Origin and evolutionary relationships of giant Galápagos tortoises

Abstract

Perhaps the most enduring debate in reptile systematics has involved the giant Galápagos tortoises (Geochelone nigra), whose origins and systematic relationships captivated Charles Darwin and remain unresolved to this day. Here we report a phylogenetic reconstruction based on mitochondrial DNA sequences from Galápagos tortoises and Geochelone from mainland South America and Africa. The closest living relative to the Galápagos tortoise is not among the larger-bodied tortoises of South America but is the relatively small-bodied Geochelone chilensis, or Chaco tortoise. The split between G. chilensis and the Galápagos lineage probably occurred 6 to 12 million years ago, before the origin of the oldest extant Galápagos island. Our data suggest that the four named southern subspecies on the largest island, Isabela, are not distinct genetic units, whereas a genetically distinct northernmost Isabela subspecies is probably the result of a separate colonization. Most unexpectedly, the lone survivor of the abingdoni subspecies from Pinta Island ("Lonesome George") is very closely related to tortoises from San Cristobal and Espanola, the islands farthest from the island of Pinta. To rule out a possible recent transplant of Lonesome George, we sequenced DNA from three tortoises collected on Pinta in 1906. They have sequences identical to Lonesome George, consistent with his being the last survivor of his subspecies. This finding may provide guidance in finding a mate for Lonesome George, who so far has failed to reproduce.

Figure 1

Map illustrating the location of the Galápagos Islands with respect to the South American coast. On the South American continent are depicted the three mainland Geochelone, from the top: G. denticulata, G. carbonaria, and G. chilensis. The enlarged area includes a map of the Galápagos Archipelago with the names of islands that have extant subspecies of tortoises. The sizes of the tortoises are in proportion to one another.

Figure 2

Phylogenetic tree based on 962 bp of the 16S ribosomal and cytb mtDNA. The tree shown is the 50% majority rule consensus tree for MP with G. pardalis as the outgroup. Numbers above branches indicate branch lengths. All lettered nodes were obtained with all weightings for MP, NJ, and ML, and by using each gene separately. See Materials and Methods for details of the analyses; see Table 2 for statistical analyses of topology robustness.

Figure 3

The 50% majority rule consensus tree for MP for the D loop of mtDNA from the four southern Isabela subspecies. The tree is unrooted, with bootstrap values on the nodes. Branch lengths are proportional to the number of steps. Three animals from a putative hybrid zone between vicina and guntheri are indicated as “v/g”. Other abbreviations: vic, vicina; gun, guntheri; van, vandenburghi; and mic, microphyes.