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'''Species''' is a [[taxonomy|taxonomic]] concept used in [[biology]] to refer to a population of [[organism|organisms]] that are in some important ways similar. The
* A '''morphological species''' is a group of organisims that have a distinctive form: for example, we can distinguish between a chicken and a duck because they have different shaped bills and the duck has webbed feet. Species have been defined in this way since well before the begining of recorded history. Although much criticised, 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.
* The '''biological species''' or '''isolation species''' concept identifies a species as a set of actually or potentially interbreeding organisims. This is generally the most useful formulation for scientists working with living examples of the higher taxa like mammals, fish, and birds, but meaningless for organisms that do not reproduce sexually. It distinguishes between the theoretical possibility of interbreeding and the actual liklihood of gene flow between populations. For example, it is possible to cross a horse with a donkey and produce offspring, however they remain seperate species—in this case for two seperate reasons: first because horses and donkeys do not norally interbreed in the wild, and second because the fruit of the union is rarely fertile. The key to defining a biological species is that there is no significant cross-flow of genetic material betwen the two populations.
* A '''mate recognition species''' is defined as a group of organisims that are known to recognise one another as potential mates. Like the isolation species concept above, it is not applicable to organisims that do not reproduce sexually.
* A '''phylogenetic species''' or '''Darwinian species''' is a group of organisms that share a common ancestor.
In practice, these definitions often coincide. Nevertheless, no species concept yet proposed is entirely objective, or can be applied in all cases without resorting to judgement.
==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. When early observers began to develop systems of organization for living things, they began to place the 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 some believed that there is no organic connection between species no matter how similar they appear; every species was created by [[God]], a view today called [[creationism]]. This approach also suggests a type of idealism, in other words, that each species exists as an ideal form. In fact, 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.
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