These concepts are not necessarily mutually exclusive.
The Essentialist Species: A species is characterized by an unchangingessence. Species are defined as groups of similar individuals that are differentfrom individuals belonging to other species.
The Nominalist Species: Only individual organisms exist, and thespecies is a man-made construct; all that classes of similar things shareis a name.
The Biological Species: A species is a reproductive communityof populations (reproductively isolated from others) that occupies a specificniche in nature (Mayr, 1982).
A Pragmatic View of Species:
Figure 1. Schematic representation of the decision-making process suggested for determining whether groups of OTUs represent distinct species. Majorcategories used in making decisions appear in large capitals. Various possibilitiesfor each category are in lower case, and outcomes are in small capitals.(From Doyen and Slobodchikoff, 1974, Systematic Zoology 23:239-247.)