Populations, Genetic Variation, and the Delimitation of Phylogenetic Species

Systematists should distinguish between cladistic analysis, i.e., a numerical procedure, and phylogenetic analysis, i.e., the reconstruction of hierarchic descent relationships. Modern cladistic analysis uses parsimony to construct hierarchic arrangements (trees) of terminal units (terminals) that have been scored for a series of attributes. The interpretation of cladistic relationships as representative of phylogenetic relationships requires two conditions, both of which were identified by Hennig (1966, Phylogenetic systematics, Univ. Illinois Press, Urbana). First, descent relationships among the terminals must be hierarchic; that is, all terminals must have been generated by the subdivision or replication of previously existing ancestors. This is a necessary condition for phylogenetic analysis, rather than an empirical discovery of it, because the results of cladistic analysis are always hierarchically structured (however poorly resolved). Because resolution of a cladistic hierarchy does not demonstrate that a hierarchic descent system underlies the character distribution pattern it reflects, additional information is necessary, in any given case, to determine that phylogenetic analysis is appropriate. Second, to have congruence between an observable attribute hierarchy and the descent hierarchy that is to be inferred, the attribute must have been transmitted from an ancestor to all of its descendants, either in its original state or in a modified state. Both conditions are met by asexual organisms and by nonrecombining genetic elements (e.g., the chloroplast and mitochondrial genomes) but not by individual sexually reproducing organisms that bear such genetic elements. Populations of sexually reproducing organisms can meet the first condition (i.e., hierarchic descent) when new populations are founded by the division of previously existing populations and individuals do not disperse among existing populations. When the first condition is met, the second also is met for genetically fixed attributes of the populations, because populations descended from an ancestral population that was fixed for an attribute also will be fixed for that attribute (in original or modified state). In contrast, attributes that are not fixed in a population may not occur in all or any descendant populations, even if descent relationships among populations are hierarchic. The occurrence of a unique fixed character combination in an extended genealogical population (phylogenetic species sensu Nixon and Wheeler, 1990, Cladistics 6:211-223) is evidence that this population has diverged from other such populations and thus that descent relationships among such populations are hierarchic. The fixed characters of phylogenetic species therefore constitute evidence that a hierarchic descent system exists and provide the means for analyzing phylogenetic relationships among these species. Phylogenetic species can be delimited by a procedure (population aggregation analysis) that involves a search for fixed differences among local populations, followed by successive rounds of aggregation of populations and previously aggregated population groups that are not distinct from each other