We have seen that the essentialist approach to classification does not apply to biological taxa (Section 3.1). Membership in the species Drosphila melanogaster, for instance, does not require that all and only the members of that species share a particular set of qualitative properties. Instead, the organisms of that species must be appropriately causally connected. Species taxa lack essences. But what about the species category? (Recall that species taxa are individual taxa, such as Homo sapiens and Drosphila melanogaster; the species category is the class of all species taxa.) Is there some property that all and only species taxa must have to be members of the category “species”? Is there, in other words, an essence to the species category?

Chapter 2 introduced no less than seven prominent species concepts. Each concept highlights a different set of biological properties that a taxon must have to be a species. Each concept, in other words, suggests a different essence of the species category. What is one to make of the diversity of species concepts in the literature? Biologists and philosophers have responded in two ways. Monists consider the species problem an unfinished debate in which the improper definitions need to be weeded from the proper one. Pluralists, on the other hand, maintain that no single correct definition of the species category exists. They suggest that we accept a number of species concepts as legitimate.

This chapter takes up the cause of species pluralism. Many authors maintain that evolution renders essentialism concerning species taxa outdated.