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Aim

To assess the genealogical relationships of widespread montane rattlesnakes in the Crotalus triseriatus species group and to clarify the role of Late Neogene mountain building and Pleistocene pine-oak forest fragmentation in driving the diversification of Mexican highland taxa.

Location

Highlands of mainland Mexico and the south-western United States (Texas, New Mexico, and Arizona).

Methods

A synthesis of inferences was used to address several associated questions about the biogeography of the Mexican highlands and the evolutionary drivers of phylogeographical diversity in co-distributed taxa. We combined extensive range-wide sampling (130 individuals representing five putative species) and mixed-model phylogenetic analyses of 2408 base pairs of mitochondrial DNA to estimate genealogical relationships and divergence times within the C. triseriatus species group. We then assessed the tempo of diversification using a maximum likelihood framework based on the birth-death process. Estimated times of divergences provided a probabilistic temporal component and questioned whether diversification rates have remained constant or varied over time. Finally, we looked for phylogeographical patterns in other co-distributed taxa.

Results

We identified eight major lineages within the C. triseriatus group, and inferred strong correspondence between maternal and geographic history within most lineages. At least one cryptic species was detected. Relationships among lineages were generally congruent with previous molecular studies, with differences largely attributable to our expanded taxonomic and geographic sampling. Estimated divergences between most major lineages occurred in the Late Miocene and Pliocene. Phylogeographical structure within each lineage appeared to have been generated primarily during the Pleistocene. Although the scale of genetic diversity recognized affected estimated rates of diversification, rates appeared to have been constant through time.

Main conclusions

The biogeographical history of the C. triseriatus group implies a dynamic history for the highlands of Mexico. The Neogene formation of the Transvolcanic Belt appears responsible for structuring geographic diversity among major lineages. Pleistocene glacial-interglacial climatic cycles and resultant expansions and contractions of the Mexican pine-oak forest appear to have driven widespread divergences within lineages. Climatic change, paired with the complex topography of Mexico, probably produced a myriad of species-specific responses in co-distributed Mexican highland taxa. The high degree of genetic differentiation recovered in our study and others suggests that the Mexican highlands may contain considerably more diversity than currently recognized.