Accurate identification of biological entities is critical to the timely and efficient preservation of biodiversity. Concepts that define segments of biological diversity--species and evolutionarily significant units (ESUs)--should reflect our current knowledge of the biological world. Conflation of different hierarchical definitions of taxa has the potential to obscure distinct biological entities in need of protection. The concept of the ESU has been criticized because it includes within its definition distinct biological entities that otherwise would be recognized as species. Herein we evaluate several versions of the evolutionary significant unit concept and provide as a case study an analysis of geographic variation of a species of freshwater mussel, the inflated heelsplitter Potamilus inflatus. We demonstrate that as currently formulated, the ESU overlaps considerably with many biological definitions of species and therefore includes, in addition to distinctive populations, entities that would be recognized as species under many contemporary species concepts. Conflation of these two hierarchically distinct entities results in the ambiguous application of these concepts and inaccurate estimations of biological diversity. Continued use of the ESU concept has ramifications for the protection of invertebrate taxa under the Endangered Species Act of 1973 as amended in 1978. Recommendations for modification of the ESU concept are presented.