Species

Abstract

Although the term “species” is widely used, there is no agreement on exactly what it means beyond the general conception of a basic taxonomic or evolutionary unit. There are a number of species concepts in use for vertebrates. In the biological species concept, a species is a group of interbreeding or potentially interbreeding individuals separated from other such groups by intrinsic barriers to gene flow. While the same operational criterion has been used, in nearly all such cases we do not know whether the groups would interbreed if the chance arose. The barrier to interbreeding may be a broad gap in range or simply a purely physical geographic one such as an isthmus or a climatic zone. In the more recently developed phylogenetic species concept (PSC), a species is “the smallest population or group of populations within which there is a parental pattern of ancestry and descent and which is diagnosable by unique combinations of character states,” with no inference about potential interbreeding. The two species concepts are actually complementary perspectives on evolution and can reinforce each other. There are additional species concepts, e.g., the cohesion species concept, under which a species is “the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability.” It is clear that the current trend is toward pragmatism in addressing species concepts. While the various species concepts are necessary and laudable attempts to model the results of actual evolutionary processes, they are human constructs, and as for all such constructs, in certain circumstances even the most commonly accepted species concepts fray at the edges. The operational criterion of at least one absolutely differentiating character that is commonly used to indicate species-level difference in morphologically based taxonomy has been extended by some to genetic characters.