Evolution of rattlesnakes (Viperidae; Crotalus) in the warm deserts of western North America shaped by Neogene vicariance and Quaternary climate change.

Molecular Ecology (2006) 15, 3353 – 3374. doi: 10.1111/j.1365-294X.2006.03007.x


During Pleistocene, the Laurentide ice sheet rearranged and diversified biotic distributionsin  eastern  North  America,  yet  had  minimal  physical  impact  in  western  North  Americawhere  lineage  diversification  is  instead  hypothesized  to  result  from  climatic  changes.  If Pleistocene climatic fluctuations impacted desert species, the latter would reflect patterns of restricted gene flow concomitant with indications of demographic bottlenecks. Accordingly, molecular evidence for refugia should be present within these distributions and for subsequent range expansions as conditions improved. We sought answers to these questions by evaluating mitochondrial DNA (mtDNA) sequences from four species of rattle-snakes [Crotalus mitchellii (speckled rattlesnake), Crotalus cerastes (sidewinder), Crotalus tigris (tiger rattlesnake), Crotalus ruber (red  diamond  rattlesnake)] with distributions restricted to desert regions of southwestern North  America. We inferred relationships using  parsimony  and  maximum likelihood, tested  intraspecific  clades  for  population expansions, applied an isolation-with-migration model to determine bi-directional migration rates (m) among  regions, and inferred divergence times for species and clades by applying a semiparametric penalized likelihood approach to our molecular data. Evidence for significant range expansion was present in two of eight  regions in  two  species (Crotalus mitchellii pyrrhus, C. tigris region north). Two species (C. cerastes, C. mitchellii) showed a distribution concomitant with northward displacement of Baja California from mainland México, followed by vicariant separation into subclades. Effects of Pleistoceneclimate  fluctuations were found in the distributions of all four species. Three regional diversification patterns were  identified: (i) shallow genetic diversity that resulted  from Pleistocene climatic events (C. tigris, C. ruber); (ii)  deep Pleistocene divisions indicating allopatric segregation of subclades within refugia (C. mitchellii, C. cerastes); and (iii) line-age diversifications that extended to Pliocene or Late Miocene (C. mitchellii, C. cerastes). Clade-diversifying and clade-constraining effects impacted the four species of rattlesnakes unequally.  We found relatively high levels of molecular  diversification in the two most broadly distributed species (C. mitchellii, C. cerastes), and lower levels of genetic diversification in the two species (C. tigris, C. ruber) whose ranges are relatively more restricted. Furthermore, in several cases, the distributions of subspecies were not congruent with our molecular information. We suggest  regional conservation perspectives for southwestern deserts cannot rely upon subspecies as biodiversity surrogates, but must instead employ a molecular and deep historical perspective as a primary mechanism to frame biodiversity reserves within this region.


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