Species delineation and the identification of evolutionarily significant units: Lessons from the freshwater mussel genus Potamilus (Bivalvia : Unionidae)

Accurate identification of biological entities is critical to the timely an d efficient preservation of biodiversity. Concepts that define segments of biological diversity-species and evolutionarily significant units (ESUs)-sh ould reflect our current knowledge of the biological world. Conflation of d ifferent hierarchical definitions of taxa has the potential to obscure dist inct biological entities in need of protection. The concept of the ESU has been criticized because it includes within its definition distinct biologic al entities that otherwise would be recognized as species. Herein we evalua te several versions of the evolutionary significant unit concept and provid e as a case study an analysis of geographic variation of a species of fresh water mussel, the inflated heelsplitter Potamilus inflatus. We demonstrate that as currently formulated, the ESU overlaps considerably with many biolo gical definitions of species and therefore includes, in addition to distinc tive populations, entities that would be recognized as species under many c ontemporary species concepts. Conflation of these two hierarchically distin ct entities results in the ambiguous application of these concepts and inac curate estimations of biological diversity. Continued use of the ESU concep t has ramifications for the protection of invertebrate taxa under the Endan gered Species Act of 1973 as amended in 1978. Recommendations for modificat ion of the ESU concept are presented.