Testing Taxon Tenacity of Tortoises: evidence for a geographical selection gradient at a secondary contact zone

• Published in: Ecology and evolution 2015-05, Vol.5 (10), p.2095-2114
• Main Authors: Edwards, Taylor, Berry, Kristin H., Inman, Richard D., Esque, Todd C., Nussear, Kenneth E., Jones, Cristina A., Culver, Melanie
• Format: Article
• Language: English
• Subjects: Cline; Clustering; Desert animals; Desert environments; Deserts; Ecological niches; Genetic analysis; Genotype & phenotype; Gopherus; Gopherus agassizii; Habitat utilization; Habitats; hybrid zone; Hybridization; Mitochondrial DNA; Mojave Desert; Mountains; Niche overlap; Original Research; Population; Precipitation; Rivers; secondary contact; Sonoran Desert; Speciation; Statistical methods; Taxa; Topography; Tortoises; Variables; Winter
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.1500

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. We present an empirical study on speciation and hybridization at a natural hybridization zone between two species of desert tortoise (Gopherus agassizii and G. morafkai) in northwestern Arizona. We utilize mtDNA and STR loci to assess hybridization and incorporate habitat suitability modeling to identify patterns of ecological niche partitioning. This research on the evolutionary history of these two tortoise species has strong conservation and management implications.