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 prio rity of an endangered organism: the sole member of a distinct family will g enerally receive a higher priority than a semi-isolated population in a pol ytypic species. When cryptic evolutionary partitions are discovered in enda ngered species, these findings are heralded as a positive step in the conse rvation process. The opposite action, demoting the taxonomic rank of an end angered organism, can be resisted by the conservation community because it is perceived as detrimental to preservation efforts. We explore the argumen ts for and against the species status of the endangered black turtle (Chelo nia 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 ba sed on size and color differences that are highly variable throughout the r ange 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 desig nation has persisted for over a century. We suggest that the maintenance of this name is based on geographical and political considerations, and we pr opose a pragmatic category for this type of taxonomy: the geopolitical spec ies. Furthermore, we argue against the practice of preserving species statu s for conservation purposes. There are several good reasons to preserve the black turtle, including morphological diversity and the possibility that i t is an incipient evolutionary lineage with novel adaptations; taxonomic ra nk, however, is not one of them.