Kristin H. Berry Richard D. Inman Todd C. Esque Kenneth E. Nussear Cristina A. Jones Melanie Culver Taylor Edwards 2015 <p>We examined a secondary contact zone between two species of desert tortoise, <i>Gopherus agassizii</i> and <i>G. morafkai</i>. 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 <i>G. agassizii</i> comes into secondary contact with <i>G. morafkai</i> 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 <i>G. agassizii</i> in California (<i>n</i> = 103), <i>G. morafkai</i> in Arizona (<i>n</i> = 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 <i>G. agassizii</i> and <i>G. morafkai</i> in the region where they co-occur using habitat suitability models. <i>Gopherus agassizii</i> and <i>G. morafkai</i> 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.</p> application/pdf 10.1002/ece3.1500 en Blackwell Pub. Ltd. Testing taxon tenacity of tortoises: evidence for a geographical selection gradient at a secondary contact zone article