Blurred lines: Scientific and legislative issues surrounding hybrids and conservation

Abstract

Hybrids are the result of interbreeding between recognized taxonomic groups such as populations, species or subspecies. This special column focuses particularly on interspecific hybridization. The blurring of lines between neatly defined species causes both academic and practical problems, especially for those still attached to the long-outdated view of species as static, rather than dynamic, entities. Species designations become blurred when previously distinct species come into secondary contact, as well as when incipient species remain connected by gene flow. Botanists are generally credited with understanding the importance of hybridization as an evolutionary process that promotes adaptation and produces novel lineages, while zoologists have been characterized as holding to more eugenic views of hybridization polluting species integrity. Conservation policy regarding hybrids has been the subject of much debate, but resolution remains elusive. In the United States, the Endangered Species Act (ESA) of 1973 did not mention hybrids, and the U.S. Fish & Wildlife Service (USFWS) developed a de facto policy of denying protection to organisms with hybrid ancestry (O’Brien and Mayr, 1991). By 1996 the USFWS, together with the National Marine Fisheries Service, proposed an intercross policy for protecting hybrids under the ESA, though it has yet to be approved (USFWS and NOAA, 1996; Allendorf et al., 2013). Beyond the U.S., endangered species legislation in other countries (with the sole exception of South Africa) also fails to consider hybrids (Haig and Allendorf, 2006). Reluctance to enact explicit policies is probably a result of the complex and idiosyncratic mix of threats and benefits posed by each case of hybridization.