Originally published as Genetics Published Articles Ahead of Print on December 30, 2005.

Genetics, Vol. 172, 1727-1744, March 2006, Copyright © 2006
doi:10.1534/genetics.105.047860

Phylogeographic History and Gene Flow Among Giant Galápagos Tortoises on Southern Isabela Island

Claudio Ciofi^*,1, Gregory A. Wilson, Luciano B. Beheregaray^*,, Cruz Marquez, James P. Gibbs^**, Washington Tapia, Howard L. Snell, Adalgisa Caccone^*, and Jeffrey R. Powell^*

^* Department of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut 06520, Department of Integrative Biology, University of California, Berkeley, California 94720, Department of Biological Sciences, Macquarie University, Sydney, New South Wales 2109, Australia, Charles Darwin Research Station, Puerto Ayora, Galápagos, Ecuador, ^** College of Environmental Science and Forestry, State University of New York, Syracuse, New York 13210, Parque Nacional de Galápagos, Puerto Ayora, Galápagos, Ecuador, Department of Biology, University of Mexico, Albuquerque, New Mexico 87131 and Yale Institute for Biospheric Studies, Yale University, New Haven, Connecticut 06520

¹ Corresponding author: Dipartimento di Biologia Animale e Genetica, Università di Firenze, Via Romana 17, 50125 Firenze, Italy.
E-mail: claudio.ciofi{at}unifi.it

Volcanic islands represent excellent models with which to study the effect of vicariance on colonization and dispersal, particularly when the evolution of genetic diversity mirrors the sequence of geological events that led to island formation. Phylogeographic inference, however, can be particularly challenging for recent dispersal events within islands, where the antagonistic effects of land bridge formation and vicariance can affect movements of organisms with limited dispersal ability. We investigated levels of genetic divergence and recovered signatures of dispersal events for 631 Galápagos giant tortoises across the volcanoes of Sierra Negra and Cerro Azul on the island of Isabela. These volcanoes are among the most recent formations in the Galápagos (<0.7 million years), and previous studies based on genetic and morphological data could not recover a consistent pattern of lineage sorting. We integrated nested clade analysis of mitochondrial DNA control region sequences, to infer historical patterns of colonization, and a novel Bayesian multilocus genotyping method for recovering evidence of recent migration across volcanoes using eleven microsatellite loci. These genetic studies illuminate taxonomic distinctions as well as provide guidance to possible repatriation programs aimed at countering the rapid population declines of these spectacular animals.

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