The evolution of viviparity and placentation revisited.

Most studies on the evolution of viviparity in squamate reptiles have focused on one or more of the following questions: (1) what selective force(s) are involved in the transition from oviparity to viviparity? (2) how does this transition occur (i.e., what changes are involved)? and (3) how and when does placentation evolve? In answer to the first of these questions, most studies have implicated cold climates (at high latitudes or high elevations) as the primary selective force involved [see Shine (1985) for a review]. Recently, in a paper published in this journal, Daniel Blackburn (1995) addressed the latter two of these questions. Specifically, Dr Blackburn tested different evolutionary models (saltation, gradualism, and punctuated equilibrium) for the evolution of viviparity, and questioned whether placentation evolves concurrent with, or subsequent to, the evolution of viviparity. In this paper, we re-examine these questions. We agree with Dr Blackburn on several main points: that squamate reproductive modes frequently exhibit apparent stasis over long periods of time, that the actual transition from oviparity to viviparity can occur quite rapidly, and that intermediate reproductive forms are relatively rare. However, we disagree, on theoretical grounds, with the manner in which Blackburn chose to couch some of his arguments. We also present further evidence for the existence of evolutionarily intermediate forms from our own studies, which were not published when Dr Blackburn surveyed the literature, and from studies that he apparently overlooked. We first discuss the evolutionary transition from oviparity (the oviposition of shelled eggs early in embryonic development) to viviparity (the retention of developing embryos within the oviducts throughout development), followed by a separate discussion of the evolution of placentation.