Using Genetics to Identify Intraspecific Conservation Units: a Critique of Current Methods

Genetic criteria have recently been proposed for identifying evolutionarily significant units (ESUs) and management units (MUs; Moritz 1994). Although this proposal has proved useful for focusing discussion about the intraspecific groupings of relevance to conservation, I am concerned that the criteria put forward have been accepted and applied without sufficient debate about their merit or limitations. My aim is to stimulate renewed debate on this subject by identifying some weaknesses in the proposed criteria. Although Moritz (1994) distinguished between the biological definitions of ESUs and MUs and the genetic criteria he proposed for recognizing such groups, many papers that cite his work erroneously refer to the recognition criteria as the definitions. An ESU is a group of organisms that has been isolated from other conspecific groups for a sufficient period of time to have undergone meaningful genetic divergence from those other groups (Ryder 1986). Moritz (1994) suggested that ESUs could be identified genetically as groups whose mtDNA lineages were reciprocally monophyletic, meaning that all lineages within each group share more-recent common ancestors than any lineage from one group shares with any lineage from the other group. In contrast, an MU is a group in which local population dynamics are determined primarily by birth and death rather than immigration and emigration (Moritz et al. 1995). Moritz (1994) suggested that MUs could be identified as populations with significant differences in allele distributions. An important point regarding both of these concepts is that they apply only when the populations under study are naturally divided into groups (Moritz et al. 1995). Of course, genetic study may also be of value in situations where populations are continuously distributed, but such populations cannot be subdivided into ESUs and MUs.