The evolution of oviparity in squamate reptiles: An adaptationist perspective

Phylogenetically based analyses can suggest directions of evolutionary transitions, based on parsimony, but can never provide unambiguous answers. To clarify the relative frequency of phylogenetic shifts from oviparity to viviparity versus the reverse, we need additional sources of evidence. Adaptationist thinking (i.e., consideration of selective forces) has revealed a great deal about the transition from oviparity to viviparity, but has rarely been employed to consider the reverse transition. An evaluation of costs and benefits identifies major obstacles to the re-evolution of oviparity. For example, even a modest decrease in the degree of embryogenesis completed in utero (i.e., a shift from viviparity back toward "normal" oviparity) requires the mother to find a suitable nest-site (often, a risky endeavor), and a minor decrease in the duration of uterine retention of eggs may not substantially reduce maternal costs (because many of those costs are minimized by maternal behavioral adaptations to pregnancy). In many climates, a small decrease in the duration of uterine retention of eggs would not allow the female to produce a second clutch within the same season; and thus, would not reduce the fecundity disadvantage of viviparity. Life-history theory thus suggests an asymmetry in the fitness consequences of the intermediate stages between oviparity and viviparity. That asymmetry facilitates the "forward" transition (based on thermally driven benefits to offspring viability) but opposes the "reverse" transition (based on lower fitness of heavily burdened females that need to seek nest-sites). These factors should constrain the re-evolution of oviparity to specific conditions (e.g., where abundant nest-sites are available within a female's usual home range, rather than requiring extensive migration).