Phylogeny as a Central Principle in Taxonomy: Phylogenetic Definitions of Taxon Names

Defining the names of taxa in terms of common ancestry, that is, using phylogenetic definitions of taxon names, departs from a tradition of character-based definitions by granting the concept of evolution a central role in taxonomy. Phylogenetic definitions bear on other taxonomic principles and practices, including the following: (1) Names cannot be applied to nonmonophyletic taxa as the result of mistaken ideas about relationships or characters. Such mistakes do not affect conclusions about the monophyly of taxa but about their content and/or diagnoses. Because nonmonophyletic taxa can only be named deliberately, they are easily avoided. (2) Definitions are clearly distinguished from descriptions and diagnoses. Definitions are ontological statements about the existence of entities that result from the relationships of common ancestry among their parts; descriptions and diagnoses are epistemological statements about how we recognize the parts of those entities. (3) By precisely specifying the clade (ancestor) with which a name is associated, phylogenetic definitions clarify the limits of taxa, and this in turn clarifies related phenomena such as time of origin and duration. (4) Unlike the case for character-based intensional definitions and enumerative extensional definitions, phylogenetic definitions provide an unambiguous criterion for synonymy of taxon names: names are synonymous if they refer to clades stemming from the same ancestor. (5) Because of their relevance to synonymy, phylogenetic definitions also are relevant to priority, of both names and authorship. In phylogenetic taxonomy, priority is based not on first use of a name at a particular rank or ranJc-group but on first use of a name in association with a particular ancestor.