HODGES, Wendy L; Univ. of California, Riverside: Testing the Cold-Climate Hypothesis: Evolution of Viviparity in Horned Lizards (Phrynosoma)

Transitions from oviparity (egg laying) to viviparity (live bearing) have occurred in many vertebrate lineages including fishes, amphibians, reptiles and mammals. The cold-climate model for the evolution of viviparity in squamates has received broad support and predicts a correlation between reproductive mode, altitude and latitude. I tested this prediction in horned lizards, genus Phrynosoma, within a phylogenetic context. I first determined whether all viviparous species formed a monophyletic group using Monte Carlo simulations. Second, I analyzed relationships between reproductive mode and minimum, midpoint, and maximum altitudes and latitudes by computing conventional correlations and phylogenetically independent contrasts. Third, I tested for the presence and relative strength of phylogenetic signal in these traits using the K statistic. Viviparous species did not form a monophyletic group, supporting the independent evolution of viviparity twice in the genus. Viviparity and minimum and midpoint altitudes showed strong phylogenetic signal and were significantly correlated with each other, while maximum altitude and latitude were not correlated with reproductive mode. Strong phylogenetic signal present in the data indicates that phylogenetic relationships are important components of these traits and suggests phylogeny plays a key role in the distribution of traits among species. This study partially supports the cold-climate model but suggests it should be re-evaluated within an explicit phylogenetic framework. An overall reanalysis of the evidence used to support the cold-climate model, especially correlations between reproductive mode, altitude and latitude, seems prudent and may help us discriminate adaptations for the evolution of viviparity from traits present by virtue of inheritance from common ancestors.