Reconstructing the evolutionary history of melanism in Elapidae: a comparative phylogenetic approach

Melanism, namely the increase of melanin-based pigmentation in skin resulting in a black colouration, is a chromatic form that is relatively common in animals, variable both within and among species. Its adaptive and evolutive significance is widely debated and has been studied mainly in isolated experimental cases. In this work we have reconstructed the evolutionary history of melanism in a family of reptiles, Elapidae, with a broad geographic distribution and a large representation in terms of species abundance. We collected georeferenced photographic records of 90 species and classified their phenotypes as melanic vs non-melanic and pooled the records together thus defining for each species whether it shows (1) or not (0) the occurrence of at least one record of melanism. Afterwards, we estimated the most likely evolutionary model of the trait to reconstruct the ancestral state at the root of the phylogenetic tree of this family; eventually we simulated the stochastic evolution of the trait by reconstructing the a posteriori probabilities of each internal node of the tree to be either in state 1 (melanic) or state 0 (non-melanic). Lastly, we tested for phylogenetic signal of the trait that would describe the probability of each species to show the same state of the trait according to the phylogenetic distance among species. The ancestral state reconstruction showed that the most likely state at the root of the tree is melanic, with an a posteriori probability of 82.5%. The phylogenetic signal we detected (K=0.54) indicates that the probability of any two species to show the same state of the trait is moderately higher than that expected only due to chance. In conclusion, melanism appears to be the ancestral trait of Elapid snakes and to be moderately maintained over time; however, it is crucial to integrate such preliminary evidence with extensive investigation on the putative effects of environmental factors on the evolutionary success of melanism over time.