Species Concepts: Assumptions, Methods, and Applications

Abstract

The assumptions, methods, and applications of various species concepts prevalent in systematics are reviewed in order to clarify issues and terminology. Species concepts are classified as mechanistic-species as participants in the process of speciation, or historical-species as the end results of processes. Mechanistic concepts lack universality and testability because they rely on a knowledge of how speciation proceeded in any particular case. Historical species concepts are preferred because they can be consistently applied and provide a context in which processes of speciation can be investigated. Three historical species concepts are compared: the Genealogical Species Concept, the Species As Taxa Concept, and the Phylogenetic Species Concept. The Gene- alogical Species Concept supports the recognition of the smallest monophyletic lineages (those that occur at the boundary between reticulating and hierarchical systems of relationship), whereas under the Species As Taxa Concept, hierarchy is considered to occur below the level of species, and any monophyletic unit may be recognized as a species. The problems presented by applying the term monophyly to reticulating systems are discussed as they apply to each concept, as are several problems specific to the Genealogical Species Concept, such as metaspecies. An alternative to mono- phyletic species concepts is the Phylogenetic Species Concept, in which the smallest diagnosable unit is recognized as a species. Monophyly is not considered demonstrable at the species level because, in contrast to Species As Taxa, an assumption of the Phylogenetic Species Concept is that species are basal taxa, and thus mark the level at which hierarchic relationships begin. Finally, the application of species concepts is reviewed by surveying the literature in Systematic Botany and Systematic Zoology/Biology during the past five years. Papers in Systematic Zoology/Biology were generally more explicit about the species concept and the criteria being used to delimit species than were papers in Systematic Botany. Because it is clear that a number of different concepts are currently in use, and these are often distinguished only vaguely, botanists are encouraged to familiarize themselves with the various concepts, and to state explicitly which concepts and criteria are being employed in recognizing species.