Taxonomy

Before taxonomists can identify, name, and classify organisms, they need to agree on a definition of the concept of species. The definition of species is not as simple as it may seem, and has been debated by biologists and philosophers alike for many years.

Most modern biologists agree that species, unlike higher taxa (genus, family, order, and so on), are authentic taxonomic units. In other words, species really do exist in nature, and are not merely artificial human constructs. Some of the most persuasive evidence in support of this is the close correspondence between the species identified by western taxonomists and those identified in the "folk taxonomy" of relatively isolated non-western societies. For example, Ernst Mayr, a noted bird taxonomist, identified 137 species of birds in his field work in the mountains of New Guinea; the folk taxonomy of the native New Guineans identifies 136 species. Similar correlations occur with plants. For example, an extensive interdisciplinary study of the native people and plants of the Chiapas highlands of Mexico showed a very close correspondence between species identified by western botanists and those identified by Chiapas natives.

Many modern biologists, particularly zoologists, define species according to the "biological species concept," a definition that has been forcibly advocated by Ernst Mayr. According to this definition, a species is a group of organisms reproductively isolated from other organisms. In other words, a species is a group of organisms that interbreed and produce fertile offspring only with one another.

Some microbiologists and botanists are dissatisfied with the biological species concept. Microbiologists have noted that single-celled organisms do not reproduce sexually in nature (although they may undergo genetic recombination), yet they can still be segregated into discrete natural groups considered species. Botanists have noted that many species of related plants can hybridize with one another in nature. For example, the different species in the sub-genus Leucobalanus (white oak) can hybridize and produce fertile offspring. Thus, rigorous application of the biological species concept would require that all of these separate species of oak be considered a single species, even though they have very different morphologies.

Most biologists basically accept the biological species concept, but argue that it is really an idealized concept, since only rarely is an actual test for reproductive compatibility performed. Thus, in practice, nearly all biologists think of species as morphologically distinct groups of organisms.