Biogeografía de tortugas de las galapagos

Evolution, 56(10), 2002, pp. 2052–2066

´ PHYLOGEOGRAPHY AND HISTORY OF GIANT GALAPAGOS TORTOISES
ADALGISA CACCONE,1,2,3 GABRIELE GENTILE,1,4 JAMES P. GIBBS,5 THOMAS H. FRITTS,6 HOWARD L. SNELL,7,8 JESSICA BETTS,1 AND JEFFREY R. POWELL1
of Ecology and Evolutionary Biology, Yale University, New Haven, Connecticut 06520-8106 Institute of Biospherics Studies, Yale University, New Haven,Connecticut 06520-8106 3 E-mail: adalgisa.caccone@yale.edu 4 Dipartimento di Biologia, Universita di Roma ‘‘Tor Vergata,’’ Rome, Italy ` 5 College of Environmental Science and Forestry, State University of New York, Syracuse, New York 13210 6 The Charles Darwin Foundation for the Galapagos Islands, 4512 McMurry Avenue, Fort Collins, Colorado 80525 ´ 7 Charles Darwin Research Station, Puerto Ayora, SantaCruz, Galapagos, Ecuador ´ 8 Department of Biology and Museum of Southwestern Biology, University of New Mexico, Albuquerque, New Mexico 87131
2 Yale 1 Department

Abstract. We examined the phylogeography and history of giant Galapagos tortoise populations based on mito´ chondrial DNA sequence data from 161 individuals from 21 sampling sites representing the 11 currently recognized extant taxa.Molecular clock and geological considerations indicate a founding of the monophyletic Galapagos lineage ´ around 2–3 million years ago, which would allow for all the diversification to have occurred on extant islands. Founding events generally occurred from geologically older to younger islands with some islands colonized more than once. Six of the 11 named taxa can be associated with monophyleticmaternal lineages. One, Geochelone porteri on Santa Cruz Island, consists of two distinct populations connected by the deepest node in the archipelago-wide phylogeny, whereas tortoises in northwest Santa Cruz are closely related to those on adjacent Pinzon Island. Volcan Wolf, the ´ northernmost volcano of Isabela Island, consists of both a unique set of maternal lineages and recent migrants fromother islands, indicating multiple colonizations possibly due to human transport or multiple colonization and partial elimination through competition. These genetic findings are consistent with the mixed morphology of tortoises on this volcano. No clear genetic differentiation between two taxa on the two southernmost volcanoes of Isabela was evident. Extinction of crucial populations by humanactivities confounds whether domed versus saddleback carapaces of different populations are mono- or polyphyletic. Our findings revealed a complex phylogeography and history for this tortoise radiation within an insular environment and have implications for efforts to conserve these endangered biological treasures. Key words. Galapagos, Geochelone nigra (elephantopus), giant tortoises, mitochondrialDNA, molecular phylogenies, ´ phylogeography. Received March 8, 2002. Accepted July 15, 2002.

Biota inhabiting relatively young, remote, oceanic archipelagoes are prized by ecologists and evolutionary biologists because they often display transparent cases of processes not readily observable in more complex and mature mainland ecosystems. Several special features of island species can beidentified. First, on remote oceanic islands, species are isolated from their mainland progenitors and often evolve into new, endemic forms. Second, for species that are not strong swimmers or fliers, populations occupying different islands are free to independently evolve, thus providing natural replicates of the evolutionary process. Third, the geological record of island formation can often provideimportant information for reconstructing the biogeographic history of species. Several examples of such island species reside on the Galapagos Archipelago including Darwin’s finches, marine and ´ land iguanas, and the subject of this report, giant tortoises. The Galapagos are volcanic islands located 1000 km west of ´ the coast of Ecuador and have never been connected to mainland South America. The...