A review of molecular genetic markers and analytical approaches that have been used for delimiting marine mammal subspecies and species

Abstract

Uncertainty in marine mammal taxonomy is increasingly being addressed using molecular genetic data. We examined 32 peer‐reviewed articles published between 1994 and 2011to review methodological practices, consistency of markers and analytical methods, and overall quality of arguments used when genetic data have been employed to delimit new species and subspecies of marine mammals. The mitochondrial DNA (mtDNA) control region was the primary genetic marker used in these studies, but analytical methods varied greatly across studies. Diagnosability, a common metric for delimiting subspecies with morphological data, was only used through citing of fixed differences in mtDNA sequences. Assignment tests based on microsatellite data were less common but were applied at both taxonomic levels. Nuclear DNA sequence data were rarely used. Basic background material needed to evaluate the strength of arguments, such as distribution and sampling maps, were often missing. For most studies, sample sizes were good, but adequate geographic sampling for broadly distributed taxa was often lacking, diminishing the strength of evidence for taxonomic distinctness. Examining these empirical cases revealed a mixture of sound and inadequate practices for genetic studies of cetacean taxonomy and suggested that improvements could be made to the field by developing standard guidelines.

Analytical approaches to subspecies delimitation with genetic data

“…To determine whether the taxa represent subspecies or full species, one must also evaluate the extent to which the taxa are separately evolving (Taylor et al . ). Cetacean taxonomists face multiple challenges when attempting to delimit subspecies and species, including recent and rapid radiations (Kingston et al .…”

“…), most cetacean studies aimed at describing new subspecies have, nonetheless, employed multiple lines of evidence in their taxonomic arguments (see review in Rosel et al . ).…”

“…We use the definitions of subspecies and species proposed by Taylor et al . (): “a species is a separately evolving lineage composed of a population or collection of populations” and “a subspecies is a population, or collection of populations, that appears to be a separately evolving lineage with discontinuities resulting from geography, ecological specialization, or other forces that restrict gene flow to the point that the population or collection of populations is diagnosably distinct.” Note that Taylor et al . 's species definition is de Queiroz's () unified species concept modified slightly to reflect the fact that several cetacean species exist as single populations rather than metapopulations.…”

“…As noted by Taylor et al . (), traditional morphological approaches to taxonomic analyses for cetaceans have proven difficult ( e.g ., Milinkovitch et al . , Reeves et al .…”

“…Most subspecies definitions, including that of Taylor et al . (), require a discontinuity between subspecies (Mallet ). We therefore also assume the researcher has done sufficient analyses to convince a reader that the units being compared are not from the ends of a cline (see Manel et al .…”