Critique of the Evolutionarily Significant Unit as a Definition for "Distinct Population Segments" under the U.S. Endangered Species Act

The U.S. Endangered Species Act grants protection to species, subspecies, and "distinct population segments" of vertebrate species. Historically, Congress included distinct population segments into endangered species legislation to enable the U.S. Fish and Wildlife Service to implement a flexible and pragmatic approach in listing populations of vertebrate species. Recently, the U.S. Fish and Wildlife Service and the National Marine Fisheries Service have proposed a policy that would narrowly define distinct population segments as evolutionarily significant units based on morphological and genetic distinctiveness between populations. Historically, the power to list species or populations as distinct population segments has been used to tailor management practices to unique circumstances; grant varied levels of protection in different parts of a species' range; protect species from extinction in significant portions of their ranges as well as to protect populations that are unique evolutionary entities. A strict redefinition of distinct population segments as evolutionarily significant units will compromise management efforts because the role of demographic and behavioral data will be reduced. Furthermore, strictly cultural, economic, or geographic justifications for listing populations as threatened or endangered will be greatly curtailed.