US Geological Survey


Date of this Version



Published in The Endangered Species Act at Thirty, Volume 2: Conserving Biodiversity in Human-Dominated Landscapes, edited by J. Michael Scott, Dale D. Goble, & Frank W. Davis (Washington: Island Press, 2006), pp. 150-163.


Hybridization (the interbreeding of individuals from genetically distinct populations, regardless of their taxonomic status) is the double-edged sword of conservation biology. On one hand, increased rates of hybridization because of human activities have led to the extinction of populations and species in plant and animal taxa throughout the world (Rhymer and Simberloff 1996; Allendorf et al. 2001). On the other, hybridization is an important and natural part of the evolutionary process. Thus, hybridization between isolated populations can be an important tool for recovery (Mansfield and Land 2002). However, it has been difficult to develop conservation policies that treat the problems caused by increasing anthropogenic hybridization and at the same time recognize the important evolutionary role of natural hybridization.

How the Endangered Species Act (ESA) should treat hybrids has been a topic of intense debate since its passage in 1973 (see box 12.1). The word "hybrid" does not occur in the definition of "species" in the ESA (sec. 3) nor are hybrids considered anywhere in the act. In fact, hybrids are not considered in endangered species legislation of any other nation (Haig, unpublished data) with the exception of the Biodiversity Act recently adopted by the Republic of South Africa (Republic of South Africa Act No. 8, 2004). In this chapter, we review the history of discussions related to listing hybrids under the Endangered Species Act, outline current legislation that may particularly address this issue, and explore new approaches to resolving this debate.