SPECIES CONCEPTS AND PLESIOMORPHIC SPECIES

Phylogenetic species concepts, in their many forms, represent an advance over previous species concepts because of their improved ability to relate species to historical patterns of evolution. A plea is made to recognize the importance of the distinction between species identifiable on the basis of uniquely derived characters (here termed apospecies) and those identifiable on the basis of a unique combination of characters, but absence of any uniquely derived character (plesiospecies). This distinction brings species concepts and phylogenetic studies more closely into accord. The ability to identify ever-finer patterns of relationship by molecular methods may result in impractical species delimitations if rigorous adherence to diagnosability or to monophyly of gene trees is the final arbiter of species. The study of gene trees and cryptic molecular characters should be used to examine processes within and among species and to clarify relationships when more readily observable characters are misleading. Operational definitions of species, based on either diagnosability or congruence among gene trees, should be viewed as hypotheses of species and as part of the process of discovering evolutionary patterns in nature that we call ''species.''