CRITIQUE OF THE EVOLUTIONARILY SIGNIFICANT UNIT AS A DEFINITION FOR DISTINCT POPULATION SEGMENTS UNDER THE US ENDANGERED SPECIES ACT

The U.S. Endangered Species Act grants protection to species, subspeci es, and ''distinct population segments'' of vertebrate species. Histor ically, Congress included distinct population segments into endangered species legislation to to enable the U.S. Fish and Wildlife Service t o implement a flexible and pragmatic approach in listing populations o f vertebrate species. Recently the U.S. Fish and Wildlife Service and the National Marine Fisheries Service have proposed a policy that woul d narrowly define distinct population segments as evolutionarily signi ficant units based on morphological and genetic distinctiveness betwee n populations. Historically, the power to list species or populations as distinct population segments has been used to tailor management pra ctices to unique circumstances; grant varied levels of protection in d ifferent parts of a species' range; protect species from extinction in significant portions of their ranges as well as to protect population s that are unique evolutionary entities. A strict redefinition of dist inct population segments as evolutionarily significant units will comp romise management efforts because the role of demographic and behavior al data will be reduced. Furthermore, strictly cultural, economic, or geographic justifications for listing populations as threatened or end angered will be greatly curtailed.