Evolution of viviparity in the lizard genus Sceloporus

Abstract

According to the cold-climate model, viviparity in squamate reptiles evolves at high elevations and latitudes. The lizard genus Sceloporus has been used to support this model

viviparous species are found at higher elevations than oviparous species. For example, in the scalaris species group, a viviparous species replaces its oviparous sister species at elevations above 3000 m. We used recent literature to evaluate four aspects of the evolution of viviparity within the genus and concluded that (1) viviparity has evolved a minimum of four times within the genus. (2) Viviparity has not evolved at high latitudes, because the oviparous species found at high latitudes lack the morphological and physiological features that facilitate prolonged egg retention, the intermediate stage between oviparity and viviparity. (3) Viviparous species have not invaded high latitudes because viviparity in Sceloporus is associated with gestation during winter months. This is a viable reproductive mode at high elevations at tropical latitudes, because ambient temperatures during the winter are similar to those in the summer. At high latitudes, in contrast, winters may be too cold to support embryogenesis, or perhaps even survival of embryos. (4) Viviparity evolved from oviparity through three major transitions. The first was the evolution of viviparity from oviparity

this transition was associated with the invasion of high elevations. The second transition was a shift from summer to winter gestation such that birth was shifted from autumn to spring. With birth in the spring, neonates are able to mature by autumn, and consequently they can produce a litter within a year of their own birth. The third transition was a shift from asynchronous reproductive cycles of males and females (mating in spring and fertilization in autumn) to synchronous reproductive cycles (mating and fertilization in the autumn).