus on the relationship of the three domains of life (Archaea, Bacteria, Eukaryota) or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of macromolecules, particularly RNA, and the behavior of complex systems. Lenovo 3000 G530 4151 laptop keyboard
The emergence of oxygenic photosynthesis (around 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, TOSHIBA MP-03433US-6984 laptop keyboard
believed to have emerged around 2 billion years ago.
In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals, the Cambrian explosion (a period of unrivaled and remarkable, but brief, organismal diversity documented in the fossils found at the Burgess Shale) saw the creation of all the major body plans, HP Mini 110-3108tu Laptop Keyboard
orphyla, of modern animals. This event is now believed to have been triggered by the development of the Hox genes. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of today's land ecosystems. SONY VAIO VGN-C2S Series Laptop Keyboard
The evolutionary process may be exceedingly slow. Fossil evidence indicates that the diversity and complexity of modern life has developed over much of the history of the earth. Geological evidence indicates that the Earth is approximately 4.6 billion years old. Studies on guppies by David Reznick at the University of California, HP Mini 110-3135dx Laptop Keyboard
Riverside, however, have shown that the rate of evolution through natural selection can proceed 10 thousand to 10 million times faster than what is indicated in the fossil record.[34] Such comparative studies however are invariably biased by disparities in the time scales over which evolutionary change is measured in the laboratory, field experiments, and the fossil record. FUJITSU Lifebook E8310 Laptop Keyboard
The word "animal" comes from the Latin word animalis, meaning "having breath".[1] In everyday colloquial usage the word incorrectly excludes humans, instead it is often used to refer only to non-human members of the kingdom Animalia. SONY VAIO VGN-C2S Series Laptop Keyboard
Sometimes, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.[2] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse assponges, jellyfish, insects, and humans.[3]
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With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, ACER TravelMate 6292 Laptop Keyboard
with one or two openings.[10] Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.[11]
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elasticglycoproteins.[12] FUJITSU Lifebook P1510 laptop keyboard
This may be calcified to form structures like shells, bones, and spicules.[13] During development, it forms a relatively flexible framework[14] upon which cells can move about and be reorganized, making complex structures possible. HP Pavilion dv6-2192el laptop keyboard
In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.[10] Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.[15] FUJITSU Amilo Mini Ui 3520 laptop keyboard
Nearly all animals undergo some form of sexual reproduction.[16] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motilespermatozoa or larger, non-motile ova.[17] HP Pavilion dv6-2143ez laptop keyboard
These fuse to form zygotes, which develop into new individuals.[18]
Many animals are also capable of asexual reproduction.[19] This may take place throughparthenogenesis, where fertile eggs are produced without mating, budding, orfragmentation.[20] HP G42-368TX laptop keyboard
A zygote initially develops into a hollow sphere, called a blastula,[21] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.[22] HP G61-420CA laptop keyboard
In most other groups, the blastula undergoes more complicated rearrangement.[23] It first invaginates to form a gastrulawith a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.[24] In most cases, a mesoderm also develops between them.[25] HP G62-237CA laptop keyboard
These germ layers then differentiate to form tissues and organs.[26]
All animals are heterotrophs, meaning that they feed directly or indirectly on other living things.[27] They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.[28] SONY Vaio VGN-FW170J/W laptop keyboard
Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked).[29] Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey.[30] The other main category of consumption is detritivory, the consumption of deadorganic matter.[31] ACER Extensa 5635Z laptop keyboard
It can at times be difficult to separate the two feeding behaviours, for example, where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.[32] Lenovo 3000 G530 4151 laptop keyboard
Most animals indirectly use the energy of sunlight by eating plants or plant-eating animals. Most plants use light to convert inorganic molecules in their environment into carbohydrates, fats, proteins and other biomolecules, characteristically containing reduced carbon in the form of carbon-hydrogen bonds. Starting with carbon dioxide (CO2) and water (H2O), SONY VAIO VGN-CR32G laptop keyboard
photosynthesis converts the energy of sunlight into chemical energy in the form of simple sugars (e.g.,glucose), with the release of molecular oxygen. These sugars are then used as the building blocks for plant growth, including the production of other biomolecules.[10] When an animal eats plants (or eats other animals which have eaten plants), TOSHIBA Satellite L755-S5107 laptop keyboard
the reduced carbon compounds in the food become a source of energy and building materials for the animal.[33] They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.[34][35] Lenovo ThinkPad T410-2537 laptop keyboard
Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight.[36] Instead chemosynthetic archaea and bacteria form the base of the food chain.[37] HP Pavilion DV7-3060us laptop keyboard
Animals are generally considered to have evolved from a flagellated eukaryote.[39] Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges.[40] ACER Aspire 3684NWXMi Laptop Keyboard
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Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists.[41] The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.[42] TOSHIBA Satellite U305-S2804 Laptop Keyboard
The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia.[43] These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.[43] SONY VAIO VGN-NW240F Laptop Keyboard
The next oldest possible animal fossils are found towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota.[44] These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.[45] HP Pavilion DV7-3057nr laptop keyboard
Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago.[46] It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible. SONY VAIO VGN-FZ19VN laptop keyboard
Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[47] Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex asearthworms.[48] SONY VAIO VGN-FS940 laptop keyboard
During the beginning of the Tonian period around 1 billion years ago, there was a decrease in Stromatolite diversity, which may indicate the appearance of grazing animals, since stromatolite diversity increased when grazing animals went extinct at the End Permian and End Ordovician extinction events, Packard Bell PEW91 laptop keyboard
and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.[49][50] TOSHIBA NB100 laptop keyboard
It has been estimated that 99.9% of animals that have ever existed are extinct.[51]
The sponges (Porifera) were long thought to have diverged from other animals early.[54] They lack the complex organization found in most other phyla.[55] Their cells are differentiated, but in most cases not organized into distinct tissues.[56]Sponges ACER Extensa 5635Z laptop keyboard
typically feed by drawing in water through pores.[57] Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.[58] However, a phylogenomic study in 2008 of 150 genes in 29 animals across 21 phyla revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, ACER Aspire 5738PZG laptop keyboard
at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.[59]
Among the other phyla, the Ctenophora and the Cnidaria, DELL Vostro PP37L laptop keyboard
which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.[60] Both have distinct tissues, but they are not organized into organs.[61] ACER Aspire 5741Z laptop keyboard
There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic.[62] The tiny placozoans are similar, but they do not have a permanent digestive chamber. SONY VGN-FE21H laptop keyboard
The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, SONY VAIO VGN NR220E laptop keyboard
a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however — for instance adultechinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures. SONY VGN-FE21H laptop keyboard
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: thedeuterostomes and the protostomes, the latter of which includes the Ecdysozoa, Platyzoa, and Lophotrochozoa. In addition, there are a few small groups of bilaterians Compaq Presario CQ56-109WM Laptop Keyboard
with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, andOrthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well. DELL Vostro PP37L Laptop Keyboard
Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes, the first opening of the gut to appear in embryological development (thearchenteron) develops into the mouth, with the anus forming secondarily. In deuterostomes the anus forms first, SONY VAIO VGN-FS742/W Laptop Keyboard
with the mouth developing secondarily.[63] In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through invagination of the endoderm, called enterocoelic pouching.[64] Deuterostome embryos undergo radial cleavage during cell division, while protostomes undergo spiral cleavage.[65] DELL Inspiron 8500 Laptop Keyboard
All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata.[66] The former are radially symmetric and exclusively marine, such as starfish, COMPAQ Presario C700 Laptop Keyboard
sea urchins, and sea cucumbers.[67] The latter are dominated by the vertebrates, animals with backbones.[68] These include fish,amphibians, reptiles, birds, and mammals.[69] Compaq Presario CQ42-228LA Laptop Keyboard
In addition to these, the deuterostomes also include the Hemichordata, or acorn worms.[70] Although they are not especially prominent today, the important fossilgraptolites may belong to this group.[71] HP Pavilion dv5-1102tu Laptop Keyboard
The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis.[72] The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. HP Mini 110-3130tu Laptop Keyboard
Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.
The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water.[73] ACER Aspire 1410-2497 Laptop Keyboard
A number are important parasites.[74] Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom. SONY VAIO VGN-CS230J/Q Laptop Keyboard
The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since in both embryos develop withspiral cleavage.
The Platyzoa include the phylum Platyhelminthes, the flatworms.[75] These were originally considered some of the most primitive Bilateria, SAMSUNG N150 Laptop Keyboard
but it now appears they developed from more complex ancestors.[76] A number of parasites are included in this group, such as the flukes and tapeworms.[75] Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.[77] TOSHIBA V000130390 Laptop Keyboard
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida,Micrognathozoa, and possibly the Cycliophora.[78] These groups share the presence of complex jaws, from which they are called the Gnathifera. TOSHIBA Mini NB 505-SP0160 Laptop Keyboard
The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida.[79][80] The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, ACER Aspire 3684NWXMi Laptop Keyboard
such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented.[81] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.[82] TOSHIBA Satellite 1800-921 Laptop Keyboard
The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a ring of ciliated tentacles around the mouth, called a lophophore.[83] These were traditionally grouped together as the lophophorates.[84] DELL NSK-D8001 Laptop Keyboard
but it now appears that the lophophorate group may be paraphyletic,[85] with some closer to the nemerteans and some to the molluscs and annelids.[86][87] They include the Brachiopoda or lamp shells, which are prominent in the fossil record, theEntoprocta, the Phoronida, and possibly the Bryozoa or moss animals.[88] ASUS X53S Laptop Keyboard
Model organisms
Main articles: Model organism and Animal testing
Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, TOSHIBA Satellite U305-S2804 laptop keyboard
the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organisms, and were among the first life-forms to be genetically sequenced. SONY VAIO VGN-NW240F laptop keyboard
This was facilitated by the severely reduced state of their genomes, but as many genes, introns, and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, HP Pavilion DV6-6000 Laptop Keyboard
and molluscan genome projects currently in progress. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.[89] HP HDX18-1024CA Laptop Keyboard
An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.[90] SONY VAIO VGN-CS230J/Q Laptop Keyboard
In biology, a species (plural: species) is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. HP Mini 110-3108tu Laptop Keyboard
While in many cases this definition is adequate, the difficulty of defining species is known as the species problem. Differing measures are often used, such as similarity of DNA, morphology, or ecological niche. Presence of specific locally adapted traits may further subdivide species into "infraspecific taxa" such as subspecies (and in botany other taxa are used, such as varieties, subvarieties, and formae). TOSHIBA NSK-TK001 Laptop Keyboard
Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, especially their DNA sequences, where available. All species are given a two-part name, a "binomial name". The first part of a binomial name is the generic name, SAMSUNG N140 Laptop Keyboard
the genus of the species. The second part is either called thespecific name (a term used only in zoology) or the specific epithet (the term used in botany, which can also be used in zoology). For example, IBM Lenovo ThinkPad R51 1833 Laptop Keyboard
Boa constrictor is one of four species of the Boa genus. The first part of the name is capitalized, and the second part has a lower case. The binomial name is written in italics when printed and underlined when handwritten. TOSHIBA Satellite L750-ST4N02 Laptop Keyboard
A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity, though other taxonomic levels such as families may be considered in broad-scale studies.[1] ACER Aspire 5742 Laptop Keyboard
Extinct species known only from fossils are generally difficult to assign precise taxonomic rankings, which is why higher taxonomic levels such as families are often used for fossil-based studies.[1][2]
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[5]
A usable definition of the word "species" and reliable methods of identifying particular species is essential for stating and testing biological theories and for measuring biodiversity. Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species. It is generally difficult to give precise taxonomic rankings to extinct species known only from fossils. HP Pavilion dv6-3178ee Laptop Keyboard
Some biologists may view species as statistical phenomena, as opposed to the traditional idea, with a species seen as a class of organisms. Lenovo 63Y0047 Laptop Keyboard
In that case, a species is defined as a separately evolving lineage that forms a single gene pool. Although properties such as DNA-sequences and morphology are used to help separate closely related lineages,[6] this definition has fuzzy boundaries.[7] However, Lenovo ThinkPad Edge E520 Laptop Keyboard
the exact definition of the term "species" is still controversial, particularly in prokaryotes,[8] and this is called the species problem.[9] Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species of concern.[9] ACER Aspire 2920Z Laptop Keyboard
The question of how best to define "species" is one that has occupied biologists for centuries, and the debate itself has become known as the species problem. Darwin wrote in chapter II of On the Origin of Species: ASUS X88S Laptop Keyboard
No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.[20] HP Pavilion DV7-3085dx laptop keyboard
But later, in The Descent of Man, when addressing "The question whether mankind consists of one or several species", Darwin revised his opinion to say:
of the term "species" is generally accepted; and the definition must not include an element that cannot possibly be ascertained, such as an act of creation.[21] HP Pavilion dv6-3050sa laptop keyboard
The modern theory of evolution depends on a fundamental redefinition of "species". Prior to Darwin, naturalists viewed species as ideal or general types, which could be exemplified by an ideal specimen bearing all the traits general to the species. Darwin's theories shifted attention from uniformity to variation and from the general to the particular. According to intellectual historian Louis Menand, SONY VAIO VGN-AR670 laptop keyboard
Once our attention is redirected to the individual, we need another way of making generalizations. We are no longer interested in the conformity of an individual to an ideal type; we are now interested in the relation of an individual to the other individuals with which it interacts. To generalize about groups of interacting individuals, TOSHIBA Satellite l25-s1215 laptop keyboard
we need to drop the language of types and essences, which is prescriptive (telling us what finches should be), and adopt the language of statistics and probability, which is predictive (telling us what the average finch, under specified conditions, is likely to do). Relations will be more important than categories; ASUS 04GNH41KUS00-2 laptop keyboard
functions, which are variable, will be more important than purposes; transitions will be more important than boundaries; sequences will be more important than hierarchies.[22]
This shift results in a new approach to "species"; Compaq Presario CQ56-109WM laptop keyboard
Darwin concluded that species are what they appear to be: ideas, which are provisionally useful for naming groups of interacting individuals. "I look at the term species", he wrote, "as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other ... DELL Vostro PP37L laptop keyboard
It does not essentially differ from the word variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for convenience sake." [22] SONY VAIO VGN-N250E/B laptop keyboard
Practically, biologists define species as populations of organisms that have a high level of genetic similarity. This may reflect an adaptation to the same niche, and the transfer of genetic material from one individual to others, DELL Vostro PP38L laptop keyboard
through a variety of possible means. The exact level of similarity used in such a definition is arbitrary, but this is the most common definition used for organisms that reproduce asexually (asexual reproduction), such as some plants andmicroorganisms. SONY VAIO VGN-CR160F laptop keyboard
This lack of any clear species concept in microbiology has led to some authors arguing that the term "species" is not useful when studying bacterial evolution. Instead they see genes as moving freely between even distantly related bacteria, with the entire bacterial domain being a single gene pool. Nevertheless, Compaq Presario CQ56-109WM laptop keyboard
a kind of rule of thumb has been established, saying that species of Bacteria or Archaea with 16S rRNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA Hybridization if they belong to the same species or not.[23] This concept has been updated recently, saying that the border of 97% was too low and can be raised to 98.7%.[24] DELL Vostro PP38L laptop keyboard
In the study of sexually reproducing organisms, where genetic material is shared through the process of reproduction, the ability of two organisms to interbreed and produce fertile offspring of both sexes is generally accepted as a simple indicator that the organisms share enough genes to be considered members of the same species. Thus a "species" is a group of interbreeding organisms. SONY VAIO VGN-SR220J/H laptop keyboard
This definition can be extended to say that a species is a group of organisms that could potentially interbreed – fish could still be classed as the same species even if they live in different lakes, as long as they could still interbreed were they ever to come into contact with each other. On the other hand, ACER Aspire 1410-2497 laptop keyboard
there are many examples of series of three or more distinct populations, where individuals of the population in the middle can interbreed with the populations to either side, but individuals of the populations on either side cannot interbreed. Thus, one could argue that these populations constitute a single species, or two distinct species. This is not a paradox; it is evidence that species are defined by gene frequencies, and thus have fuzzy boundaries. SONY VAIO VGN-CS230J/Q laptop keyboard
Consequently, any single, universal definition of "species" is necessarily arbitrary. Instead, biologists have proposed a range of definitions; which definition a biologists uses is a pragmatic choice, depending on the particularities of that biologist's research. SONY VAIO VGN-NR50B laptop keyboard
In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgment. Given the complexity of life, some have argued[who?] HP Pavilion dm1-1028tu laptop keyboard
that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition.
For most vertebrates, this is the biological species concept (BSC), and to a lesser extent (or for different purposes) the phylogenetic species concept (PSC). ASUS A3Hf laptop keyboard
Many BSC subspecies are considered species under the PSC; the difference between the BSC and the PSC can be summed up insofar as that the BSC defines a species as a consequence of manifest evolutionary history, while the PSC defines a species as a consequence of manifest evolutionary potential. DELL Vostro 3550 Laptop Keyboard
Thus, a PSC species is "made" as soon as an evolutionary lineage has started to separate, while a BSC species starts to exist only when the lineage separation is complete. Accordingly, there can be considerable conflict between alternative classifications based upon the PSC versus BSC, SONY 147915321 Laptop Keyboard
as they differ completely in their treatment of taxa that would be considered subspecies under the latter model (e.g., the numerous subspecies of honey bees).
The naming of a particular species may be regarded as a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, HP Mini 110-3000ea laptop keyboard
the hypothesis may be confirmed or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. TOSHIBA Satellite l20-157 laptop keyboard
When two named species are discovered to be of the same species, the older species name is usually retained, and the newer species name dropped, a process called synonymization, or colloquially, as lumping. HP Pavilion dv6-3013cl laptop keyboard
Dividing a taxon into multiple, often new, taxons is called splitting. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognizing differences or commonalities between organisms (see lumpers and splitters). Acer eMachines G730ZG laptop keyboard
Traditionally, researchers relied on observations of anatomical differences, and on observations of whether different populations were able to interbreed successfully, to distinguish species; both anatomy and breeding behavior are still important to assigning species status. As a result of the revolutionary (and still ongoing) HP Pavilion dv6-3125ej laptop keyboard
advance in microbiological research techniques, including DNA analysis, in the last few decades, a great deal of additional knowledge about the differences and similarities between species has become available. HP Pavilion dv6-3050tx laptop keyboard
Many populations formerly regarded as separate species are now considered a single taxon, and many formerly grouped populations have been split. Any taxonomic level (species, genus, family, etc.) can be synonymized or split, and at higher taxonomic levels, these revisions have been still more profound. SONY VAIO VGN-CS31S/R laptop keyboard
From a taxonomical point of view, groups within a species can be defined as being of a taxon hierarchically lower than a species. In zoology only the subspeciesis used, while in botany the variety, subvariety, HP Pavilion dv6-3125ej laptop keyboard
and form are used as well. In conservation biology, the concept of evolutionary significant units (ESU) is used, which may define either species or smaller distinct population segments. Identifying and naming species is the providence of alpha taxonomy. ACER Aspire 5320 laptop keyboard
In the earliest works of science, a species was simply an individual organism that represented a group of similar or nearly identical organisms. No other relationships beyond that group were implied. SONY VAIO VGN-FZ21S laptop keyboard
Aristotle used the words genus and species to mean generic and specific categories. Aristotle and other pre-Darwinian scientists took the species to be distinct and unchanging, with an "essence", like the chemical elements. SONY VAIO VGN-NW18H/T laptop keyboard
When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. Many of these early delineation schemes would now be considered whimsical and these included consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants). TOSHIBA Tecra A9-50N laptop keyboard
John Ray (1686), an English naturalist, was the first to give a biological definition of the term species.[50]
In the 18th century Swedish scientist Carolus Linnaeus classified organisms according to differences in the form ofreproductive apparatus. Although his system of classification sorts organisms according to degrees of similarity, COMPAQ Presario CQ60-151EP laptop keyboard
it made no claims about the relationship between similar species. At that time, it was still widely believed that there was no organic connection between species, no matter how similar they appeared. ASUS MP-06833US-D57 laptop keyboard
This approach also suggested a type of idealism: the notion that each species existed as an "ideal form". Although there are always differences (although sometimes minute) between individual organisms, Linnaeus considered such variation problematic. He strove to identify individual organisms that were exemplary of the species, and considered other non-exemplary organisms to be deviant and imperfect. HP Pavilion DV7-3065dx Laptop Keyboard
By the 19th century most naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. Jean-Baptiste Lamarck, in his 1809 Zoological Philosophy, offered one of the first logical arguments against creationism. The new emphasis was on determining how a species could change over time. HP 488590-001 Laptop Keyboard
Lamarck suggested that an organism could pass on an acquired trait to its offspring, i.e., the giraffe's long neck was attributed to generations of giraffes stretching to reach the leaves of higher treetops (this well-known and simplistic example, however, does not do justice to the breadth and subtlety of Lamarck's ideas). DELL Precision M4500 Laptop Keyboard
With the acceptance of the natural selection idea ofCharles Darwin in the 1860s, however, Lamarck's view of goal-oriented evolution, also known as a teleological process, was eclipsed. Recent interest in inheritance of acquired characteristics centers around epigenetic processes, e.g. methylation, Compaq Presario CQ56-103LA Laptop Keyboard
that do not affect DNA sequences, but instead alter expression in an inheritable manner. Thus, Neo-Lamarckism, as it is sometimes termed, is not a challenge to the theory of evolution by natural selection. GATEWAY M-6337 Laptop Keyboard
Charles Darwin and Alfred Wallace provided what scientists now consider as the most powerful and compelling theory of evolution. Darwin argued that it was populations that evolved, not individuals. HP Mini 210-1099TU Laptop Keyboard
His argument relied on a radical shift in perspective from that of Linnaeus: rather than defining species in ideal terms (and searching for an ideal representative and rejecting deviations), Darwin considered variation among individuals to be natural. He further argued that variation, far from being problematic, actually provides the explanation for the existence of distinct species. Lenovo ThinkPad Edge E520 Laptop Keyboard
Darwin's work drew on Thomas Malthus' insight that the rate of growth of a biological population will always outpace the rate of growth of the resources in the environment, such as the food supply. As a result, Darwin argued, not all the members of a population will be able to survive and reproduce. Those that did will, ACER Aspire 5810T-8929 Laptop Keyboard
on average, be the ones possessing variations—however slight—that make them slightly better adapted to the environment. If these variable traits are heritable, then the offspring of the survivors will also possess them. Thus, over many generations, adaptive variations will accumulate in the population, while counter-adaptive traits will tend to be eliminated. HP G72-a20EM Laptop Keyboard
Whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the "fight for existence") that selects the traits to be passed on. This is the theory of evolution by natural selection. IHP AEUT3U00140 Laptop Keyboard
n this model, the length of a giraffe's neck would be explained by positing that proto-giraffes with longer necks would have had a significant reproductive advantage to those with shorter necks. Over many generations, the entire population would be a species of long-necked animals. Compaq Presario CQ42-228LA Laptop Keyboard
In 1859, when Darwin published his theory of natural selection, the mechanism behind the inheritance of individual traits was unknown. Although Darwin made some speculations on how traits are inherited (pangenesis), his theory relies only on the fact that inheritable traits exist, and are variable (which makes his accomplishment even more remarkable.) ACER TravelMate 6292 Laptop Keyboard
Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. It was not until 1900 that his work was rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak, who realised that the "inheritable traits" in Darwin's theory aregenes. HP 317443-001 Laptop Keyboard
The theory of the evolution of species through natural selection has two important implications for discussions of species—consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, ACER Aspire 4920 Laptop Keyboard
it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that all species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different, and over time species change. HP Mini 110-1079TU Laptop Keyboard
This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals, but one can no longer think of species as independent and immutable. HP Pavilion DV6-6000 Laptop Keyboard
The rise of a new species from a parental line is called speciation. There is no clear line demarcating the ancestral species from the descendant species.
Although the current scientific understanding of species suggests that there is no rigorous and comprehensive way to distinguish between different species inall cases, HP HDX18-1024CA Laptop Keyboard
biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring of both sexes, then they are in different species. This definition captures a number of intuitive species boundaries, SONY 147915321 Laptop Keyboard
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but it remains imperfect. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: ACER Aspire 7745G Laptop Keyboard
there are examples where members of one population can produce fertile offspring of both sexes with a second population, and members of the second population can produce fertile offspring of both sexes with members of a third population, but members of the first and third population cannot produce fertile offspring, HP Mini 210-1001SA Laptop Keyboard
or can only produce fertile offspring of the homozygous sex. Consequently, some people reject this definition of a species.
Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, DELL Studio 1458 Laptop Keyboard
both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118). However, most if not all taxonomists would strongly disagree[citation needed]. For example, in manyamphibians, SAMSUNG NP-Q320-FS01UK Laptop Keyboard
most notably in New Zealand's Leiopelma frogs, the genome consists of "core" chromosomes that are mostly invariable and accessory chromosomes, of which exist a number of possible combinations. HP Pavilion dv2-1030ea Laptop Keyboard
Even though the chromosome numbers are highly variable between populations, these can interbreed successfully and form a single evolutionary unit. In plants, polyploidy is extremely commonplace with few restrictions on interbreeding; as individuals with an odd number of chromosome sets are usually sterile, DELL Precision M4500 Laptop Keyboard
depending on the actual number of chromosome sets present, this results in the odd situation where some individuals of the same evolutionary unit can interbreed with certain others and some cannot, with all populations being eventually linked as to form a common gene pool. HP Mini 210-1014TU Laptop Keyboard
The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on molecular markers, starting with the comparatively crude blood plasma precipitation assays in the mid-20th century to Charles Sibley's ground-breaking DNA-DNA hybridization studies in the 1970s leading to DNA sequencing techniques. HP 640436-001 Laptop Keyboard
The results of these techniques caused revolutionary changes in the higher taxonomic categories (such as phyla and classes), resulting in the reordering of many branches of the phylogenetic tree (see also: molecular phylogeny). TOSHIBA Satellite L300 Laptop Keyboard
For taxonomic categories below genera, the results have been mixed so far; the pace of evolutionary change on the molecular level is rather slow, yielding clear differences only after considerable periods of reproductive separation. DNA-DNA hybridization results have led to misleading conclusions, SAMSUNG N150 Laptop Keyboard
the Pomarine Skua – Great Skuaphenomenon being a famous example. Turtles have been determined to evolve with just one-eighth of the speed of other reptiles on the molecular level, and the rate of molecular evolution in albatrosses is half of what is found in the rather closely related storm-petrels. Lenovo ThinkPad Edge E520 Laptop Keyboard
The hybridization technique is now obsolete and is replaced by more reliable computational approaches for sequence comparison. Molecular taxonomy is not directly based on the evolutionary processes, HP Pavilion DV7-3065dx Laptop Keyboard
but rather on the overall change brought upon by these processes. The processes that lead to the generation and maintenance of variation such as mutation, crossover and selection are not uniform (see also molecular clock). Lenovo 63Y0047 Laptop Keyboard
DNA is only extremely rarely a direct target of natural selection rather than changes in the DNA sequence enduring over generations being a result of the latter; for example, silent transition-transversion combinations would alter the melting point of the DNA sequence, HP Pavilion DV7-3173nr laptop keyboard
but not the sequence of the encoded proteins and thus are a possible example where, for example in microorganisms, a mutation confers a change in fitness all by itself. COMPAQ Presario CQ60-420us laptop keyboard
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influenced by humans. A geographical area is regarded as a natural environment.
It is difficult to find absolutely natural environments, and it is common that the naturalness varies in a continuum, TOSHIBA Satellite U305-S2804 Laptop Keyboard
from ideally 100% natural in one extreme to 0% natural in the other. More precisely, we can consider the different aspects or components of an environment, and see that their degree of naturalness is not uniform.[2] SAMSUNG X20 Laptop Keyboard
If, for instance, we take an agricultural field, and consider the mineralogic composition and the structure of its soil, we will find that whereas the first is quite similar to that of an undisturbed forest soil, the structure is quite different. SONY 148084721 Laptop Keyboard
Natural environment is often used as a synonym for habitat. For instance, when we say that the natural environment of giraffes is the savanna.
Earth science generally recognizes 4 spheres, the lithosphere, the hydrosphere, theatmosphere, and the biosphere[3] GATEWAY M-6813M Laptop Keyboard
as correspondent to rocks, water, air, and life. Some scientists include, as part of the spheres of the Earth, the cryosphere (corresponding toice) as a distinct portion of the hydrosphere, as well as the pedosphere (corresponding to soil) as an active and intermixed sphere. Earth science (also known as geoscience, the geosciences or the Earth Sciences), TOSHIBA Satellite U305-S2804 Laptop Keyboard
is an all-embracing term for the sciences related to the planet Earth.[4] There are four major disciplines in earth sciences, namely geography,geology, geophysics and geodesy. These major disciplines use physics, chemistry, SONY A1731834A Laptop Keyboard
biology,chronology and mathematics to build a qualitative and quantitative understanding of the principal areas or spheres of the Earth system. The Earth's crust, or lithosphere, is the outermost solid surface of the planet and is chemically and mechanically different from underlying mantle. Lenovo Thinkpad T420 Laptop Keyboard
It has been generated largely by igneous processes in which magma cools and solidifies to form solid rock. Beneath the lithosphere lies the mantle which is heated by thedecay of radioactive elements. HP Mini 210-1109TU Laptop Keyboard
The mantle though solid is in a state of rheic convection. This convection process causes the lithospheric plates to move, albeit slowly. The resulting process is known as plate tectonics.[5][6][7] Volcanoes result primarily from the melting of subducted crust material or of rising mantle at mid-ocean ridges and mantle plumes. HP Pavilion DV3000 Laptop Keyboard
An ocean is a major body of saline water, and a component of the hydrosphere. Approximately 71% of the Earth'ssurface (an area of some 362 million square kilometers) is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas. SONY Vaio PCG-K23 Laptop Keyboard
More than half of this area is over 3,000 meters (9,800 ft) deep. Average oceanic salinity is around 35 parts per thousand (ppt) (3.5%), and nearly all seawater has a salinity in the range of 30 to 38 ppt. Though generally recognized as several 'TOSHIBA Satellite U305-S2804 Laptop Keyboard
separate' oceans, these waters comprise one global, interconnected body of salt water often referred to as the World Ocean or global ocean.[8][9] This concept of a global ocean as a continuous body of HP 640892-001 Laptop Keyboard
water with relatively free interchange among its parts is of fundamental importance to oceanography.[10] The major oceanic divisions are defined in part by thecontinents, various archipelagos, and other criteria: these divisions are (in descending order of size) the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Southern Ocean and the Arctic Ocean. Compaq Presario CQ35-110TU Laptop Keyboard
A river is a natural watercourse,[11] usually freshwater, flowing toward an ocean, a lake, a sea or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. TOSHIBA Satellite U305-S2804 Laptop Keyboard
Small rivers may also be termed by several other names, including stream, creek and brook. In the United States a river is generally classified as a watercourse more than 60 feet (18 metres) wide. The water in a river is usually in a channel, made up of a stream bed between banks. In larger rivers there is also a wider floodplain shaped by waters over-topping the channel. HP G61-450SG Laptop Keyboard
Flood plains may be very wide in relation to the size of the river channel. Rivers are a part of thehydrological cycle. Water within a river is generally collected from precipitationthrough surface runoff, groundwater recharge, springs, and the release of water stored inglaciers and snowpacks. HP Envy 15-1100 Series Laptop Keyboard
A lake (from Latin lacus) is a terrain feature, a body of water that is localized to the bottom of basin. A body of water is considered a lake when it is inland, is not part of an ocean, is larger and deeper than a pond, and is fed by a river.[13][14] SONY VAIO VGN-CS115J/Q laptop keyboard
Natural lakes on Earth are generally found in mountainous areas, rift zones, and areas with ongoing or recent glaciation. Other lakes are found in endorheic basins or along the courses of mature rivers. In some parts of the world, there are many lakes because of chaotic drainage patterns left over from the last Ice Age. TOSHIBA 9j.n7482.201 laptop keyboard
All lakes are temporary over geologic time scales, as they will slowly fill in with sediments or spill out of the basin containing them.
The atmosphere of the Earth serves as a key factor in sustaining the planetary ecosystem. The thin layer of gases that envelops the Earth is held in place by the planet's gravity. Dry air consists of 78% nitrogen, 21% oxygen, TOSHIBA Tecra M10-S1001 laptop keyboard
1% argon and otherinert gases, such as carbon dioxide. The remaining gases are often referred to as trace gases,[16] among which are thegreenhouse gases such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Compaq Presario CQ35 laptop keyboard
Air also contains a variable amount of water vapor and suspensions of water droplets andice crystals seen as clouds. Many natural substances may be present in tiny amounts in an unfiltered air sample, includingdust, pollen and spores, sea spray, volcanic ash, and meteoroids. DELL NSK-D8001 laptop keyboard
Various industrial pollutants also may be present, such aschlorine (elementary or in compounds), fluorine compounds, elemental mercury, and sulphur compounds such as sulphur dioxide[SO2]. TOSHIBA Satellite L755-SP5167CM laptop keyboard
The ozone layer of the Earth's atmosphere plays an important role in depleting the amount of ultraviolet (UV) radiation that reaches the surface. As DNA is readily damaged by UV light, this serves to protect life at the surface. The atmosphere also retains heat during the night, thereby reducing the daily temperature extremes. HP G61-450SG laptop keyboard
Weather is a set of all the phenomena occurring in a given atmospheric area at a given time.[22] Most weather phenomena occur in the troposphere,[23][24] just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate is the term for the average atmospheric conditions over longer periods of time.[25]When used without qualification, "weather" is understood to be the weather of Earth. FUJITSU Siemens M1010 laptop keyboard
Weather occurs due to density (temperature and moisture) differences between one place and another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. The strong temperature contrast between polar and tropical air gives rise to the jet stream. TOSHIBA MP-06873US-6983 laptop keyboard
Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. SONY VAIO VGN-C2S Series Laptop Keyboard
On the Earth's surface, temperatures usually range ±40 °C (100 °F to −40 °F) annually. Over thousands of years, changes in the Earth's orbit have affected the amount and distribution of solar energy received by the Earth and influence long-term climate SAMSUNG Q330-JS01 Laptop Keyboard
Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes due to differences in compressional heating. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. SONY VAIO VGN-NW20SF Laptop Keyboard
The atmosphere is a chaotic system, and small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout human history, and there is evidence that human activity such as agriculture and industry has inadvertently modified weather patterns. TOSHIBA Satellite L750-ST4N02 Laptop Keyboard
Evidence suggests that life on Earth has existed for about 3.7 billion years.[26] All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. SONY VAIO VGN-N250E/W Laptop Keyboard
Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life is characterized by organization,metabolism, growth, adaptation, response to stimuli and reproduction.[27] Life may also be said to be simply the characteristic state of organisms. SONY Vaio PCG-K415S laptop keyboard
In biology, the science of living organisms, "life" is the condition which distinguishes active organisms frominorganic matter, including the capacity for growth, functional activity and the continual change preceding death.[28][29] SONY VAIO VGN-CR123E laptop keyboard
A diverse variety of living organisms (life forms) can be found in the biosphere onEarth, and properties common to these organisms—plants, animals, fungi, protists,archaea, and bacteria—are a carbon- and water-based cellular form with complexorganization and heritable genetic information. IBM ThinkPad T40-2373 laptop keyboard
Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. SONY VAIO VGN-FS395VP laptop keyboard
More complex living organisms can communicate through various means.
An ecosystem (also called as environment) is a natural unit consisting of all plants, animals and micro-organisms (bioticfactors) in an area functioning together with all of the non-living physical (abiotic) factors of the environment.[30] HP AEUT5U00010 laptop keyboard
Central to the ecosystem concept is the idea that living organisms are continually engaged in a highly interrelated set of relationships with every other element constituting the environment in which they exist. Eugene Odum, one of the founders of the science of ecology, stated: "Any unit that includes all of the organisms (ie: ASUS X53S laptop keyboard
the "community") in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e.: exchange of materials between living and nonliving parts) within the system is an ecosystem."[31] HP Pavilion DV7t-3100 laptop keyboard
The human ecosystem concept is then grounded in the deconstruction of the human/naturedichotomy, and the emergent premise that all species are ecologically integrated with each other, TOSHIBA 9J.N9282.W01 laptop keyboard
as well as with the abiotic constituents of their biotope.
A greater number or variety of species or biological diversity of an ecosystem may contribute to greater resilience of an ecosystem, because there are more species present at a location to respond to change and thus "absorb" or reduce its effects. DELL Inspiron 1731 laptop keyboard
This reduces the effect before the ecosystem's structure is fundamentally changed to a different state. This is not universally the case and there is no proven relationship between the species diversity of an ecosystem and its ability to provide goods and services on a sustainable level. Packard Bell Easynote TK37 laptop keyboard
The term ecosystem can also pertain to human-made environments, such as human ecosystems and human-influenced ecosystems, and can describe any situation where there is relationship between living organisms and their environment. TOSHIBA 9J.N9282.W01 laptop keyboard
Fewer areas on the surface of the earth today exist free from human contact, although some genuine wilderness areas continue to exist without any forms of human intervention.
Biomes are terminologically similar to the concept of ecosystemsLenovo 3000 G530 4151 laptop keyboard
and are climatically and geographically defined areas of ecologically similar climatic conditions on the Earth, such as communities of plants, animals, and soil organisms, often referred to as ecosystems. Biomes are defined on the basis of factors such as plant structures (such as trees, shrubs, and grasses), TOSHIBA 9J.N9282.W01 laptop keyboard
leaf types (such as broadleaf and needleleaf), plant spacing (forest, woodland, savanna), and climate. Unlikeecozones, biomes are not defined by genetic, taxonomic, or historical similarities. Biomes are often identified with particular patterns of ecological succession and climax vegetation. ACER Aspire 5562WXMi laptop keyboard
Wilderness is generally defined as a natural environment on Earth that has not been significantly modified by human activity. The WILD Foundation goes into more detail, defining wilderness as: "The most intact, undisturbed wild natural areas left on our planet - those last truly wild places that humans do not control and have not developed with roads, SONY 148084122 laptop keyboard
pipelines or other industrial infrastructure."[33] Wilderness areas and protected parks are considered important for the survival of certain species, ecological studies, conservation, solitude, and recreation. Wilderness is deeply valued for cultural, spiritual, moral, and aesthetic reasons. Some nature writers believe wilderness areas are vital for the human spirit and creativity.[34] ACER Extensa 5635Z laptop keyboard
The word, "wilderness", derives from the notion of wildness; in other words that which is not controllable by humans. The word's etymology is from the Old English wildeornes, which in turn derives from wildeor meaning wild beast (wild + deor = beast, deer).[35] From this point of view, it is the wildness of a place that makes it a wilderness. TOSHIBA Satellite l25-s121 laptop keyboard
The mere presence or activity of people does not disqualify an area from being "wilderness." Many ecosystems that are, or have been, inhabited or influenced by activities of people may still be considered "wild." This way of looking at wilderness includes areas within which natural processes operate without very noticeable human interference. SONY KFRGBA028B laptop keyboard
Wildlife includes all non-domesticated plants, animals and other organisms. Domesticating wild plant and animal species for human benefit has occurred many times all over the planet, and has a major impact on the environment, both positive and negative. Wildlife can be found in all ecosystems. Deserts, rain forests, HP Pavilion DV7t-3300 CTO laptop keyboard
plains, and other areas—including the most developed urban sites—all have distinct forms of wildlife. While the term in popular culture usually refers to animals that are untouched by human factors, most scientists agree that wildlife around the world is impacted by human activities. SONY VAIO VGN-FS215M laptop keyboard
In biology, an organism is any contiguous living system (such as animal, fungus, micro-organism, or plant). In at least some form, all types of organisms are capable of responding to stimuli, reproduction, growth and development, and maintenance ofhomeostasis as a stable whole. HP Pavilion dv6-3134nr laptop keyboard
An organism may be either unicellular (a single cell) or, as in the case of humans, comprise many trillions of cells grouped into specialized tissues and organs. The term multicellular (many cells) describes any organism made up of more than one cell. An organism may be either a prokaryote or a eukaryote. TOSHIBA MP-06873US-6983 laptop keyboard
Prokaryotes are represented by two separate domains, the Bacteria and Archaea. Eukaryotic organisms are characterized by the presence of a membrane-bound cell nucleus and contain additional membrane-bound co HP Pavilion dv6-2170ee laptop keyboard
mpartments called organelles (such asmitochondria in animals and plants and plastids in plants and algae, all generally considered to be derived fromendosymbiotic bacteria).[1] Fungi, animals and plants are examples of kingdoms of organisms within the eukaryotes. FUJITSU Lifebook S7110 laptop keyboard
In 2002 Thomas Cavalier-Smith proposed a clade, Neomura, which groups together the Archaea and Eukarya. Neomura is thought to have evolved from Bacteria, more specifically from Actinobacteria.[2] FUJITSU Siemens M1010 laptop keyboard
See the article: Branching order of bacterial phyla (Cavalier-Smith, 2002).
The word organism may broadly be defined as an assembly of molecules functioning as a more or less stable whole that exhibits the properties of life. However, many sources propose definitions that exclude viruses and theoretically possible man-made non-organic life forms.[18] Viruses are dependent on the biochemical machinery of a host cell for reproduction. SAMSUNG N128 laptop keyboard
Chambers Online Reference provides a broad definition: "any living structure, such as a plant, animal, fungus or bacterium, capable of growth and reproduction".[19]
In multicellular terms, "organism" usually describes the whole hierarchical assemblage of systems (for example circulatory, SAMSUNG NP-N145-JP01 laptop keyboard
digestive, or reproductive) themselves collections of organs; these are, in turn, collections of tissues, which are themselves made of cells. In some plants and the nematode Caenorhabditis elegans, individual cells are totipotent. TOSHIBA Satellite l10 laptop keyboard
A superorganism is an organism consisting of many individuals working together as a single functional or social unit.
Viruses are not typically considered to be organisms because they are incapable of autonomous reproduction, SONY 013-000A-8715-C laptop keyboard
growth or metabolism. This controversy is problematic because some cellular organisms are also incapable of independent survival (but not of independent metabolism and procreation) and live as obligatory intracellular parasites. HP Pavilion dv6-3050tx laptop keyboard
Naturally, this rules out autonomous reproduction and they can only be passively replicated by the machinery of the host cell. In this sense they are similar to inanimate matter. While viruses sustain no independent metabolism, and thus are usually not accounted organisms, they do have their own genes and they do evolve by similar mechanisms by which organisms evolve. HP Pavilion dv6-3093ca laptop keyboard
The most common argument in support of viruses as living organisms is their ability to undergo evolution and replicate through self-assembly. Some scientists argue that viruses neither evolve, nor self- reproduce. In fact, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. DELL 0PVDG3 laptop keyboard
If host cells did not exist, viral evolution would be impossible. This is not true for cells. If viruses did not exist, the direction of evolution could be different; however, the ability to evolve would not be affected. HP Mini 210-1170NR Laptop Keyboard
As for the reproduction, viruses totally rely on hosts' machinery to replicate themselves.[20] The discovery of viral megagenomes with genes coding for energy metabolism and protein synthesis fueled the debate about whether viruses belong on the tree of life. The presence of these genes suggested that viruses could metabolize in the past. HP Compaq NC8430 Laptop Keyboard
It was found later that the genes coding for energy and protein metabolism have cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts.[20]
To give an example, SAMSUNG R19 Laptop Keyboard
Homo sapiens is the Latin binomial equating to modern humans. All members of the species sapiens are, at least in theory, genetically able to interbreed. Several species may belong to a genus, but the members of different species within a genus are usually unable to interbreed to produce fertile offspring. GATEWAY MT6920 Laptop Keyboard
(The red wolf Canis lupus rufus, however, may be a result of interbreeding between the grey wolf Canis lupus and the coyote Canis latrans.) Homo only has one surviving species (sapiens), Homo erectus, Homo neanderthalensis, LENOVO IdeaPad S10 20015 Laptop Keyboard
etc. having become extinct thousands of years ago; some scientists argue for interbreeding between H. sapiens and H. neanderthalensis with fertile progeny. Several genera belong to the same family and so on up the hierarchy. Eventually, the relevant kingdom (Animalia, HP G62-a43SE Laptop Keyboard
in the case of humans) is placed into one of the three domains depending upon certain genetic and structural characteristics. SONY VAIO VGN-CR22/W Laptop Keyboard
All living organisms known to science are given classification by this system such that the species within a particular family are more closely related and genetically similar than the species within a particular phylum. DELL Vostro 3500 Laptop Keyboard
Since viruses are not living organisms, their classification is a challenging task. At first, viruses were classified according to their hosts: plant viruses, animal viruses, bacteriophages. Later, they were classified by the disease that they cause. For example, respiratory viruses, enterics. Now, viruses are classified based on the nucleic acid content, capsid symmetry and the presence or absence of the envelope. SONY 148096222 Laptop Keyboard
Organisms are complex chemical systems, organized in ways that promote reproduction and some measure of sustainability or survival. The same laws that govern non-living chemistry govern the chemical processes of life. lENOVO IdeaPad G550 Laptop Keyboard
It is generally the phenomena of entire organisms that determine their fitness to an environment and therefore the survivability of their DNA-based genes.
Organisms clearly owe their origin, HP 407218-001 Laptop Keyboard
metabolism, and many other internal functions to chemical phenomena, especially the chemistry of large organic molecules. Organisms are complex systems of chemical compounds that, through interaction and environment, play a wide variety of roles. TOSHIBA Satellite l25-s121 laptop keyboard
Organisms are semi-closed chemical systems. Although they are individual units of life (as the definition requires), they are not closed to the environment around them. To operate they constantly take in and release energy. Autotrophs produce usable energy (in the form of organic compounds) using light from the sun or inorganic compounds while heterotrophs take in organic compounds from the environment. SONY VAIO VGN-SZ210P/B laptop keyboard
The primary chemical element in these compounds is carbon. The chemical properties of this element such as its great affinity for bonding with other small atoms, including other carbon atoms, and its small size making it ASUS X53S laptop keyboard
capable of forming multiple bonds, make it ideal as the basis of organic life. It is able to form small three-atom compounds (such as carbon dioxide), as well as large chains of many thousands of atoms that can store data (nucleic acids), hold cells together, and transmit information (protein). HP G61-420CA Laptop Keyboard
The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells, SONY 148738121 Laptop Keyboard
and cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
There are two types of cells, TOSHIBA Satellite L300 Laptop Keyboard
eukaryotic and prokaryotic. Prokaryotic cells are usually singletons, while eukaryotic cells are usually found in multicellular organisms. Prokaryotic cells lack a nuclear membrane so DNA is unbound within the cell, eukaryotic cells have nuclear membranes. HP G42-415DX Laptop Keyboard
All cells, whether prokaryotic or eukaryotic, have a membrane, which envelops the cell, separates its interior from its environment, regulates what moves in and out, and maintains the electric potential of the cell. HP Pavilion G6-1C58DX Laptop Keyboard
Inside the membrane, a salty cytoplasm takes up most of the cell volume. All cells possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. HP Mini 110-1212NR Laptop Keyboard
In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool. Evidence for common descent may be found in traits shared between all living organisms. HP HDX18 Laptop Keyboard
In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds have wings, even those that do not fly.
Today, there is debate over whether or not all organisms descended from a common ancestor, or a "last universal ancestor" (LUA), HP 597635-001 Laptop Keyboard
also called the "last universal common ancestor" (LUCA), the hypothetical unicellular organism or single cell that gave rise to all life on Earth 3.5 to 3.8 billion years ago.[22] The universality of genetic coding suggests common ancestry. For example, HP Mini 210-1130LA Laptop Keyboard
every living cell makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins, although exceptions to the basic twenty amino acids have been found. However, throughout history groupings based on appearance or function of species have sometimes been polyphyletic due to convergent evolution. Lenovo ThinkPad T410-2537 Laptop Keyboard
It was once thought that the genetic code was invariably universal (see: Universal genetic code), but many variations have been discovered[23] including various alternative mitochondrial codes.[24] Back in the early 1970s, IBM Lenovo ThinkPad R50 2883 Laptop Keyboard
evolutionary biologists thought that a given piece of DNA specified the same protein subunit in every living thing, and that the genetic code was thus universal. This was interpreted as evidence that every organism had inherited its genetic code from a single common ancestor, aka, an LUCA. In 1979, however, HP Pavilion DV7-3065dx Laptop Keyboard
exceptions to the code were found in mitochondria, the tiny energy factories inside cells. Researchers studying human mitochondrial genes discovered that they used an alternative code, and many slight variants have been discovered since,[23] SONY VGN-FE670G Laptop Keyboard
including various alternative mitochondrial codes,[24] as well as small variants such as Mycoplasma translating the codon UGA as tryptophan. Biologists subsequently found exceptions in bacteria and in the nuclei of algae and single-cell animals. For example, certain proteins may use alternative initiation (start) SAMSUNG N150 Laptop Keyboard
codons not normally used by that species.[25] In certain proteins, non-standard amino acids are substituted for standard stop codons, depending upon associated signal sequences in the messenger RNA: UGA can code for selenocysteine and UAG can code for pyrrolysine. Packard Bell PEW91 laptop keyboard
Selenocysteine is now viewed as the 21st amino acid, and pyrrolysine is viewed as the 22nd. A detailed description of variations in the genetic code can be found at the NCBI web site.
Sexual reproduction (Sex) is widespread among current eukaryotes, and evidence suggests that it is a primordial and fundamental characteristic of eukaryotes.[26] HP Mini 210-1170NR laptop keyboard
Dacks and Rogers[27] proposed, on the basis of a phylogenetic analysis, that sex was present in the common ancestor of all eukaryotes. The finding of a core set of genes necessary for meiosis in the descendants of lineages that diverged early form the eukaryotic evolutionary tree also led Ramesh et al.[28] and Malik et al.[29] TOSHIBA Satellite U305-S2808 laptop keyboard
to suggest that sex was present in a common ancestor of all eukaryotes. Furthermore, this view is supported by evidence that eukaryotes previously regarded as “ancient asexuals”, such as amoeba, were likely sexual in the past, and that most present day asexual amoeboid lineages likely arose recently and independently.[30] HP Compaq NC8430 laptop keyboard
In prokaryotes, natural bacterial transformation involves the transfer of DNA from one bacterium to another and integration of the donor DNA into the recipient chromosome by recombination. Natural bacterial transformation is considered to be a primitive sexual process and occurs in both bacteria and archaea, ACER TravelMate 2403 Series laptop keyboard
although it has been studied mainly in bacteria. Transformation is clearly a bacterial adaptation and not an accidental occurrence, because it depends on numerous gene products that specifically interact with each other to enter a state of natural competence to perform this complex process.[31] SONY VAIO VGN-CR22/W laptop keyboard
Transformation is a common mode of DNA transfer, and over 60 prokaryotic species are known to be naturally competent for transformation.[32] Transformation in prokaryotes has basic similarities to sex in eukaryotes, and it has been proposed that eukaryotic sex evolved from prokaryotic sex.[33] HP AEAT8TPU017 laptop keyboard
Little is known about the earliest developments in life. However, all existing organisms share certain traits, including cellular structure and genetic code. Most scientists interpret this to mean all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, SONY Vaio VGN-AW390J laptop keyboard
but there is no scientific consensus on the relationship of the three domains of life (Archaea, Bacteria, Eukaryota) or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of macromolecules, particularly RNA, and the behavior of complex systems. Lenovo 3000 G530 4151 laptop keyboard
The emergence of oxygenic photosynthesis (around 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, TOSHIBA MP-03433US-6984 laptop keyboard
believed to have emerged around 2 billion years ago.
In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals, the Cambrian explosion (a period of unrivaled and remarkable, but brief, organismal diversity documented in the fossils found at the Burgess Shale) saw the creation of all the major body plans, HP Mini 110-3108tu Laptop Keyboard
orphyla, of modern animals. This event is now believed to have been triggered by the development of the Hox genes. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of today's land ecosystems. SONY VAIO VGN-C2S Series Laptop Keyboard
The evolutionary process may be exceedingly slow. Fossil evidence indicates that the diversity and complexity of modern life has developed over much of the history of the earth. Geological evidence indicates that the Earth is approximately 4.6 billion years old. Studies on guppies by David Reznick at the University of California, HP Mini 110-3135dx Laptop Keyboard
Riverside, however, have shown that the rate of evolution through natural selection can proceed 10 thousand to 10 million times faster than what is indicated in the fossil record.[34] Such comparative studies however are invariably biased by disparities in the time scales over which evolutionary change is measured in the laboratory, field experiments, and the fossil record. FUJITSU Lifebook E8310 Laptop Keyboard
The word "animal" comes from the Latin word animalis, meaning "having breath".[1] In everyday colloquial usage the word incorrectly excludes humans, instead it is often used to refer only to non-human members of the kingdom Animalia. SONY VAIO VGN-C2S Series Laptop Keyboard
Sometimes, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.[2] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse assponges, jellyfish, insects, and humans.[3]
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With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, ACER TravelMate 6292 Laptop Keyboard
with one or two openings.[10] Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.[11]
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elasticglycoproteins.[12] FUJITSU Lifebook P1510 laptop keyboard
This may be calcified to form structures like shells, bones, and spicules.[13] During development, it forms a relatively flexible framework[14] upon which cells can move about and be reorganized, making complex structures possible. HP Pavilion dv6-2192el laptop keyboard
In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.[10] Also, unique to animal cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.[15] FUJITSU Amilo Mini Ui 3520 laptop keyboard
Nearly all animals undergo some form of sexual reproduction.[16] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motilespermatozoa or larger, non-motile ova.[17] HP Pavilion dv6-2143ez laptop keyboard
These fuse to form zygotes, which develop into new individuals.[18]
Many animals are also capable of asexual reproduction.[19] This may take place throughparthenogenesis, where fertile eggs are produced without mating, budding, orfragmentation.[20] HP G42-368TX laptop keyboard
A zygote initially develops into a hollow sphere, called a blastula,[21] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.[22] HP G61-420CA laptop keyboard
In most other groups, the blastula undergoes more complicated rearrangement.[23] It first invaginates to form a gastrulawith a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.[24] In most cases, a mesoderm also develops between them.[25] HP G62-237CA laptop keyboard
These germ layers then differentiate to form tissues and organs.[26]
All animals are heterotrophs, meaning that they feed directly or indirectly on other living things.[27] They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.[28] SONY Vaio VGN-FW170J/W laptop keyboard
Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked).[29] Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey.[30] The other main category of consumption is detritivory, the consumption of deadorganic matter.[31] ACER Extensa 5635Z laptop keyboard
It can at times be difficult to separate the two feeding behaviours, for example, where parasitic species prey on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.[32] Lenovo 3000 G530 4151 laptop keyboard
Most animals indirectly use the energy of sunlight by eating plants or plant-eating animals. Most plants use light to convert inorganic molecules in their environment into carbohydrates, fats, proteins and other biomolecules, characteristically containing reduced carbon in the form of carbon-hydrogen bonds. Starting with carbon dioxide (CO2) and water (H2O), SONY VAIO VGN-CR32G laptop keyboard
photosynthesis converts the energy of sunlight into chemical energy in the form of simple sugars (e.g.,glucose), with the release of molecular oxygen. These sugars are then used as the building blocks for plant growth, including the production of other biomolecules.[10] When an animal eats plants (or eats other animals which have eaten plants), TOSHIBA Satellite L755-S5107 laptop keyboard
the reduced carbon compounds in the food become a source of energy and building materials for the animal.[33] They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.[34][35] Lenovo ThinkPad T410-2537 laptop keyboard
Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight.[36] Instead chemosynthetic archaea and bacteria form the base of the food chain.[37] HP Pavilion DV7-3060us laptop keyboard
Animals are generally considered to have evolved from a flagellated eukaryote.[39] Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges.[40] ACER Aspire 3684NWXMi Laptop Keyboard
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Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists.[41] The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.[42] TOSHIBA Satellite U305-S2804 Laptop Keyboard
The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia.[43] These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.[43] SONY VAIO VGN-NW240F Laptop Keyboard
The next oldest possible animal fossils are found towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota.[44] These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.[45] HP Pavilion DV7-3057nr laptop keyboard
Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago.[46] It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible. SONY VAIO VGN-FZ19VN laptop keyboard
Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.[47] Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex asearthworms.[48] SONY VAIO VGN-FS940 laptop keyboard
During the beginning of the Tonian period around 1 billion years ago, there was a decrease in Stromatolite diversity, which may indicate the appearance of grazing animals, since stromatolite diversity increased when grazing animals went extinct at the End Permian and End Ordovician extinction events, Packard Bell PEW91 laptop keyboard
and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.[49][50] TOSHIBA NB100 laptop keyboard
It has been estimated that 99.9% of animals that have ever existed are extinct.[51]
The sponges (Porifera) were long thought to have diverged from other animals early.[54] They lack the complex organization found in most other phyla.[55] Their cells are differentiated, but in most cases not organized into distinct tissues.[56]Sponges ACER Extensa 5635Z laptop keyboard
typically feed by drawing in water through pores.[57] Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.[58] However, a phylogenomic study in 2008 of 150 genes in 29 animals across 21 phyla revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, ACER Aspire 5738PZG laptop keyboard
at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.[59]
Among the other phyla, the Ctenophora and the Cnidaria, DELL Vostro PP37L laptop keyboard
which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.[60] Both have distinct tissues, but they are not organized into organs.[61] ACER Aspire 5741Z laptop keyboard
There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic.[62] The tiny placozoans are similar, but they do not have a permanent digestive chamber. SONY VGN-FE21H laptop keyboard
The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, SONY VAIO VGN NR220E laptop keyboard
a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however — for instance adultechinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures. SONY VGN-FE21H laptop keyboard
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: thedeuterostomes and the protostomes, the latter of which includes the Ecdysozoa, Platyzoa, and Lophotrochozoa. In addition, there are a few small groups of bilaterians Compaq Presario CQ56-109WM Laptop Keyboard
with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, andOrthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well. DELL Vostro PP37L Laptop Keyboard
Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes, the first opening of the gut to appear in embryological development (thearchenteron) develops into the mouth, with the anus forming secondarily. In deuterostomes the anus forms first, SONY VAIO VGN-FS742/W Laptop Keyboard
with the mouth developing secondarily.[63] In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through invagination of the endoderm, called enterocoelic pouching.[64] Deuterostome embryos undergo radial cleavage during cell division, while protostomes undergo spiral cleavage.[65] DELL Inspiron 8500 Laptop Keyboard
All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata.[66] The former are radially symmetric and exclusively marine, such as starfish, COMPAQ Presario C700 Laptop Keyboard
sea urchins, and sea cucumbers.[67] The latter are dominated by the vertebrates, animals with backbones.[68] These include fish,amphibians, reptiles, birds, and mammals.[69] Compaq Presario CQ42-228LA Laptop Keyboard
In addition to these, the deuterostomes also include the Hemichordata, or acorn worms.[70] Although they are not especially prominent today, the important fossilgraptolites may belong to this group.[71] HP Pavilion dv5-1102tu Laptop Keyboard
The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis.[72] The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. HP Mini 110-3130tu Laptop Keyboard
Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.
The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water.[73] ACER Aspire 1410-2497 Laptop Keyboard
A number are important parasites.[74] Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom. SONY VAIO VGN-CS230J/Q Laptop Keyboard
The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since in both embryos develop withspiral cleavage.
The Platyzoa include the phylum Platyhelminthes, the flatworms.[75] These were originally considered some of the most primitive Bilateria, SAMSUNG N150 Laptop Keyboard
but it now appears they developed from more complex ancestors.[76] A number of parasites are included in this group, such as the flukes and tapeworms.[75] Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.[77] TOSHIBA V000130390 Laptop Keyboard
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida,Micrognathozoa, and possibly the Cycliophora.[78] These groups share the presence of complex jaws, from which they are called the Gnathifera. TOSHIBA Mini NB 505-SP0160 Laptop Keyboard
The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida.[79][80] The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, ACER Aspire 3684NWXMi Laptop Keyboard
such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented.[81] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.[82] TOSHIBA Satellite 1800-921 Laptop Keyboard
The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a ring of ciliated tentacles around the mouth, called a lophophore.[83] These were traditionally grouped together as the lophophorates.[84] DELL NSK-D8001 Laptop Keyboard
but it now appears that the lophophorate group may be paraphyletic,[85] with some closer to the nemerteans and some to the molluscs and annelids.[86][87] They include the Brachiopoda or lamp shells, which are prominent in the fossil record, theEntoprocta, the Phoronida, and possibly the Bryozoa or moss animals.[88] ASUS X53S Laptop Keyboard
Model organisms
Main articles: Model organism and Animal testing
Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, TOSHIBA Satellite U305-S2804 laptop keyboard
the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organisms, and were among the first life-forms to be genetically sequenced. SONY VAIO VGN-NW240F laptop keyboard
This was facilitated by the severely reduced state of their genomes, but as many genes, introns, and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, HP Pavilion DV6-6000 Laptop Keyboard
and molluscan genome projects currently in progress. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.[89] HP HDX18-1024CA Laptop Keyboard
An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.[90] SONY VAIO VGN-CS230J/Q Laptop Keyboard
In biology, a species (plural: species) is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. HP Mini 110-3108tu Laptop Keyboard
While in many cases this definition is adequate, the difficulty of defining species is known as the species problem. Differing measures are often used, such as similarity of DNA, morphology, or ecological niche. Presence of specific locally adapted traits may further subdivide species into "infraspecific taxa" such as subspecies (and in botany other taxa are used, such as varieties, subvarieties, and formae). TOSHIBA NSK-TK001 Laptop Keyboard
Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, especially their DNA sequences, where available. All species are given a two-part name, a "binomial name". The first part of a binomial name is the generic name, SAMSUNG N140 Laptop Keyboard
the genus of the species. The second part is either called thespecific name (a term used only in zoology) or the specific epithet (the term used in botany, which can also be used in zoology). For example, IBM Lenovo ThinkPad R51 1833 Laptop Keyboard
Boa constrictor is one of four species of the Boa genus. The first part of the name is capitalized, and the second part has a lower case. The binomial name is written in italics when printed and underlined when handwritten. TOSHIBA Satellite L750-ST4N02 Laptop Keyboard
A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity, though other taxonomic levels such as families may be considered in broad-scale studies.[1] ACER Aspire 5742 Laptop Keyboard
Extinct species known only from fossils are generally difficult to assign precise taxonomic rankings, which is why higher taxonomic levels such as families are often used for fossil-based studies.[1][2]
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[5]
A usable definition of the word "species" and reliable methods of identifying particular species is essential for stating and testing biological theories and for measuring biodiversity. Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species. It is generally difficult to give precise taxonomic rankings to extinct species known only from fossils. HP Pavilion dv6-3178ee Laptop Keyboard
Some biologists may view species as statistical phenomena, as opposed to the traditional idea, with a species seen as a class of organisms. Lenovo 63Y0047 Laptop Keyboard
In that case, a species is defined as a separately evolving lineage that forms a single gene pool. Although properties such as DNA-sequences and morphology are used to help separate closely related lineages,[6] this definition has fuzzy boundaries.[7] However, Lenovo ThinkPad Edge E520 Laptop Keyboard
the exact definition of the term "species" is still controversial, particularly in prokaryotes,[8] and this is called the species problem.[9] Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species of concern.[9] ACER Aspire 2920Z Laptop Keyboard
The question of how best to define "species" is one that has occupied biologists for centuries, and the debate itself has become known as the species problem. Darwin wrote in chapter II of On the Origin of Species: ASUS X88S Laptop Keyboard
No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.[20] HP Pavilion DV7-3085dx laptop keyboard
But later, in The Descent of Man, when addressing "The question whether mankind consists of one or several species", Darwin revised his opinion to say:
of the term "species" is generally accepted; and the definition must not include an element that cannot possibly be ascertained, such as an act of creation.[21] HP Pavilion dv6-3050sa laptop keyboard
The modern theory of evolution depends on a fundamental redefinition of "species". Prior to Darwin, naturalists viewed species as ideal or general types, which could be exemplified by an ideal specimen bearing all the traits general to the species. Darwin's theories shifted attention from uniformity to variation and from the general to the particular. According to intellectual historian Louis Menand, SONY VAIO VGN-AR670 laptop keyboard
Once our attention is redirected to the individual, we need another way of making generalizations. We are no longer interested in the conformity of an individual to an ideal type; we are now interested in the relation of an individual to the other individuals with which it interacts. To generalize about groups of interacting individuals, TOSHIBA Satellite l25-s1215 laptop keyboard
we need to drop the language of types and essences, which is prescriptive (telling us what finches should be), and adopt the language of statistics and probability, which is predictive (telling us what the average finch, under specified conditions, is likely to do). Relations will be more important than categories; ASUS 04GNH41KUS00-2 laptop keyboard
functions, which are variable, will be more important than purposes; transitions will be more important than boundaries; sequences will be more important than hierarchies.[22]
This shift results in a new approach to "species"; Compaq Presario CQ56-109WM laptop keyboard
Darwin concluded that species are what they appear to be: ideas, which are provisionally useful for naming groups of interacting individuals. "I look at the term species", he wrote, "as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other ... DELL Vostro PP37L laptop keyboard
It does not essentially differ from the word variety, which is given to less distinct and more fluctuating forms. The term variety, again, in comparison with mere individual differences, is also applied arbitrarily, and for convenience sake." [22] SONY VAIO VGN-N250E/B laptop keyboard
Practically, biologists define species as populations of organisms that have a high level of genetic similarity. This may reflect an adaptation to the same niche, and the transfer of genetic material from one individual to others, DELL Vostro PP38L laptop keyboard
through a variety of possible means. The exact level of similarity used in such a definition is arbitrary, but this is the most common definition used for organisms that reproduce asexually (asexual reproduction), such as some plants andmicroorganisms. SONY VAIO VGN-CR160F laptop keyboard
This lack of any clear species concept in microbiology has led to some authors arguing that the term "species" is not useful when studying bacterial evolution. Instead they see genes as moving freely between even distantly related bacteria, with the entire bacterial domain being a single gene pool. Nevertheless, Compaq Presario CQ56-109WM laptop keyboard
a kind of rule of thumb has been established, saying that species of Bacteria or Archaea with 16S rRNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA Hybridization if they belong to the same species or not.[23] This concept has been updated recently, saying that the border of 97% was too low and can be raised to 98.7%.[24] DELL Vostro PP38L laptop keyboard
In the study of sexually reproducing organisms, where genetic material is shared through the process of reproduction, the ability of two organisms to interbreed and produce fertile offspring of both sexes is generally accepted as a simple indicator that the organisms share enough genes to be considered members of the same species. Thus a "species" is a group of interbreeding organisms. SONY VAIO VGN-SR220J/H laptop keyboard
This definition can be extended to say that a species is a group of organisms that could potentially interbreed – fish could still be classed as the same species even if they live in different lakes, as long as they could still interbreed were they ever to come into contact with each other. On the other hand, ACER Aspire 1410-2497 laptop keyboard
there are many examples of series of three or more distinct populations, where individuals of the population in the middle can interbreed with the populations to either side, but individuals of the populations on either side cannot interbreed. Thus, one could argue that these populations constitute a single species, or two distinct species. This is not a paradox; it is evidence that species are defined by gene frequencies, and thus have fuzzy boundaries. SONY VAIO VGN-CS230J/Q laptop keyboard
Consequently, any single, universal definition of "species" is necessarily arbitrary. Instead, biologists have proposed a range of definitions; which definition a biologists uses is a pragmatic choice, depending on the particularities of that biologist's research. SONY VAIO VGN-NR50B laptop keyboard
In practice, these definitions often coincide, and the differences between them are more a matter of emphasis than of outright contradiction. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgment. Given the complexity of life, some have argued[who?] HP Pavilion dm1-1028tu laptop keyboard
that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition.
For most vertebrates, this is the biological species concept (BSC), and to a lesser extent (or for different purposes) the phylogenetic species concept (PSC). ASUS A3Hf laptop keyboard
Many BSC subspecies are considered species under the PSC; the difference between the BSC and the PSC can be summed up insofar as that the BSC defines a species as a consequence of manifest evolutionary history, while the PSC defines a species as a consequence of manifest evolutionary potential. DELL Vostro 3550 Laptop Keyboard
Thus, a PSC species is "made" as soon as an evolutionary lineage has started to separate, while a BSC species starts to exist only when the lineage separation is complete. Accordingly, there can be considerable conflict between alternative classifications based upon the PSC versus BSC, SONY 147915321 Laptop Keyboard
as they differ completely in their treatment of taxa that would be considered subspecies under the latter model (e.g., the numerous subspecies of honey bees).
The naming of a particular species may be regarded as a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, HP Mini 110-3000ea laptop keyboard
the hypothesis may be confirmed or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. TOSHIBA Satellite l20-157 laptop keyboard
When two named species are discovered to be of the same species, the older species name is usually retained, and the newer species name dropped, a process called synonymization, or colloquially, as lumping. HP Pavilion dv6-3013cl laptop keyboard
Dividing a taxon into multiple, often new, taxons is called splitting. Taxonomists are often referred to as "lumpers" or "splitters" by their colleagues, depending on their personal approach to recognizing differences or commonalities between organisms (see lumpers and splitters). Acer eMachines G730ZG laptop keyboard
Traditionally, researchers relied on observations of anatomical differences, and on observations of whether different populations were able to interbreed successfully, to distinguish species; both anatomy and breeding behavior are still important to assigning species status. As a result of the revolutionary (and still ongoing) HP Pavilion dv6-3125ej laptop keyboard
advance in microbiological research techniques, including DNA analysis, in the last few decades, a great deal of additional knowledge about the differences and similarities between species has become available. HP Pavilion dv6-3050tx laptop keyboard
Many populations formerly regarded as separate species are now considered a single taxon, and many formerly grouped populations have been split. Any taxonomic level (species, genus, family, etc.) can be synonymized or split, and at higher taxonomic levels, these revisions have been still more profound. SONY VAIO VGN-CS31S/R laptop keyboard
From a taxonomical point of view, groups within a species can be defined as being of a taxon hierarchically lower than a species. In zoology only the subspeciesis used, while in botany the variety, subvariety, HP Pavilion dv6-3125ej laptop keyboard
and form are used as well. In conservation biology, the concept of evolutionary significant units (ESU) is used, which may define either species or smaller distinct population segments. Identifying and naming species is the providence of alpha taxonomy. ACER Aspire 5320 laptop keyboard
In the earliest works of science, a species was simply an individual organism that represented a group of similar or nearly identical organisms. No other relationships beyond that group were implied. SONY VAIO VGN-FZ21S laptop keyboard
Aristotle used the words genus and species to mean generic and specific categories. Aristotle and other pre-Darwinian scientists took the species to be distinct and unchanging, with an "essence", like the chemical elements. SONY VAIO VGN-NW18H/T laptop keyboard
When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. Many of these early delineation schemes would now be considered whimsical and these included consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants). TOSHIBA Tecra A9-50N laptop keyboard
John Ray (1686), an English naturalist, was the first to give a biological definition of the term species.[50]
In the 18th century Swedish scientist Carolus Linnaeus classified organisms according to differences in the form ofreproductive apparatus. Although his system of classification sorts organisms according to degrees of similarity, COMPAQ Presario CQ60-151EP laptop keyboard
it made no claims about the relationship between similar species. At that time, it was still widely believed that there was no organic connection between species, no matter how similar they appeared. ASUS MP-06833US-D57 laptop keyboard
This approach also suggested a type of idealism: the notion that each species existed as an "ideal form". Although there are always differences (although sometimes minute) between individual organisms, Linnaeus considered such variation problematic. He strove to identify individual organisms that were exemplary of the species, and considered other non-exemplary organisms to be deviant and imperfect. HP Pavilion DV7-3065dx Laptop Keyboard
By the 19th century most naturalists understood that species could change form over time, and that the history of the planet provided enough time for major changes. Jean-Baptiste Lamarck, in his 1809 Zoological Philosophy, offered one of the first logical arguments against creationism. The new emphasis was on determining how a species could change over time. HP 488590-001 Laptop Keyboard
Lamarck suggested that an organism could pass on an acquired trait to its offspring, i.e., the giraffe's long neck was attributed to generations of giraffes stretching to reach the leaves of higher treetops (this well-known and simplistic example, however, does not do justice to the breadth and subtlety of Lamarck's ideas). DELL Precision M4500 Laptop Keyboard
With the acceptance of the natural selection idea ofCharles Darwin in the 1860s, however, Lamarck's view of goal-oriented evolution, also known as a teleological process, was eclipsed. Recent interest in inheritance of acquired characteristics centers around epigenetic processes, e.g. methylation, Compaq Presario CQ56-103LA Laptop Keyboard
that do not affect DNA sequences, but instead alter expression in an inheritable manner. Thus, Neo-Lamarckism, as it is sometimes termed, is not a challenge to the theory of evolution by natural selection. GATEWAY M-6337 Laptop Keyboard
Charles Darwin and Alfred Wallace provided what scientists now consider as the most powerful and compelling theory of evolution. Darwin argued that it was populations that evolved, not individuals. HP Mini 210-1099TU Laptop Keyboard
His argument relied on a radical shift in perspective from that of Linnaeus: rather than defining species in ideal terms (and searching for an ideal representative and rejecting deviations), Darwin considered variation among individuals to be natural. He further argued that variation, far from being problematic, actually provides the explanation for the existence of distinct species. Lenovo ThinkPad Edge E520 Laptop Keyboard
Darwin's work drew on Thomas Malthus' insight that the rate of growth of a biological population will always outpace the rate of growth of the resources in the environment, such as the food supply. As a result, Darwin argued, not all the members of a population will be able to survive and reproduce. Those that did will, ACER Aspire 5810T-8929 Laptop Keyboard
on average, be the ones possessing variations—however slight—that make them slightly better adapted to the environment. If these variable traits are heritable, then the offspring of the survivors will also possess them. Thus, over many generations, adaptive variations will accumulate in the population, while counter-adaptive traits will tend to be eliminated. HP G72-a20EM Laptop Keyboard
Whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the "fight for existence") that selects the traits to be passed on. This is the theory of evolution by natural selection. IHP AEUT3U00140 Laptop Keyboard
n this model, the length of a giraffe's neck would be explained by positing that proto-giraffes with longer necks would have had a significant reproductive advantage to those with shorter necks. Over many generations, the entire population would be a species of long-necked animals. Compaq Presario CQ42-228LA Laptop Keyboard
In 1859, when Darwin published his theory of natural selection, the mechanism behind the inheritance of individual traits was unknown. Although Darwin made some speculations on how traits are inherited (pangenesis), his theory relies only on the fact that inheritable traits exist, and are variable (which makes his accomplishment even more remarkable.) ACER TravelMate 6292 Laptop Keyboard
Although Gregor Mendel's paper on genetics was published in 1866, its significance was not recognized. It was not until 1900 that his work was rediscovered by Hugo de Vries, Carl Correns and Erich von Tschermak, who realised that the "inheritable traits" in Darwin's theory aregenes. HP 317443-001 Laptop Keyboard
The theory of the evolution of species through natural selection has two important implications for discussions of species—consequences that fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, ACER Aspire 4920 Laptop Keyboard
it suggests that species are not just similar, they may actually be related. Some students of Darwin argue that all species are descended from a common ancestor. Second, it supposes that "species" are not homogeneous, fixed, permanent things; members of a species are all different, and over time species change. HP Mini 110-1079TU Laptop Keyboard
This suggests that species do not have any clear boundaries but are rather momentary statistical effects of constantly changing gene-frequencies. One may still use Linnaeus' taxonomy to identify individual plants and animals, but one can no longer think of species as independent and immutable. HP Pavilion DV6-6000 Laptop Keyboard
The rise of a new species from a parental line is called speciation. There is no clear line demarcating the ancestral species from the descendant species.
Although the current scientific understanding of species suggests that there is no rigorous and comprehensive way to distinguish between different species inall cases, HP HDX18-1024CA Laptop Keyboard
biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation; if two creatures cannot reproduce to produce fertile offspring of both sexes, then they are in different species. This definition captures a number of intuitive species boundaries, SONY 147915321 Laptop Keyboard
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but it remains imperfect. It has nothing to say about species that reproduce asexually, for example, and it is very difficult to apply to extinct species. Moreover, boundaries between species are often fuzzy: ACER Aspire 7745G Laptop Keyboard
there are examples where members of one population can produce fertile offspring of both sexes with a second population, and members of the second population can produce fertile offspring of both sexes with members of a third population, but members of the first and third population cannot produce fertile offspring, HP Mini 210-1001SA Laptop Keyboard
or can only produce fertile offspring of the homozygous sex. Consequently, some people reject this definition of a species.
Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, DELL Studio 1458 Laptop Keyboard
both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118). However, most if not all taxonomists would strongly disagree[citation needed]. For example, in manyamphibians, SAMSUNG NP-Q320-FS01UK Laptop Keyboard
most notably in New Zealand's Leiopelma frogs, the genome consists of "core" chromosomes that are mostly invariable and accessory chromosomes, of which exist a number of possible combinations. HP Pavilion dv2-1030ea Laptop Keyboard
Even though the chromosome numbers are highly variable between populations, these can interbreed successfully and form a single evolutionary unit. In plants, polyploidy is extremely commonplace with few restrictions on interbreeding; as individuals with an odd number of chromosome sets are usually sterile, DELL Precision M4500 Laptop Keyboard
depending on the actual number of chromosome sets present, this results in the odd situation where some individuals of the same evolutionary unit can interbreed with certain others and some cannot, with all populations being eventually linked as to form a common gene pool. HP Mini 210-1014TU Laptop Keyboard
The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on molecular markers, starting with the comparatively crude blood plasma precipitation assays in the mid-20th century to Charles Sibley's ground-breaking DNA-DNA hybridization studies in the 1970s leading to DNA sequencing techniques. HP 640436-001 Laptop Keyboard
The results of these techniques caused revolutionary changes in the higher taxonomic categories (such as phyla and classes), resulting in the reordering of many branches of the phylogenetic tree (see also: molecular phylogeny). TOSHIBA Satellite L300 Laptop Keyboard
For taxonomic categories below genera, the results have been mixed so far; the pace of evolutionary change on the molecular level is rather slow, yielding clear differences only after considerable periods of reproductive separation. DNA-DNA hybridization results have led to misleading conclusions, SAMSUNG N150 Laptop Keyboard
the Pomarine Skua – Great Skuaphenomenon being a famous example. Turtles have been determined to evolve with just one-eighth of the speed of other reptiles on the molecular level, and the rate of molecular evolution in albatrosses is half of what is found in the rather closely related storm-petrels. Lenovo ThinkPad Edge E520 Laptop Keyboard
The hybridization technique is now obsolete and is replaced by more reliable computational approaches for sequence comparison. Molecular taxonomy is not directly based on the evolutionary processes, HP Pavilion DV7-3065dx Laptop Keyboard
but rather on the overall change brought upon by these processes. The processes that lead to the generation and maintenance of variation such as mutation, crossover and selection are not uniform (see also molecular clock). Lenovo 63Y0047 Laptop Keyboard
DNA is only extremely rarely a direct target of natural selection rather than changes in the DNA sequence enduring over generations being a result of the latter; for example, silent transition-transversion combinations would alter the melting point of the DNA sequence, HP Pavilion DV7-3173nr laptop keyboard
but not the sequence of the encoded proteins and thus are a possible example where, for example in microorganisms, a mutation confers a change in fitness all by itself. COMPAQ Presario CQ60-420us laptop keyboard
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