Analytical approaches to subspecies delimitation with genetic data

The vast and remote distributions and large body size of most cetaceans make it difficult to obtain and maintain morphological collections adequate for advancing sound taxonomic arguments. Consequently, genetic data are playing an increasingly important role in cetacean species and subspecies delimitation. We review seven categories of analytical methods useful in delimiting subspecies based on genetic data. For each category, we summarize its utility in evaluating putative subspecies, the types of markers to which it can be applied, and potential challenges in interpreting results in a taxonomic context. We focus on the utility of each type of method to evaluate the critical features distinguishing subspecies from populations and species: the degree of diagnosability between putative taxa and the extent to which the putative taxa have diverged along separate evolutionary pathways. We conclude that diagnosability is best estimated with either assignment tests or multivariate methods, while evaluating the degree of divergence requires a synthesis of multiple lines of evidence derived from different analytical methods and different data types, including nongenetic data.