Diversification rates and phenotypic evolution in venomous snakes (Elapidae)

Abstract

The relationship between rates of diversification and of body size change (a common proxy for phenotypic evolution) was investigated across Elapidae, the largest radiation of highly venomous snakes. Time-calibrated phylogenetic trees for 175 species of elapids (more than 50% of known taxa) were constructed using seven mitochondrial and nuclear genes. Analyses using these trees revealed no evidence for a link between speciation rates and changes in body size. Two clades (Hydrophis, Micrurus) show anomalously high rates of diversification within Elapidae, yet exhibit rates of body size evolution almost identical to the general elapid 'background' rate. Although correlations between speciation rates and rates of body size change exist in certain groups (e.g. ray-finned fishes, passerine birds), the two processes appear to be uncoupled in elapid snakes. There is also no detectable shift in diversification dynamics associated with the colonization of Australasia, which is surprising given that elapids appear to be the first clade of venomous snakes to reach the continent.

Keywords: body size; macroevolution; phylogenetics; reptiles; speciation rates.

Figure 1.

Datedphylogenetic tree of elapids, with rates of speciation inferred using BAMM (warmer colours = faster rates; scale bar on right represents speciation rate per Ma). Circled numbers indicate the two branches with the highest probability of rate shifts, i.e. Hydrophis and Micrurus. Tree with posterior probabilities of each clade is shown in electronic supplementary material, figure S1, rates of extinction are shown in electronic supplementary material, figure S4, and the nine rate shift configurations (for diversification) in the 95% credibility shift set are shown in electronic supplementary material, figure S5.

Figure 2.

Datedphylogenetic tree of elapids, with rates of body size evolution inferred using BAMM (warmer colours = faster rates; scale bar on right in divergence per Ma); this is the same tree as figure 1 but with six taxa lacking size data excluded. Circled numbers indicate the two sizeable clades with the highest probability of rate shifts. Note that Hydrophis and Micrurus (figure 1) are not characterized by unusually fast rates of body size evolution. The three rate shift configurations (for size evolution) in the 95% credibility shift set are shown in electronic supplementary material, figure S7.