Origin and evolutionary relationships of giant Galápagos tortoises

Abstract

Perhaps the most enduring debate in reptile systematics has involved the giant Galá pagos 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 Galá pagos tortoises and Geochelone from mainland South America and Africa. The closest living relative to the Galá pagos 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 Galá pagos lineage probably occurred 6 to 12 million years ago, before the origin of the oldest extant Galá pagos 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 Cristó bal and Españ ola, 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.

Giant tortoises are not so slow: Rapid diversification and biogeographic consensus in the Galápagos

“…Previous molecular studies (11,(14)(15)(16) of giant Galápagos tortoises revealed marked genetic differentiation among most populations and a general pattern of colonization from geologically older to younger islands. However, due to the recent age of the radiation and incomplete lineage sorting these surveys presented insufficient information about relationships among recently diverged lineages.…”

“…Giant Galápagos tortoises are the largest terrestrial chelonians in the world and represent the only surviving group of giant tortoises where evolutionary divergence is evident among populations (13). The group is monophyletic and is derived from a mainland South American lineage (14). The radiation is comprised of 11 extant and four extinct taxa endemic to different islands, or is restricted to each of Isabela's five major volcanoes (ref.…”

Historical DNA analysis reveals living descendants of an extinct species of Galápagos tortoise

“…are renowned both for their uniqueness and for their contribution to the development of Darwin's theory of natural selection (1). Fifteen formally described taxa of giant Galápa-gos tortoises are generally recognized, four of which are extinct from the islands of Floreana (G. elephantopus), Santa Fe (Geochelone sp.…”

Turtles of the Tiputini Biodiversity Station with remarks on the diversity and distribution of the Testudines from Ecuador

“…Pritchard (1996) suggested that the four southern Isla Isabela taxa do not warrant separate status. Caccone et al (1999) found that those populations lack genetic differentiation; but latter, Caccone et al (2002) found that microphyes (Volcan Darwin) and vandenburghi (Volcan Alcedo) are genetically distinct, and deserve separation; but they can not refute the suggestion that the two southern most subspecies are not genetically distinct.…”