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 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 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
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