Species

Species are the basic taxonomic units of biological classification. This grouping of individuals and populations of organisms of "like kind" into discrete and stable units has been traced at least from the time of Plato and Aristotle. Nonetheless, today biologists lack consensus in how to define the term and what constitutes a species. Among the several definitions of species, the most commonly used is the biological species definition first coined by Ernst Mayr: Species are "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." (However, see other definitions of species below.)

This basic taxonomic unit is remarkably stable. Species tend to remain the same throughout their geological history. As noted by eminent evolutionist Stephen Jay Gould, the macroevolutionry patterns of species are typically ones of morphological stability during their existence, a phenomena known as "stasis." In presenting the theory of punctuated equilibrium, Niles Eldridge and Gould noted: "Most species, during their geological history, either do not change in any appreciable way, or else they fluctuate mildly in morphology, with no apparent direction." Once a species appears, the fossil record does not change much during its existence, which may be several million years. This view accords well with the view of Unification Thought, which references a clear-cut boundary between species and stability during their existence.

File:Ivorybilledwoodpecker.jpg

The Ivory-billed Woodpecker (Campephilus principalis) is an endangered species.

The concept of species is important. For one, environmental law is framed in terms of species. Indeed, many countries have laws proscribing special protection to species considered endangered to prevent their extinction. The term species also is central to evolutionary studies, and is generally presented as the principal unit of evolution. Ernst Mayr maintains that one cannot even write about evolution, or most aspects of the philosophy of biology, without understanding the meaning of the biological species.

Both the singular and plural forms of the noun are rendered by the word "species." The term derives from the Latin "specere" (to look at, to behold), with the meaning of "kind," "quality," "appearance," "shape," or "a pecular sort." Note that the word "specie" is NOT the singular of "species." It refers to coined money.

Scientific name

In taxonomy, a species is assigned a two-part scientific name. The genus is listed first (and capitalized) followed by a species epithet (which is not capitalized). For example, humans belong to the genus Homo, and are in the species Homo sapiens. Tigers, lions, leopards, and jaguars are different species, but each are similar enough to belong to the same genus (Panthera). The name of the species is the whole binomial not just the second term of the binomial (the specific epithet). This system was set up by Swedish botanist Carolus Linnaeus.

The scientific name of a species is properly typeset in italics. When an unknown species is being referred to this may be done by using the abbreviation "sp." in the singular or "spp." in the plural in the place of the second part of the scientific name.

Groups within a species can be defined as being of a taxon hierarchically lower than a species. In zoology only the subspecies is used, while in botany the variety, subvariety, and form are used as well.

The idea of species has a long history. It is one of the most important levels of classification, for several reasons:

• It often corresponds to what lay people treat as the different basic kinds of organism - dogs are one species, cats another.
• It is the standard binomial nomenclature (or trinomial nomenclature) by which scientists typically refer to organisms.
• It is the only taxonomic level which has empirical content, in the sense that asserting that two animals are of different species is saying something more than classificatory about them.

After thousands of years of use, the concept remains central to biology and a host of related fields, and yet also remains at times ill-defined and controversial.

Definitions of species

Several, different concepts are employed in identifying species:

• Typological (or "morphological") species concept. Historically, species were viewed as collections of individuals that share a common phenotype, including morphology, behavior, and ecological relationships with their environment. This method was used as a "classical" method of determining species. For example, continental North American savanna sparrows were differentiated from savanna sparrows from Sable Island, Nova Scotia if they were sufficiently different in morphological characters. A chicken and a duck can be distinguished because they have different shaped bills and the duck has webbed feet. This traditional method is useful in the work of taxonomy such as cataloging species and creating identification keys. It is also applicable in paleontology, where all there is is morphology (such as snail shells in fossil beds). Indeed, the concept of morphological species remains the single most widely used species concept in everyday life, and retains an important place within the biological sciences, particularly in the case of plants. Shortcomings. However, there are also important shortcomings with the typological species concept of distinguishing species. For example, different phenotypes do not always constitute different species (ie: a 4-winged Drosphila born to a 2-winged mother is not a different species). It is not uncommon to find pronounced morphological differences among individuals within one interbreeding population due to individual genetic variation, sexual dimorphism (males, females, immatures) and life stage (catepillars and butterflies), differences that may be much more evident than between clearly different species. Furthermore, there is the challenge of oversplitting taxa, whereby each varient is called a new species.

• Biological (or "isolation") species concept. This concept identifies a species as a set of actually or potentially interbreeding organisms. Or, as stated by Ernst Mayr, "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." The biological species concept (BCS), which developed in the second half of the 19th Century and was greatly advanced by Mayr in the 20th Century, involves thinking of species in terms of variable populations rather than fixed types. This is generally the most useful and common formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds. Shortcomings. The BSC is meaningless for organisms that do not reproduce sexually. It also does not distinguish between the theoretical possibility of interbreeding and the actual likelihood of gene flow between populations and is thus impractical in many instances of allopatric (geographically isolated) populations. For example, it is possible to cross a horse with a donkey and produce offspring, however they remain separate species—in this case for two different reasons: first because horses and donkeys do not normally interbreed in the wild, and second because the fruit of the union is rarely fertile. Does one successful hybridization invalidate species distinction? The key to defining a biological species is that there is no significant cross-flow of genetic material between the two populations. Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding. Similarly, how does one utilize the BCS to delineate paleospecies (extinct or fossil species)?

• Mate-recognition species concept. A mate-recognition species is defined as a group of organisms that share a common fertilization system and are known to recognise one another as potential mates. Shorcoming. Like the isolation species concept above, it applies only to organisms that reproduce sexually.

• Phylogenetic species concept. The phylogenetic species concept, which has several versions, essentially defines a species as a group of organisms bound by a unique ancestry. Devised by paleontologists Niles Eldredge and Joel Craft, it is an attempt to define species by their relationships to other species, involving uncovering their genealogical relationships. That is, it views a species as the evolutionary unit identified by evidence for patterns of ancestry and descent. A formal definition given by Joel Cracraft is: "A species is the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent." Thus, according to this concept, diagnosable geographic forms of the same basic "kind" of bird should be treated as distinct species, because these forms have evolved separately, and have unique evolutionary histories. For example, a population of sparrows that had a unique heritable character, such as a particular allele (form of a particular gene) would be considered a separate species from those that do not have that partiular character. This method is applicable even to unusual reproductive modes, other than sexual reproduction. Shortcomings. Application of this concept is problematic in those cases where morphologically different populations are connected by gene flow, and such morphological variation among populations is not uncommon within living species. For example, humans have substantial morphological variation from continent to continent. Fragmentary specimens collected by a paleontolgist on different continents, which show clinal variation, would appear to be unique. Ernst Mayr also criticizes the phylogenetic species concept as nothing more than the revival of a purely morphological species concept.

• Evolutionary (or "Darwinian") species concept. An evolutionary species is a group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. This "evolutionary species concept" (ESC) is often associated with George Gaylord Simpson, a mammalian paleontologist, who stated "a species is a series of ancestor-descendent populations passing through time and space independent of other populations, each of which possesses its own evolutionary tendencies and historical fate." The ESC is the most popular concept among paleontologists, and is used extensively in comparative biology and phylogenetic systematics. It has similarities with the phylogentic species concept, but the ESC combines the genealogical basis of the phylogenetic species concept with the genetic basis of the biological species concept. An evolutionary species is a lineage of interbreeding organisms, reproductively isolated from other lineages, that has a beginning, an end, and a distinct evolutionary trajectory (Wiley 1978). Specifically, the ESC uses a wider range of characters to make the species determination. Shortcomings. Mayr criticized this concept for several reasons: (1) it replaces the clear-cut criterion of reproductive isolation of the BCS with such vague terms as "maintains its identity," "evolutionary tendencies," and "historical fate"; (2) it is applicable only to monotypic species, and geographic isolates would have to be treated as different species; (3) there are no empirical criteria by which evolutionary tendency of historical fate can be observed in a given fossibl sample; and (4) the ESC does not help in the lower or upper demarcation of chronospecies, even though the concept was apparently introduced to deal with the time dimension.

• The ecological species concept defines a species as a group of organisms that share a distinct ecological niche. Shortcoming. This concept, which is based on the niche occuped by a species, is problematic because widespread species generall have local populations that differ in their niche occupation, which would require they be recognized as different species, even though based on all other criteria they would not be. As noted by Mayr, "fatal for the ecological species concept are the tropic species of cichlids," which differentiate niche within a single set of offspring from the same parents. There are also common cases where two sympatric species seem to occupy the same niche.

These are just a few of more than a dozen common methods of delineating species among biologists. One might also include, for example, Templeton's "cohesion species concept" that attempts to combine several component of species concepts, but which is likewise critcized as failing to resolve the resulting conflicts. 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 judgement. Given the complexity of life, some have argued that such an objective definition is in all likelihood impossible, and biologists should settle for the most practical definition.

Origin of species

No one knows how many species exist today. Approximately 1.5 million species have been identified, but some biologists estimate there may be as many as 50 million species of living things on the earth. How have so many species come to exist?

Deal here with the concept of stasis and speciation --- mainly a link to an article on speciation — and link to religious point of view, and to origin of species by Darwin Deal with religious view How connected to evolution

There are approximately ** species that have been identified, and perhaps 50 million species. How have these new species been created?

Speciation refers to the process by which new biological species arise.

Basic orthodoxy view is that created gradually............. Origin of species ... gradual transformation .....

Recently puncuational modesl ......

stasis .....

Formation of new and distinct species, whereby a single evolutionary line splits into two or more genetically independent ones.

One of the fundamental processes of evolution, speciation may occur in many ways. Investigators formerly found evidence for speciation in the fossil record by tracing sequential changes in the structure and form of organisms. Genetic studies now show that such changes do not always accompany speciation, since many apparently identical groups are in fact reproductively isolated (i.e., they can no longer produce viable offspring through interbreeding). Polyploidy (see ploidy) is a means by which the beginnings of new species are created in just two or three generations.

There are three main ideas concerning the creation of new species (Speciation mechanisms), each based on the degree to which populations undergoing this process are geographically isolated from one another. Speciation mechanisms include allopatric speciation, sympatric speciation, and parapatric speciation.

[edit] Speciation mechanisms Ernst Mayr proposed a speciation mechanism referred to as allopatric speciation. In allopatric speciation, a population splits into two geographically isolated allopatric populations (for example, by habitat fragmentation or emigration). The isolated populations then undergo genotypic and/or phenotypic divergence as they a) become subjected to dissimilar selective pressures and b) they independently undergo genetic drift. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging genes.

In sympatric speciation, species diverge while remaining sympatric (i.e. the diverging populations are not isolated geographically). Examples of sympatric speciation are found in insects which become dependent on different host plants in the same area. Increased ploidy levels, i.e. Polyploidy, is a mechanism often attributed to causing some speciation events in sympatry. Polyploidy is observed in many plant species, a well studied example being that of wheat. However, it should be noted that not all polyploids are completely reproductively isolated from their parental plants, so an increase in chromosome number may not result in the complete cessation of gene flow between the incipient polyploids and their parental diploids.

In parapatric speciation, the zones of two diverging populations abut but do not overlap. There is only partial separation afforded by geography, so individuals of each species may come in contact or cross the barrier from time to time, but reduced fitness of the heterozygote leads to selection for behaviours or mechanisms which prevent breeding between the two species.

All forms of speciation have actually taken place over the course of evolution, though it still remains a subject of debate as to the relative importance of each mechanism in driving biodiversity.

There is some debate as to the rate at which speciation events occur over geologic time. While some evolutionary biologists claim that speciation events have remained relatively constant over time, some palaeontologists such as Niles Eldredge and Stephen Jay Gould have argued that species usually remain unchanged over long stretches of time, and that speciation occurs only over relatively brief intervals, a view known as punctuated equilibrium.

The isolation species concept in more detail

In general, for large, complex, organisms that reproduce sexually (such as mammals and birds), one of several variations on the isolation or biological species concept is employed. Often, the distinction between different species, even quite closely related ones, is simple. Horses (Equus caballus) and donkeys (Equus asinus) are easily told apart even without study or training, and yet are so closely related that they can interbreed after a fashion. Because the result, a mule or hinny, is not usually fertile, they are clearly separate species.

But many cases are more difficult to decide. This is where the isolation species concept diverges from the evolutionary species concept. Both agree that a species is a lineage that maintains its integrity over time, that is diagnosably different to other lineages (else we could not recognise it), is reproductively isolated (else the lineage would merge into others, given the chance to do so), and has a working intra-species recognition system (without which it could not continue). In practice, both also agree that a species must have its own independent evolutionary history—otherwise the characteristics just mentioned would not apply. The species concepts differ in that the evolutionary species concept does not make predictions about the future of the population: it simply records that which is already known. In contrast, the isolation species concept refuses to assign the rank of species to populations that, in the best judgement of the researcher, would recombine with other populations if given the chance to do so.

The isolation question

There are, essentially, two questions to resolve. First, is the proposed species consistently and reliably distinguishable from other species? Secondly, is it likely to remain so in the future? To take the second question first, there are several broad geographic possibilities.

• The proposed species are sympatric—they occupy the same habitat. Observation of many species over the years has failed to establish even a single instance of two diagnostically different populations that exist in sympatry and have then merged to form one united population. Without reproductive isolation, population differences cannot develop, and given reproductive isolation, gene flow between the populations cannot merge the differences. This is not to say that cross breeding does not take place at all, simply that it has become negligible. Generally, the hybrid individuals are less capable of successful breeding than pure-bred individuals of either species.

• The proposed species are allopatric—they occupy different geographical areas. Obviously, it is not possible to observe reproductive isolation in allopatric groups directly. Often it is not possible to achieve certainty by experimental means either: even if the two proposed species interbreed in captivity, this does not demonstrate that they would freely interbreed in the wild, nor does it always provide much information about the evolutionary fitness of hybrid individuals. A certain amount can be inferred from other experimental methods: for example, do the members of population A respond appropriately to playback of the recorded mating calls of population B? Sometimes, experiments can provide firm answers. For example, there are seven pairs of apparently almost identical marine snapping shrimp (Altheus) populations on either side of the Isthmus of Panama, which did not exist until about 3 million years ago. Until then, it is assumed, they were members of the same 7 species. But when males and females from opposite sides of the isthmus are placed together, they fight instead of mating. Even if the isthmus were to sink under the waves again, the populations would remain genetically isolated: therefore they are now different species. In many cases, however, neither observation nor experiment can produce certain answers, and the determination of species rank must be made on a 'best guess' basis from a general knowledge of other related organisms.

• The proposed species are parapatric—they have breeding ranges that abut but do not overlap. This is fairly rare, particularly in temperate regions. The dividing line is often a sudden change in habitat (an ecotone) like the edge of a forest or the snow line on a mountain, but can sometimes be remarkably trivial. The parapatry itself indicates that the two populations occupy such similar ecological roles that they cannot coexist in the same area. Because they do not crossbreed, it is safe to assume that there is a mechanism, often behavioral, that is preventing gene flow between the populations, and that therefore they should be classified as separate species.

• There is a hybrid zone where the two populations mix. Typically, the hybrid zone will include representatives of one or both of the 'pure' populations, plus first-generation and back-crossing hybrids. The strength of the barrier to genetic transmission between the two pure groups can be assessed by the width of the hybrid zone relative to the typical dispersal distance of the organisms in question. The dispersal distance of oaks, for example, is the distance that a bird or squirrel can be expected to carry an acorn; the dispersal distance of Numbats is about 15 kilometres, as this is as far as young Numbats will normally travel in search of vacant territory to occupy after leaving the nest. The narrower the hybrid zone relative to the dispersal distance, the less gene flow there is between the population groups, and the more likely it is that they will continue on separate evolutionary paths. Nevertheless, it can be very difficult to predict the future course of a hybrid zone; the decision to define the two hybridizing populations as either the same species or as separate species is difficult and potentially controversial.

• The variation in the population is clinal—at either extreme of the population's geographic distribution, typical individuals are clearly different, but the transition between them is seamless and gradual. For example, the Koalas of northern Australia are clearly smaller and lighter in colour than those of the south, but there is no particular dividing line: the further south an individual Koala is found, the larger and darker it is likely to be; Koalas in intermediate regions are intermediate in weight and colour. In contrast, over the same geographic range, black-backed (northern) and white-backed (southern) Australian Magpies do not blend from one type to another: northern populations have black backs, southern populations white backs, and there is an extensive hybrid zone where both 'pure' types are common, as are crossbreeds. The variation in Koalas is clinal (a smooth transition from north to south, with populations in any given small area having a uniform appearance), but the variation in magpies is not clinal. In both cases, there is some uncertainty regarding correct classification, but the consensus view is that species rank is not justified in either. The gene flow between northern and southern magpie populations is judged to be sufficiently restricted to justify terming them subspecies (not full species); but the seamless way that local Koala populations blend one into another shows that there is substantial gene flow between north and south. As a result, experts tend to reject even subspecies rank in this case.

The difference question

Obviously, when defining a species, the geographic circumstances become meaningful only if the populations groups in question are clearly different: if they are not consistently and reliably distinguishable from one another, then we have no grounds for believing that they might be different species. The key question in this context, is "how different is different?" and the answer is usually "it all depends".

In theory, it would be possible to recognise even the tiniest of differences as sufficient to delineate a separate species, provided only that the difference is clear and consistent (and that other criteria are met). There is no universal rule to state the smallest allowable difference between two species, but in general, very trivial differences are ignored on the twin grounds of simple practicality, and genetic similarity: if two population groups are so close that the distinction between them rests on an obscure and microscopic difference in morphology, or a single base substitution in a DNA sequence, then a demonstration of restricted gene flow between the populations will probably be difficult in any case.

More typically, one or other of the following requirements must be met:

• It is possible to reliably measure a quantitative difference between the two groups that does not overlap. A population has, for example, thicker fur, rougher bark, longer ears, or larger seeds than another population, and although this characteristic may vary within each population, the two do not grade into one another, and given a reasonably large sample size, there is a definite discontinuity between them. Note that this applies to populations, not individual organisms, and that a small number of exceptional individuals within a population may 'break the rule' without invalidating it. The less a quantitative difference varies within a population and the more it varies between populations, the better the case for making a distinction. Nevertheless, borderline situations can only be resolved by making a 'best-guess' judgement.

• It is possible to distinguish a qualitative difference between the populations; a feature that does not vary continuously but is either entirely present or entirely absent. This might be a distinctively shaped seed pod, an extra primary feather, a particular courting behaviour, or a clearly different DNA sequence.

Sometimes it is not possible to isolate a single difference between species, and several factors must be taken in combination. This is often the case with plants in particular. In eucalypts, for example, Corymbia ficifolia cannot be reliably distinguished from its close relative Corymbia calophylla by any single measure (and sometimes individual trees cannot be definitely assigned to either species), but populations of Corymbia can be clearly told apart by comparing the colour of flowers, bark, and buds, number of flowers for a given size of tree, and the shape of the leaves and fruit.

When using a combination of characteristics to distinguish between populations, it is necessary to use a reasonably small number of factors (if more than a handful are needed, the genetic difference between the populations is likely to be insignificant and is unlikely to endure into the future), and to choose factors that are functionally independent (height and weight, for example, should usually be considered as one factor, not two).

Historical development of the species concept

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. 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. When early observers began to develop systems of organization for living things, they began to place formerly isolated species into a context. To the modern mind, many of the schemes delineated are whimsical at best, such as those that determined consanguinity based on color (all plants with yellow flowers) or behavior (snakes, scorpions and certain biting ants).

In the 18th century Carolus Linnaeus classified organisms according to differences in the form of reproductive apparatus. Although his system of classification sorts organisms according to degrees of similarity, it made no claims about the relationship between similar species. At the time, it was still widely believed that there is no organic connection between species, no matter how similar they appear; every species was individually created by God, a view today called creationism. This approach also suggested a type of idealism: the notion that each species exists 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.

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. As such, the new emphasis was on determining how a species could change over time. Jean-Baptiste 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).

Lamarck's most important insight may have been that species can be extraordinarily fluid; his 1809 Zoological Philosophy contained one of the first logical refutations of creationism. With the acceptance of the work of Charles Darwin in the 1860s, Lamarck's view of evolution was quickly eclipsed. It was not until the late 20th century that his work began to be reexamined, and took its place as a fundamental stepping stone to the modern theory of adaptive mutation. Lamarck's long-discarded ideas of the goal-oriented evolution of species, also known the teleological process, have also received renewed attention, particularly by proponents of artificial selection.

Charles Darwin and Alfred Wallace provided what scientists now consider the most powerful and compelling theory of evolution. Basically, Darwin argued that it is populations that evolve, not individuals. His argument relies on a radical shift in perspective from 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.

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, 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 will be eliminated.

It should be emphasized that 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. In 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.

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.) 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 are genes.

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, 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. 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.

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 there is no rigorous and comprehensive way to distinguish between different species in all cases, 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, then they are in different species. This definition captures a number of intuitive species boundaries, but nonetheless has some problems, however. 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: there are examples where members of one population can produce fertile offspring with a second population, and members of the second population can produce fertile offspring with members of a third population, but members of the first and third population cannot produces fertile offspring. Consequently, some people reject this notion of species.

Richard Dawkins defines two organisms as conspecific if and only if they have the same number of chromosomes and, for each chromosome, both organisms have the same number of nucleotides (The Blind Watchmaker, p. 118).

The classification of species has been profoundly affected by technological advances that have allowed researchers to determine relatedness based on genetic markers. The results have been nothing short of revolutionary, resulting in the reordering of vast expanses of the phylogenetic tree (see also: molecular phylogeny).

See also

• Speciation
• Cryptic species complex
• Ring species

References

ISBN links support NWE through referral fees

Mayr, Ernst. 1996. What is a species, and what is not? Phhilosophy of Science 63:262-277.

Wiley EO. 1978. The evolutionary species concept reconsidered. Systematic Zoology 27:17-26.

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