EVOLUTION OF VIVIPARITY IN THE LIZARD GENUS SCELOPORUS

Abstract

We compared modes of reproduction in the lizard genus Sceloporus with ele- vation, latitude, environmental moisture, habits, and body size. Viviparous species of Sceloporus more commonly are found at high elevation than at low elevation, and tend to exhibit arboreal or saxicolous habits. Our findings suggest the following hypothesis: Within oviparous species of Sceloporus, animals exhibiting egg retention have a selective advantage over conspecifics in the cold environments of high elevation or latitude. Egg retention leads to ovoviviparity in which eggs hatch in utero. Once ovoviviparity has evolved, the placenta evolves primarily as an ad- aptation to supply oxygen to the developing embryo. Viviparous forms then can assume an arboreal or saxicolous habit as an anti-predatory and/or thermoregulatory adaptation. Finally, viviparity evolves as an irreversible trait. Subsequent radiation of the viviparous forms produces species that can live in environments not originally a factor in the evolution of the viviparous mode of reproduction.