Species are not theoretical objects, or, On what there is in biology

Wilkins, John S. (2009) Species are not theoretical objects, or, On what there is in biology.

Abstract

It is often claimed that species are the units of evolution, but this is not defined or clearly explained. In this paper I will argue that species are phenomenal objects that stand in need of explanation, but that they are not objects required by any theory of biology. I further define, or rather describe, species as the genealogical cluster of various lineages at the genetic, haplotype, genomic, organismic, and population level, in keeping with my previous discussions.