Evolution, Evidence, and the Role of Species Concepts in Phylogenetics

Three general families of species concepts have been proposed within the field of phylogenetic systematics, each with the goal of identifying the minimal taxa within phylogenetic systems as Under the Phylogenetic Species Concept species are the minimal elements of hierarchic descent systems. In terms of population-level processes, phylogenetic species are the population systems that result from cladogenesis, which occurs when an ancestral population system becomes subdivided into two or more descendant systems. The descendant population systems that arise from events of this sort have ceased to exchange genes with each other, but they are not individually monophyletic; the minimal group is a group of two or more phylogenetic species. The Monophyletic Species Concept was formulated with the goal of recognizing minimal monophyletic groups of individuals as species. Monophyly is a unified concept in hierarchic descent systems, wherein three key attributes are correlated in occurrence, these being common ancestry of members of a group (i.e., the group includes an ancestor and all of its descendants), exclusivity of kinship (i.e., every member of the group is more closely related to every other member than to any nonmember), and nesting of such groups (i.e., if there is any overlap in the membership of two different groups, one of the groups is completely included in the other). These attributes are disengaged in occurrence in reticulate descent systems, and none of them by itself constitutes monophyly; proponents of different versions of the Monophyletic Species Concept have variously emphasized one or another or a combination of these attributes. Under one proposed definition the set of descendants of any mating pair in a population is considered a monophyletic group, and many monophyletic groups of individuals, of partially overlapping membership, therefore coexist within a sexual population. Under the terms of this definition a monophyletic population can absorb immigrants indefinitely; all offspring derived from crosses between immigrants and existing members of the monophyletic group are themselves members of the monophyletic group, and population monophyly therefore is regained as soon as the immigrants (which are not members of the monophyletic group) die. Some former proponents of the Monophyletic Species Concept have abandoned the notion of monophyletic species and have proposed an alternative, the Genealogical Species Concept, in which species are recognized on the basis of character descent relationships, rather than organismal descent relationships. The Genealogical Species Concept disallows incongruence between gene trees and species trees by operationally expanding the limits of species until all incongruence among gene trees is accommodated within species. Under the terms of the Genealogical Species Concept there can be any amount of discoverable phylogenetic structure within genealogical species, so individual genealogical species are not minimal elements of phylogenetic systems.