Species as natural entities, instrumental units and ranked taxa: new perspectives on the grouping and ranking problems

This paper addresses the two problems that together constitute the species problem: the grouping problem (the question: on what basis should organisms be allocated to groups that are candidate species?) and the ranking problem (the question: on what basis should such groups be ranked as species?). We inquire what it means to say that species are real or that species are natural units, and argue for solutions to the grouping and ranking problems that allow us to be realists about species while at the same time avoiding the problems of a naive realism about species. In the account presented here, theoretical relevance is what constitutes the reality of species.