How does the biological species concept define separate species?

The ancient philosophical terms genus and species were used in logic to classify objects and ideas as well as living organisms. The species was a collection of objects that had a common underlying "essence"; in fact the Greek term for species and essence were the same (). The genus was a group of such species, with its own, broader essence. For biological organisms, this essentialism fitted well with the biblical story of creation, and Linnaeus adopted this essentialist view of classification in his ambitious project to catalogue every organism known at the time (18th Century). Linnaeus pioneered the use of binomial nomenclature, Latin scientific names which serve as a shorthand for descriptions of species. As an example, the scientific name of the yellow-shafted flicker (a woodpecker) is Colaptes auratus, where Colaptes is the generic and auratus the specific epithet.

The rise of evolutionary ideas meant that the old creationist essentialism was no longer tenable. Charles Darwin was interested in showing that species evolved, but to do so he obviously had to develop a species concept which depended neither on creation nor on evolution. If they evolved, varieties and species could be continuous in space and time. Thus Darwin wrote: "hereafter, we shall be compelled to acknowledge that the only distinction between species and well-marked varieties is, that the latter are known, or believed, to be connected at the present day by intermediate gradations, whereas species were formerly thus connected." To Darwin, the origin of species became the origin of the morphological gaps between populations. This species concept has been called the morphological species concept, although it emphasizes the clustering of members of the same species in morphological space, rather than the fact that morphological characters were used in its implementation.

As the geographic representation of well-organized museum collections increased, a major revolution in this Darwinian species concept began to take place (1890-1920). It was shown that some apparently good species of birds and butterflies blended together in areas of overlap or hybrid zones. Related forms that hybridize and replace each other geographically became downgraded to subspecies and were referred to by a novel extension of the Linnaean system, which now could consist of a trinomial: genus-species-subspecies. For example, the red shafted flicker (formerly Colaptes cafer) is now usually referred to as Colaptes auratus cafer, or the red-shafted race of the common flicker. This species also includes the yellow-shafted flicker, C. auratus auratus. Species with more than one subspecies became known as polytypic species. At the same time, other, more trivial forms or varieties within local populations became excluded from the formal Linnaean taxonomy.

In the 1930s and 1940s, evolutionists became dissatisfied with the Linnaean/Darwinian character-based definitions of species; they felt that species designations could reflect a real underlying biological phenomenon rather than remaining merely as categories for taxonomic convenience, and they wished to formalize this reality by specifying their idea of the important biological process. Drawing on ideas from Buffon and other early biologists, EB Poulton, T Dobzhansky and E Mayr proposed what is now known as the biological species concept, in which species are thought of as populations which do not interbreed, and are therefore reproductively isolated from other species. These ideas were developed along with (though do not necessarily require) the idea that species were important units of evolution, and that isolating mechanisms were protective devices to maintain the genetic integrity of the species.

The biological species concept seems to have been largely accepted by zoologists for about 30 years. On the other hand, botanists never fully accepted the idea because plants often had high rates of hybridization, local variability, and environmentally-induced plasticity. In recent years, however, any semblance of agreement about species concepts, even among zoologists, has been shattered. An explosion of new ideas has occurred. One movement, spearheaded by PR Ehrlich and PH Raven has claimed that populations, rather than species, are the important and real biological units of evolution. Others claim that biological processes do underlie species, but each has supported a different type of process as the important one. Examples include L Van Valen's ecological species concept, in which species are defined by their ecological niches, and HEH Paterson's recognition concept of species, in which species are defined by sexual signalling or specific mate recognition systems within species (see also isolating mechanisms). The cohesion concept of species was proposed by A Templeton to combine reproductive isolation, ecological selection, and reproductive compatibility within a single species concept. The major advantage of this idea was that both hybridizing and asexual species which, which could not be classified under the biological species concept, could be included.

A completely different approach to species concepts has been to include the idea of evolutionary history as opposed to merely the maintenance of current species. The evolutionary species concept, in which a species is a lineage evolving separately from others, was proposed by GG Simpson to allow fossils to be classified as species as well as living organisms. This idea has been formalized recently in various types of phylogenetic species concept, in which the individuals that belong to a species contain all the descendents of a single population of ancestors, that is they are monophyletic. This group of ideas was developed by J Cracraft and others specifically in response to an increase in the use of cladistics in classification. In cladistics, only apomorphies (uniquely derived traits) are used to unite groups; reproductive compatibility and free hybridization supposedly cannot be used in species definitions because they are primitive or plesiomorphic traits. Unfortunately, hybridization may also allow genes to pass from one taxon to another, and so different genes within groups of organisms may in fact have different phylogenies (phylogenies of single genes are called genealogies). To get around this problem of conflicting data, DL Baum and KL Shaw have suggested a variant phylogenetic species concept based on the consensus of many estimated genealogies of different genes; this is called the genealogical species concept. Finally, A Templeton has recently added phylogenetic and genealogical considerations, as well as ecology and reproductive isolation, to his cohesion concept of species.

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