1. Museum of Vertebrate Zoology and Department of Integrative Biology, University of California, Berkeley, CA 94720-3160, USA
2. Current address: Department of Ecology and Evolutionary Biology, Yale University, New Haven, CT 06520, USA
3. Center for Population Biology, Section of Evolution and Ecology, University of California, Davis, CA 95616-5270, USA
4. Current address: Genome Center and Section of Evolution and Ecology, University of California, Davis, CA 95616, USA