Evolutionary patterns in life-history traits of lizards of the genus Xenosaurus

Life histories are directly related to fitness and, hence, are the focus of strong selective pressures. However, different life-history traits may evolve at different paces and may respond differentially to particular selective pressures. We examined patterns of evolutionary change in the following life-history traits of xenosaurid lizards: size at maturity, average size of adult females, litter size, neonate size, and relative litter mass. We used a phylogenetic hypothesis of the genus Xenosaurus and different phylogenetic comparative methods to search for evolutionary relationships between traits as well as to estimate ancestral states, rates of evolution, and the amount of phylogenetic signal on each trait. In addition, we searched for differences in these life-history traits among the different environments where these lizards inhabit (cloud forest, tropical forest, oak-pine forest, and xeric scrub). We found an evolutionary relationship between size at maturity and average adult size, with larger species maturing at larger sizes. We also found an evolutionary trade-off between litter size and neonate size. Ancestral state reconstructions revealed differences among traits in the relative timing of diversification. Litter size and neonate size began diversification early in the history of the genus. In contrast, size at maturity and relative litter mass remained phenotypically invariant for a long time period before diverging into distinct phenotypic values. Litter size exhibited significant phylogenetic signal because the diversification history of this trait has tracked the phylogeny closely. The observed variation among species in neonate size also showed some trace of the phylogenetic relationships. The remaining three traits diverged throughout time without a clear phylogenetic pattern. In addition, litter size and relative litter mass exhibited the highest evolutionary rates whereas average adult size and neonate size exhibited the lowest rates. Litter size was the only trait that differed significantly among environments, with largest litters in cloud forests. We discuss potential hypotheses to explain the observed differences among life-history traits in the tempo and mode of evolution.