Recent Developments in Species Delimitation and Taxonomy Considered in the Context of the Unified Species Concept

Species delimitation and species taxonomy are important to most subfields of herpetology. I have argued that a previous controversy about the definition and concept of species resulted in a large part from different authors treating different properties that lineages acquire during divergence as necessary properties of species and therefore treating the species category as a stage in the evolution of lineages. I also proposed that a unified species concept could be achieved by not treating any of the properties in question as necessary properties of species so that all separately evolving metapopulation lineages are species. Although the proposed unified species concept has become increasingly accepted, the traditional interpretation persists. I discuss two cases of such persistence in the field of species delimitation. In one case, a protracted speciation model assumes the traditional interpretation of species by not considering separately evolving lineages to be “true” species until they cross some “conversion” threshold. Use of this model has led to the conclusion that species delimitation methods based on the multispecies coalescent model delimit intraspecific population structure rather than species. Consideration of the results upon which that conclusion was based in the context of the unified concept reveals that the method in question accurately delimited species. In another case, a proposed heuristic criterion for delimiting species, the genealogical divergence index, adopts the traditional interpretation of species by treating an index value above a certain threshold as a necessary property of species—one that is closely related to the traditional monophyly and exclusivity criteria. In the context of the unified species concept, this index is more appropriately used as a coalescent measure of lineage divergence than as a species criterion. In the field of species taxonomy, I have recently proposed a revised concept of subspecies based on the unified species concept. The revised concept makes the subspecies category more significant biologically and avoids problems associated with previous uses that caused some authors to reject that category. Nonetheless, recent authors have continued to avoid recognizing subspecies taxa, causing them to adopt to misleading taxonomies that obscure the existence of lineage structure within many species.