Conservation Value of Independently Evolving Units: Sacred Cow or Testable Hypothesis?

Authors who attempt to identify genetically distinct conservation units based on molecular marker data publish frequently in scientific journals, including Conservation Biology (Haig et al. 1994; Sinclair et al. 1996; Evans et al. 1997; Gibbs et al. 1997; Prior et al. 1997; Rosenblum et al. 1997). Some authors have used molecular marker data to advocate the value of one or more study populations for the conservation of genetic diversity (Kimberling et al. 1996; Castilla et al. 1998; Duvernell & Turner 1998; Lammi et al. 1999). These claims of conservation value assume a relationship between patterns at marker loci and the distribution of adaptive variation among populations. These assumptions often are not explicitly acknowledged, and are rarely tested, before some scientists make policy recommendations. Therefore, these recommendations may be premature. Conservation biologists need to address the question of how well population structure at molecular marker loci serves as a shortcut to indicate the conservation value of among-population differences. I question whether population differences based on molecular data are sufficient for the designation of conservation units because of their evolutionary distinctiveness. If we believe that natural selection is the principal mechanism generating adaptation to the environment and, by extrapolation, that adaptation through natural selection (or the failure thereof ) is the primary determinant of persistence in the face of environmental change, then we need experiments and other studies to determine whether patterns of molecular differentiation can provide a quick, valid assessment of a population’s conservation value. Specifically, if molecular markers are to serve as a shortcut indicator of among-population variation with conservation value, we need empirical studies to determine when patterns of molecular marker variation indicate genetic variation that contributes to the evolutionary potential and persistence of populations. Recognition of problems inherent in determining conservation value on the basis of population subdivision, identified with molecular markers, will spur empirical research of broad interest. Maintenance of diversity among populations is recognized as a central conservation problem (Hughes et al. 1997). General considerations for identifying and prioritizing populations for listing under the U.S. Endangered Species Act have been suggested based on copious ecological and genetic data (Waples 1995; Crandall et al. 2000). In the process of developing scientific grounds for the conservation of species of concern, some authors have also recommended conservation of wild populations based on interpopulation divergence at molecular marker loci (e.g., Kimberling et al. 1996; Evans et al. 1997). Others have concluded that particular populations are genetically undifferentiated based on the lack of spatial patterns in molecular markers and, explicitly or implicitly, suggest that conservation of all the populations under consideration is not necessary to maintain genetic diversity (e.g., Haig et al. 1994; Sinclair et al. 1996) or that translocation of individuals among regions presents no particular problem (e.g., Haig et al. 1994; Barratt et al. 1999). These authors imply that if there is no difference at marker loci among populations, then there is little among-population genetic variation important to the species’ persistence. The presence of interpopulation variation in the frequency or diversity of molecular variants is used to indicate whether genetic variation among populations constitutes something of conservation value, and thus whether protection efforts are merited for one or more populations on genetic grounds. This approach omits consideration of the relationship between molecular genetic patterns and “conservation value,” an often used but poorly defined phrase.