Integrative taxonomy at the nexus of population divergence and speciation in insular speckled rattlesnakes

We describe two diminutive species of rattlesnakes (Genus _Crotalus_) from small nearshore islands off the coast of Baja California in the western Gulf of California, Mexico. In order to test the hypothesis that some island populations represent cohesive species entities, we applied linear discriminant analysis and uniform validation procedures to multiple classes of intrinsic trait data. By using previously recognised species to establish a threshold for species recognition, we found that assignment of specimens to both new species was as probable as with other established rattlesnake species within the speckled rattlesnake, _Crotalus mitchellii_, complex. We also found that assignment of specimens from other island populations was not as probable as for the established species, and these populations are referable to _C. pyrrhus_. The species endemic to Piojo Island is most closely related to other island and mainland populations of _C. pyrrhus_ whereas the species endemic to Cabeza de Caballo Island is apparently most closely related to _C. angelensis_, a nearby island endemic of large body size. However, patterns from both mitochondrial and nuclear phylogenies, and phenotypic variation indicate that evolutionary trajectories of both of these species have been influenced by introgression from _C. angelensis_. We speculate that collective evidence based on contrasting patterns of nuclear and mitochondrial evolution supports a hybrid origin of the species from Cabeza de Caballo Island followed by exceptionally rapid mitochondrial evolution. Consistent with small body size, both species show a reduction in various scale counts relative to other species of the _C. mitchellii_ species complex, suggesting that dwarfism is not simply a plastic response to insular conditions.