The Evolutionary Species Concept Reconsidered

Wiley, E. O. (Division of Fishes, Museum of Natural History, University of Kansas, Lawrence, KS 66045). 1978. Syst. Zool. 27:17–26.—The concept of species (as taxa) adopted by an investigator will influence his perception of the processes by which species originate. The concept adopted should have as universal applicability as current knowledge permits. Simpson's definition of a species is modified to state: a species is a lineage of ancestral descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate. This definition is defended as that which has widest applicability given current knowledge of evolutionary processes. Four corollaries are deduced and discussed relative to other species concepts: (1) all organisms, past and present, belong to some evolutionary species; (2) reproductive isolation must be effective enough to permit maintenance of identity from other contemporary lineages; (3) morphological distinctiveness is not necessary; and (4) no presumed (hypothesized) single lineage may be subdivided into a series of ancestral-descendant “species.” The application of the evolutionary species concept to allopatric demes and to asexual species is discussed and it is concluded that the lack of evolutionary divergence forms the basis for grouping such populations into single species. It is suggested that some ecological species definitions lead to under-estimations of the rate of extinction due to interspecific competition because their logical framework excludes unsuccessful species from being species. Finally, the implications of accepting an evolutionary species concept to the field of phylogeny reconstruction are discussed.