Evolutionary origins of viviparity consistent with palaeoclimate and lineage diversification

It is of fundamental importance for the field of evolutionary biology to understand when and why major evolutionary transitions occur. Live-bearing young (viviparity) is a major evolutionary change and has evolved from egg-laying (oviparity) independently in many vertebrate lineages and most abundantly in lizards and snakes. Although contemporary viviparous squamate species generally occupy cold climatic regions across the globe, it is not known whether viviparity evolved as a response to cold climate in the first place. Here, we used available published time-calibrated squamate phylogenies and parity data on 3,498 taxa. We compared the accumulation of transitions from oviparity to viviparity relative to background diversification and a simulated binary trait. Extracting the date of each transition in the phylogenies and informed by 65 my of global palaeoclimatic data, we tested the nonexclusive hypotheses that viviparity evolved under the following: (a) cold, (b) long-term stable climatic conditions and (c) with background diversification rate. We show that stable and long-lasting cold climatic conditions are correlated with transitions to viviparity across squamates. This correlation of parity mode and palaeoclimate is mirrored by background diversification in squamates, and simulations of a binary trait also showed a similar association with palaeoclimate, meaning that trait evolution cannot be separated from squamate lineage diversification. We suggest that parity mode transitions depend on environmental and intrinsic effects and that background diversification rate may be a factor in trait diversification more generally.