The process of evolution is continuous through time but yields in space discontinuous groups of organisms. The continuity of the process links the myriad living organisms with the last universal common ancestor, from which all living organisms descend. Organisms evolve differences because of the haphazard mutation process, adaptation to different environmental circumstances, interaction with other organisms, constraints imposed by the organisms’ past evolutionary history, and the like. The discontinuities are encompassed in the Linnean system of classification, which is hierarchical, with gradually more inclusive categories: species, genus, family, order, and so on.

“Species” is a category of classification, the most basic, within which are placed groups of organisms designated by specific names such as Homo sapiens, Mus musculus, Drosophila melanogaster, or Quercus borealis. But species have a biological reality that is lacking in more inclusive groups of organisms. In sexually reproducing organisms, individual members of a species are able to interbreed and thus share in a common gene pool. Collectively, there is variation among the members of a species, but there is also continuity in space and time. Species are evolutionary units. Because of these properties, some philosophers have affirmed that species, but not more inclusive groups of organisms, are metaphysical individuals. According to Hull (1977), “species fit as naturally into the idealized category of spatiotemporally localized individuals as do particular organisms” (Ghiselin, 1974).

Dobzhansky (1935a,b) pointed out in 1935 this double biological reality of the concept of species: (i) as a category of classification, just like genus, family, and other categories, a logical construct pragmatically necessary for organizing the enormous diversity of the living world and (ii) as a category with “an attribute peculiar only to itself” (1935b), because a species is a natural entity, a collectivity that has biological continuity defined, in sexually reproducing organisms, by the capacity to interbreed among individuals of the same species and their incapacity to interbreed with individuals of other species. The biological species concept, as it came to be known, defines species precisely by these two attributes: ability to interbreed within the species and reproductive isolation from other species. The evolutionary process of speciation, by which one species splits into two, is equivalent to the evolutionary emergence of reproductive isolation.