Reptilian viviparity and Dollo's law

It has been suggested repeatedly that the evolutionary transition from oviparity (egg-laying) to viviparity (live-bearing) m reptiles is irreversible. However, these adaptive arguments have yet to be tested by detailed examination of the phylogenetic distribution of oviparity and viviparity across a broad range of taxa. Using available data on reproductive modes and phylogenetic relationships within reptiles, we here quantify the numbers and directions of evolutionary transitions between oviparity and viviparity. Phylogenetic relationships among three diverse squamate groups (scincid lizards, colubrid snakes, elapid snakes) are currently inadequately known for inclusion in this study. Among the remaining reptiles, oviparity has given rise to viviparity at least 35 times. Five possible instances of 'reversals' (from viviparity to oviparity) are identified, but closer examination indicates that all have weak empirical support (i.e., they could be 'unreversed' with little loss in parsimony, and/or are based on poorly substantiated phylogenetic hypotheses). Viviparity is clearly more frequently (and presumably easily) gained than lost in several disparate groups so far examined (reptiles, fishes, polychaete worms); this evolutionary bias should be considered when reproductive mode is optimized on a phylogeny or employed in phylogenetic reconstruction.