Parallel selective pressures drive convergent phenotypic diversification in the morphologically and ecologically diverse Pythons and Boas

Abstract

Convergent evolution, or the multiple independent origin of a biological trait, can be expressed at many levels, from populations to adaptive radiations. Pythons and boas are globally distributed and are famous for their remarkable phenotypic and ecological diversity, spanning dwarfs to giants, and burrowers to tree-climbers. Each of the pythons and boas clades are species-rich adaptive radiations and provide a novel system to explore convergent evolution and macroevolutionary patterns at an intercontinental scale and between distantly related clades. Our main hypothesis is that strong and widespread convergent evolution in head shape is driven by parallel adaptation to similar ecological niches between pythons, boas and their close relatives. We classified each species into one of six ecotype categories based on their habitat use. With a recently published phylogeny for the group and using two-dimensional geometric morphometrics, we applied an explicitly phylogenetic framework to our analyses, and used three main approaches. First, we reconstructed phylomorphospace for the geometric morphometric data, then we used a new method called SURFACE to identify convergent phenotypic regimes, and then we tested the strength of convergence for all alternative ecomorph groupings using the Wheatsheaf index. We found that pythons and boas group according to their ecotype in phylomorphospace, overriding phylogenetic effects. Moreover, we found five convergent phenotypic regimes, each of which has a different predominant ecotype, suggesting that the evolution of head shape in these species has a strong adaptive component related to habitat use. Because replicated adaptive radiations at an intercontinental level and between distantly related taxa are rarely demonstrated, pythons and boas provide and excellent new model for the study of ecological and phenotypic convergence at deep macroevolutionary levels. Show more