Species

This chapter studies the systematists' perspective on species concepts and the role of species in systematics. No matter how sophisticated the tools and methods enhancing the conceptualization of reality may become in the future, systematists will still be constrained by their perceptions. In their more modest, empirical view, systematists embrace their perceived reality and prefer species concepts that incorporate tools for identifying and delimiting species as empirical hypotheses, thereby providing them with efficacious working terminal elements for phylogenetic analysis and classification of more inclusive taxa. It is fortunate that cladists employed the notion of a “phylogenetic” species concept based on diagnosability before more metaphysically inclined authors appropriated the term for concepts founded on monophyly or common ancestors. As noted, Willi Hennig's species concept was a version of the “biological” species concept, and it fell to his followers to develop a species concept that is well suited to cladistic principles. Among the earliest of the post-Hennigian empiricists was American Museum ichthyologist Donn Rosen. Rosen's concept, sometimes called the apomorphic concept because of its requirement that every recognized species must have its own derived character state, accomplished two key advances for systematics: it proposed a cladistic criterion for recognizing species, and it defined species as the minimal units of analysis, as far as taxonomy is concerned, thus setting a lower bound for systematic inquiry.