Examining metrics and magnitudes of molecular genetic differentiation used to delimit cetacean subspecies based on mitochondrial DNA control region sequences

Abstract

Cetacean taxonomy continues to be in flux and molecular genetic analyses examining alpha taxonomy in cetaceans have relied heavily on the mitochondrial DNA control region. However, there has been little consistency across studies; a variety of metrics and levels of divergence have been invoked when delimiting new cetacean species and subspecies. Using control region sequences, we explored, across pairs of well‐recognized cetacean populations, subspecies, and species, a suite of metrics measuring molecular genetic differentiation to examine which metrics best categorize these taxonomic units. Nei's estimate of net divergence (dA) and percent diagnosability performed best. All but a single, recently diverged species were unambiguously identified using these metrics. Many subspecies were found at intermediate values as expected, allowing separation from both populations and species, but several had levels of divergence equivalent to populations, resulting in underclassification errors using this single marker. Coupling dA with additional measures, such as percent diagnosability, examining appropriate nuclear genetic markers, and interpreting results in a broader biological context will improve taxonomic investigations in cetaceans.

Analytical approaches to subspecies delimitation with genetic data

“…However, Rosel et al . () showed that the number of fixed differences is not a good predictor of the current taxonomic status of cetacean subspecies and species.…”

“…Rosel et al . () found that for cetaceans, d A calculated from control region sequence was very effective at distinguishing between populations, subspecies, and species, with species generally exhibiting values of d A greater than 0.02 and populations exhibiting values less than 0.004. Taylor et al .…”

“…Rosel et al . () apply many of the analytical methods we discuss to a large number of pairs of cetacean populations, subspecies, and species. Based on these comparisons, Taylor et al .…”

“…Rosel et al . () applied a similar comparative approach using a broader suite of genetic divergence estimates in an attempt to identify metrics that could be used to distinguish subspecies from species and populations. Unlike Hey and Pinho (), Rosel et al .…”

“…Unlike Hey and Pinho (), Rosel et al . () limited their study to a single taxonomic group (cetaceans), took greater care to ensure that their categorizations of units as populations, subspecies, or species were well‐accepted, and calculated all metrics for a comparable locus. They also found broad overlap in estimates of most metrics between comparisons at different taxonomic levels.…”