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.

A legacy of geo‐climatic complexity and genetic divergence along the lower Colorado River: Insights from the geological record and 33 desert‐adapted animals

“…Species with extralimital populations east of the river include two mammals ( Chaetodipus penicillatus , Ovis canadensis ), seven reptiles ( Chionactis annulata , Chionactis occipitalus , Crotaphytus bicintores , Gopherus spp. ; based on microsatellite data; Edwards et al, , Lichanura trivirgata , Phrynosoma platyrhinos , Uma scoparia ), two arachnids ( Smeringurus spp. Hadrurus arizonensis Clade III), and two insects ( Prodoxys sordidus, P. weethumpi ).…”

“…These species also hybridize at the Mojave‐Sonoran ecotone east of the Mohave Valley (Figure b) where precipitation patterns are intermediate (Figure S1). In this contact zone the hybrids show an intermediate habitat preference to the two parent species (Edwards et al, ). The Ambrosia shrub (not studied here) has different drought mortality in the Mojave versus Sonoran deserts that is best explained by a natal precipitation index, also suggesting adaptation to different precipitation regimes (McAuliffe & Hamerlynk, ).…”

Local niche differences predict genotype associations in sister taxa of desert tortoise

“…A third category, if coinciding with regions containing hybrids, might suggest that hybridized individuals select habitat in ways that are locally different than either of the two pure genotypes. Previous work has shown that these hybrids do occupy habitats with characteristics that span those of both their Mojave and Sonoran parental lineages in terms of topographic, surface textural and vegetation characteristics (Edwards et al, ). Our delineation of two categories does not counter these findings because here we explore local niche differences, that is local differences in species–environment relationships, rather than differences in occupied habitat as was explored by Edwards et al ().…”

“…In order to compare niches of G. agassizii and G. morafkai , a common set of explanatory variables is needed; we therefore considered a set of 13 explanatory variables (Table ) that represented a suite of physiographic, vegetative and climatic characteristics hypothesized to influence the distribution of both species (Edwards et al, ; Inman et al, ; Nussear et al, ). These variables were chosen from among 18 by removing variables with Pearson's correlation values greater than 0.6 to reduce multicollinearity.…”

“…This situation is further complicated by evidence of natural hybridization between G. agassizii and G. morafkai occurring in this secondary contact zone, and by the lack of a clear definition of habitat for this population of G. agassizii east of the river. Recent work has also suggested that this population occupies habitat with intermediate characteristics to that of the pure lineages (Edwards et al, ). Habitat for G. agassizii and G. morafkai outside the contact zone is better defined, with known habitat ranging from valley bottoms and alkaline areas surrounding playas in the Mojave, to bajadas and alluvial fans, arroyos, rocky slopes and ridges in the upland regions in the Sonoran desert (Nussear & Tuberville, ).…”