Evolutionary significant units versus geopolitical taxonomy:: Molecular systematics of an endangered sea turtle (genus Chelonia)

Taxonomic rank is an important criterion in assessing the conservation priority of an endangered organism: the sole member of a distinct family will generally receive a higher priority than a semi-isolated population in a polytypic species. When cryptic evolutionary partitions are discovered in endangered species, these findings are heralded as a positive step in the conservation process. The opposite action, demoting the taxonomic rank of an endangered organism, can be resisted by the conservation community because it is perceived as detrimental to preservation efforts. We explore the arguments for and against the species status of the endangered black turtle (Chelonia agassizii) and contribute an additional data set based on DNA sequences of single-copy nuclear loci. These data are concordant with previous mtDNA surveys in indicating no evolutionary distinction between C. agassizii and adjacent green turtle (C. mydas) populations. Although the black turtle is morphologically identifiable at a low level, much of its distinction is based on size and color differences that are highly variable throughout the range of C. mydas. Thus the black turtle would be more accurately classified at the subspecific or population level. There is no strong scientific case available to defend the species status of C. agassizii, and yet that designation has persisted for over a century. We suggest that the maintenance of this name is based on geographical and political considerations, and we propose a pragmatic category for this type of taxonomy: the geopolitical species. Furthermore, we argue against the practice of preserving species status for conservation purposes. There are several good reasons to preserve the black turtle, including morphological diversity and the possibility that it is an incipient evolutionary lineage with novel adaptations; taxonomic rank, however, is not one of them.