Abstract

Phylogenetic analyses of reproductive patterns have clarified several aspects of the evolution of viviparity in reptiles and mammals, and offer useful comparisons to that pattern in fishes. In amniotes, viviparity, placentation, and matrotrophy have been assumed to evolve as three successive stages in a continuous, unilinear transformation, with yolk sac placentas originating prior to chorioallantoic placentas. However, the phylogenetic distribution of reproductive characters in mammals indicates that obligative uterine matrotrophy actually evolved under conditions of oviparity, and that viviparity and placentation evolved subsequently. In contrast, in lizards and snakes, viviparity frequently has evolved simultaneously with placentation and incipient matrotrophy, apparently in accord with a punctuated equilibrium model of evolutionary change. In squamates (and quite possibly, mammals), placentas formed from the chorioallantois and yolk sac have evolved simultaneously. Phylogenetic analysis indicates that reptilian viviparity has originated independently in over 100 separate lineages, often at low taxonomic levels and in geologically recent times. At least four of these clades subsequently have evolved substantial matrotrophy. Mammals, squamates and fishes have evolved viviparity and matrotrophy convergently, but apparently by different historical sequences and different anatomical and physiological specializations. These findings reveal the difficulties in making evolutionary generalizations about viviparity across higher taxonomic boundaries, and the importance of employing quantitative phylogenetic analyses.

Citations