Sky-islands diversification: The case of the alpine garter snakes (Natricidae: Thamnophis), their evolutionary and biogeographic history in the Mexican highlands

The Mexican highlands, a biodiversity hotspot characterized by complex topography and dynamic paleoenvironments, function as "Sky Islands," isolating temperate-adapted species and promoting in situ diversification. This pattern is exemplified by the Mexican Highland Clade (MHLC) of Thamnophis, a genus exhibiting high ecological and phylogenetic diversity in North America. We hypothesize that MHLC diversification is the result of ecological specialization and divergence driven by the environmental heterogeneity of the Mexican highlands. Through an integrative approach, combining phylogenetics, morphological analyses, and ecological niche modelling, we reconstruct the clade’s evolutionary and biogeographic history to identify key drivers of diversification. Our results indicate that MHLC diversification was driven by the geologic and climatic heterogeneity of the highlands, with initial divergence in the Late Miocene (~5.62 Ma) consistent with Trans-Mexican Volcanic Belt activity. These patterns align with other highland-endemic taxa, suggesting shared biogeographic processes. Niche specialization and correlated shifts in head morphology suggest possible adaptive responses to habitat variation, potentially reflecting ecomorphological convergence independent of phylogenetic constraints. These findings highlight the Mexican highlands as a crucial area for evolutionary processes in North America, containing significant undocumented biodiversity and confirming their importance as a global endemism hotspot.