A cryptic taxon of Galápagos tortoise in conservation peril

Abstract

As once boldly stated, 'bad taxonomy can kill', highlighting the critical importance of accurate taxonomy for the conservation of endangered taxa. The concept continues to evolve almost 15 years later largely because most legal protections aimed at preserving biological diversity are based on formal taxonomic designations. In this paper we report unrecognized genetic divisions within the giant tortoises of the Galápagos. We found three distinct lineages among populations formerly considered a single taxon on the most populous and accessible island of Santa Cruz; their diagnosability, degree of genetic divergence and phylogenetic placement merit the recognition of at least one new taxon. These results demonstrate the fundamental importance of continuing taxonomic investigations to recognize biological diversity and designate units of conservation, even within long-studied organisms such as Galápagos tortoises, whose evolutionary heritage and contribution to human intellectual history warrant them special attention.

Figure 1

Distribution maps of Galápagos tortoises throughout the archipelago and on Santa Cruz. Shaded islands indicate presence of extant tortoise populations and italized names represent current subspecific designations; island names indicated in all capitals, with distinct populations on Isabela specified by name; triangles represent volcanoes and circles indicate sampled populations throughout the archipelago. Shaded and unshaded tortoise caricatures signify ‘domed’ and ‘saddleback’ morphologies, respectively. The current distributions of tortoises (shaded yellow) and the agricultural corridor (shaded black) on Santa Cruz (Pritchard 1996) are plotted on a satellite map (MODOS Rapid Response Project, NASA/GSFC). Scale bar specified for the Galápagos archipelago (above) and Santa Cruz (below).

Figure 2

Bayesian phylogenetic tree of extant and extinct Galápagos tortoise taxa. Distinct haplogroups of becki, guntheri and vicina are indicated with numbers and follow figure 1; island of origin for each taxon is shown on the right; an asterisk (*) signifies an extinct taxon or population and arrows highlight the relative phylogenetic placement of the three populations of giant tortoises on Santa Cruz. Bayesian posterior probabilities and maximum likelihood bootstrap proportions (greater than 50%) are indicated above and below the branches, respectively. For illustration purposes, accurate branch lengths leading to out-group taxa are not shown (indicated by dashed line).