Using a null hypothesis framework to test expectations of disparity in an adaptive radiation

Adaptive radiations are expected to generate striking differences in species and morphological diversity between closely related groups. Not all hypotheses in evolutionary biology, including these observed disparities, are amenable to experimental manipulation or comparative phylogenetics. In some cases, the use of null hypotheses offers a way to test these scientific conjectures such that their premise can be rejected. We examine the Hydrophis sea snakes (Hydrophiinae: Hydrophiini), a putative example of adaptive radiation based on both its species and morphological diversity. We compared its observed species richness and morphological disparity with the closely related, yet species and morphologically depauperate clade, Aipysurus-Emydocephalus. We used phylogenetic null models and a phylomorphospace approach to test for significant differences in diversification rate and in morphological disparity between these two major sea snake lineages. We found no diversification rate differences between the Hydrophis and Aipysurus-Emydocephalus groups under an equal-rates Markov model of diversification. However, Hydrophis species occupy a significantly larger region of the morphospace than the Aipysurus-Emydocephalus group, consistent with previous conclusions that ecological specialization causes increased levels of morphological disparity. While the Hydrophis and Aipysurus-Emydocephalus lineages do not significantly differ in species numbers, the significant morphological disparity among Hydrophis species further upholds its adaptive radiation.