Abstract
Biodiversity in southwestern North America has a complex biogeographic history involving tectonism interspersed with climatic fluctuations. This yields a contemporary pattern replete with historic idiosyncrasies often difficult to interpret when viewed from through the lens of modern ecology. The Aspidoscelis tigris (Tiger Whiptail) complex (Squamata: Teiidae) is one such group in which taxonomic boundaries have been confounded by a series of complex biogeographic processes that have defined the evolution of the clade. To clarify this situation, we first generated multiple taxonomic hypotheses, which were subsequently tested using mitochondrial DNA sequences (ATPase 8 and 6) evaluated across 239 individuals representing five continental members of this complex. We then evaluated the manner by which our models parsed phylogenetic and biogeographic patterns. We found considerable variation among species ‘hypotheses,’ which we interpret as reflecting inflated levels of inter-population genetic divergence caused by historical demographic expansion and contraction cycles. Inter-specific boundaries with A. marmoratus juxtaposed topographically with the Cochise Filter Barrier that separates Sonoran and Chihuahuan deserts (interpreted herein as an example of ‘soft’ allopatry). Patterns of genetic divergence were consistent across the Cochise Filter Barrier, regardless of sample proximity. Surprisingly, this also held true for intraspecific comparisons that spanned the Colorado River. These in turn suggest geomorphic processes as a driver of speciation in the A. tigris complex, with intraspecific units governed by local demographic processes.
Highlights
The Colorado Plateau (~337,000 km2 at ~2000m asl) is a tectonically intact landform of relatively linear sedimentary rock that contrasts markedly with its eroding margin, ongoing since the Late Miocene (Levander et al 2011)
We evaluated a complex of four continental subspecies (A. t. mundus, A. t. punctilinealis, A. t. septentrionalis, and A. t. tigris; Fig. 1), plus a recently elevated putative sister species, A. marmoratus (Hendricks and Dickson 1986, Reeder et al 2002, Tucker et al 2016)
We evaluated a total of N=206 samples representing five members of the A. tigris complex (A. marmoratus, A. tigris mundus, A. t. punctilinealis, A. t. septentrionalis, and A. t. tigris), their putative secondary intergrades, plus two out-groups (Tupinambis rufus and Aspidoscelis gularis) (Fig. 1, Table S1)
Summary
The Colorado Plateau (~337,000 km at ~2000m asl) is a tectonically intact landform of relatively linear sedimentary rock that contrasts markedly with its eroding margin, ongoing since the Late Miocene (Levander et al 2011). It stands in sharp contrast with the more fractured geomorphic features of neighboring regions: The Rocky Mountains (immediately north and east), Rio Grande Uplift (southeast), and the Basin and Range Province (immediately west) (Fig. 1 of Flowers 2010). The Pliocene brought decreased seasonality as well as elevated temperatures and humidity, a climatic regime subsequently reversed by Quaternary glaciation (Thompson 1991)
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