  In listing the topics I would discuss under the heading of neo-Darwinism, I distinguished the question of the ontological status of species from the epistemological status of the species concept. Though they are closely related questions, it is important to keep them distinct. As will become clear as we proceed, this distinction is rarely honored. Moreover, it is equally important to distinguish the species concept from the categories of features that belong in a definition of species (Rheins 2011). Advances in our theoretical understanding may lead us to reconsider the sorts of attributes that are most important for determining whether a group of organisms is a species, and thus whether it deserves to be assigned a name at that taxonomic level. It should not be assumed that such changes constitute a change in the species concept, though at least some such changes may lead us to restrict or expand the range of taxa that are designated as species. In his contribution to the Neo-Darwinian Synthesis, Systematics and the Origin of Species, Ernst Mayr titled chapter five ‘The Systematic Categories and the New Species Concept’. Recall that Darwin made a point of treating the species category as continuous with ‘well-marked variety’ and ‘sub-species’, and made the radical suggestion that its boundaries would be just as fluid. Without explicitly acknowledging Darwin, Mayr takes the same tack, discussing ‘individual variants’ and ‘sub-species’ as a preliminary to discussing the species concept. Mayr notes that for someone studying the evolutionary process, speciation is a critical juncture; “…his interpretation of the speciation process depends largely on what he considers to be the final stage of this process, the species.” (Mayr 1942/1982, 113) With this in mind, he offers the following definition, the so-called ‘biological species concept’ (BSC):