The Evolution of Big Snakes: A New Perspective on Their Biogeography and Morphological Diversification

Last April, Noonan and Chippindale published a wonderful new phylogeny of boid snakes (a group that includes the Boinae [i.e., all of the well-known boas from the New World, Madagascar, and the Pacific] and the Erycinae [i.e., the African sand boas plus North America's rubber and rosy boas]) with fascinating implications for the group's biogeographic history and morphological evolution. I can't find anything to complain about in their phylogenetic analyses, which include thorough parsimony, likelihood, and Bayesian analyses of four nuclear and one mitochondrial gene. Their results should be considered the definitive modern statement on phylogenetic relationships in the Boinae.

In addition to supporting a late Cretaceous (~90 Ma) Antarctic land bridge between the New World with Madagascar (yes, that's right, an Antartic land bridge), Noonan and Chippindale's results have some mind-blowing implications for the evolution of morphological diversity in the Boinae. Two particularly stunning examples are noteworthy. First, they find moderate support for the paraphyly of Epicrates with respect to anacondas (Eunectes). Thus, it appears that the shared morphological and ecological features of Caribbean and mainland Epicrates have experienced prolonged evolutionary stasis during a longer interval of time than that required for the evolution of their distinctive evolutionary cousins (the anacondas). A second example shows how similar selective pressures can drive independent evolution of remarkably convergent morphologies. This point is made by the sister group relationship between the burrowing African genus Calabaria with the larger terrestrial/arboreal species on Madagascar (Sanzinia and Acrantophis). This is noteworthy, of course, because Calabaria now appears only distantly related to other, morphologically similar, burrowing taxa. Needless to say, these results drive another nail into the coffin of Kluge's ill-considered taxonomic revision that unified New World Boa with Madagascar's endemic genera. Kluge's results, which were based on morphological analyses, confound not only biogeography, but also a fascinating story of convert evolution.