Taxonomic hypotheses and the biogeography of speciation in the Tiger Whiptail complex (Aspidoscelis tigris: Squamata, Teiidae)
Authors: Chafin Tyler K. , Douglas Marlis R. , Anthonysamy Whitney J.B. , Sullivan Brian K. , Walker James M. , Cordes James E. , Douglas Michael E. 
Published: October 2020

(225)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. TheAspidoscelis tigris(Tiger Whiptail) complex (Squamata: Teiidae) is one such group in which potential 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. To do so, we evaluated the manner by which our models parsed phylogenetic and biogeographic patterns. We found considerable variation among species ‘hypotheses’, which we suggest in part reflects inflated levels of inter-population genetic divergence caused by historical demographic expansion and contraction cycles. Inter-specific boundaries withA. marmoratusjuxtaposed topographically with the Cochise Filter Barrier that separates Sonoran and Chihuahuan deserts (interpreted herein as case 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 thetigriscomplex, with intraspecific units governed locally by demographic processes.

HIGHLIGHTS

Phylogeographies of vertebrates within the southwestern deserts of North America have been shaped by climatic fluctuations imbedded within broad geomorphic processes.

The resulting synergism drives evolutionary processes, such as an expansion of within-species genetic divergence over time. Taxonomic inflation often results (i.e., an increase in recognized taxa due to arbitrary delineations), such as when morphological divergences fail to juxtapose with biogeographic hypotheses.

However, isolated groups can be discriminated within-species by mapping genetic variability onto geographic distances. This approach can often diagnose ‘hard’ barriers to dispersal, or alternatively, strong selection acting against hybridization. On the other hand, elevated genetic divergences among groups less-isolated would underscore isolation-by-distance (i.e., an increase in genetic differentiation concomitant with geographic distance).

The biogeographic patterns we identified in Tiger Whiptail are largely synonymous with those found in other regional species, particularly given the geomorphic separation of Mohave and Sonoran deserts by the Colorado River, and Sonoran/ Chihuahuan deserts by the Cochise Filter Barrier.

Our results for the Tiger Whiptail complex broaden and extend the context within which polytypic species are conserved and managed, particularly those that reflect an incongruence among molecular and morphological standards.