A bare-bones scheme to choose between the species, subspecies, and ‘evolutionarily significant unit’ categories in taxonomy and conservation

Abstract

Species are the fundamental unit of analysis in biodiversity. They are often divided into subspecies, or into ‘evolutionarily significant units’ (ESUs). The imprecise demarcation between these categories causes disagreements and practical difficulties in taxonomy and conservation. I argue that it is possible to unequivocally choose which category to use for referring to any plant or animal population on the basis of a simple classificatory scheme, here proposed, resulting from the combination of stages of morphological and genetic differentiation. This scheme is the first to consider the three categories together in a unified frame of reference. Numerous definitions have been proposed for specific and infraspecific groupings. Most of them are redundant, or unnecessarily conflate the properties of species, subspecies, and ESUs. Based on feedback from the scheme, I reword general definitions for these categories to make them complementary, both conceptually and operationally.