This article deals with the the biological usage of the term. See combinatorial species for a mathematical usage.
Species is a taxonomic concept used in biology to refer to a population of organisms that are in some important ways similar. The idea of species has a long history. 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. There are several main lines of thought in the definition of species:
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.
The naming of a particular species should be regarded as a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be confirmed or refuted. As a result of the revolutionary (and still ongoing) advance in microbiological research techniques in the later years of the 20th century, a great deal of extra knowledge about the differences and similarities between species has become available. Many populations which were formerly regarded as separate species are now considered to be a single biological unit, and many formerly grouped populations have been split. At higher taxonomic levels, these changes have been still more profound.
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.
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.
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:
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).
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 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 common to believe 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. Lamarck suggested that an organism could pass on an acquired trait to its offspring. As an example, imagine an animal that repeatedly stretches its neck in order to reach the treetops: the longer neck that it has acquired would then, according to this theory, be passed on to its offspring. This well-known and simplistic example, however, does not do justice to the breadth and subtly 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 advent of Darwin, Lamarck's reputation suffered gravely. 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 such variation, far from being problematic, is actually a good thing.
Following Thomas Malthus, he suggested that population would often exceed the amount of food available, and that some organisms would die. Darwin suggested that those organisms that would die would be those less adapted to their environment, and that those that survived -- and reproduced -- would be those best adapted to their environment. Variation among members of a species is important because different and changing environments favor different traits (i.e. there is no ideal trait; whether a trait is beneficial or not depends on the environment).
These survivors would not pass acquired traits on to their offspring; they would pass their inherited traits on to their offspring. But since the environment effectively selected which organisms would live to reproduce, the environment would select which traits would be passed on. This is the theory of evolution by "natural selection." For example, among a group of animals some have longer necks, others have shorter necks. If all the leaves are high up, those with shorter necks will die; those with longer necks will thrive. This process is evident today as resistant strains of bacteria evolve.
The development of the field of genetics (many years after Darwin) has revealed the mechanisms that generate variation as well as those through which traits are passed on from generation to generation.
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 principled, black and white 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 is it 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).
A species name can be:
There are several common species names. Most of these are adjectives.
Linnaean taxonomy discusses how the category "species" meshes with other classification categories, such as "kingdom" and "genus".
Compare with race.