title: Testing Taxon Tenacity of Tortoises: evidence for a geographical selection gradient at a secondary contact zone.
abstract: We examined a secondary contact zone between two species of desert tortoise, Gopherus agassizii and G. morafkai. The taxa were isolated from a common ancestor during the formation of the Colorado River (4-8 mya) and are a classic example of allopatric speciation. However, an anomalous population of G. agassizii comes into secondary contact with G. morafkai east of the Colorado River in the Black Mountains of Arizona and provides an opportunity to examine reinforcement of species' boundaries under natural conditions. We sampled 234 tortoises representing G. agassizii in California (n - 103), G. morafkai in Arizona (n - 78), and 53 individuals of undetermined assignment in the contact zone including and surrounding the Black Mountains. We genotyped individuals for 25 STR loci and determined maternal lineage using mtDNA sequence data. We performed multilocus genetic clustering analyses and used multiple statistical methods to detect levels of hybridization. We tested hypotheses about habitat use between G. agassizii and G. morafkai in the region where they co-occur using habitat suitability models. Gopherus agassizii and G. morafkai maintain independent taxonomic identities likely due to ecological niche partitioning, and the maintenance of the hybrid zone is best described by a geographical selection gradient model.
authors.affiliation: School of Natural Resources and the Environment, The University of Arizona Tucson, Arizona, 85721 ; University of Arizona Genetics Core, University of Arizona 1657 E. Helen Street, Tucson, Arizona, 85721.
authors.forename: Taylor
authors.initials: T
authors.lastname: Edwards
authors.affiliation: Western Ecological Research Center, U.S. Geological Survey Falcon Business Park, 21803 Cactus Avenue, Suite F, Riverside, California, 92518.
authors.forename: Kristin H
authors.initials: KH
authors.lastname: Berry
authors.affiliation: Western Ecological Research Center, U.S. Geological Survey 160 North Stephanie St., Henderson, Nevada, 89074.
authors.forename: Richard D
authors.initials: RD
authors.lastname: Inman
authors.affiliation: Western Ecological Research Center, U.S. Geological Survey 160 North Stephanie St., Henderson, Nevada, 89074.
authors.forename: Todd C
authors.initials: TC
authors.lastname: Esque
authors.affiliation: Western Ecological Research Center, U.S. Geological Survey 160 North Stephanie St., Henderson, Nevada, 89074.
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Nussear
authors.affiliation: School of Natural Resources and the Environment, The University of Arizona Tucson, Arizona, 85721 ; Arizona Game and Fish Department, Nongame Wildlife Branch Phoenix, Arizona, 85086.
authors.forename: Cristina A
authors.initials: CA
authors.lastname: Jones
authors.affiliation: School of Natural Resources and the Environment, The University of Arizona Tucson, Arizona, 85721 ; Arizona Cooperative Fish and Wildlife Research Unit, U.S. Geological Survey, University of Arizona Tucson, Arizona, 85721.
authors.forename: Melanie
authors.initials: M
authors.lastname: Culver
keywords: Cline; Gopherus; Mojave Desert; Sonoran Desert; hybrid zone; secondary contact