Geographic variation in life history phenotypes between populations of a single species is often assumed to reflect genetic divergence caused by natural selection. The relative contribution of genetic and environmental sources of phenotypic variation is rarely determined, especially for vertebrates. However, distinguishing between phenotypic plasticity induced by proximate environmental variation and genetic divergence is fundamental to understanding the ecological and evolutionary significance of geographic variation. We used a reciprocal transplant experiment to uncover the relative importance of population-specific (genetic) and environmental sources of variation in individual growth rates between two populations of the fence lizard, Sceloporus undulatus. Our study revealed a population x environment interaction, consistent with a genotype x environment interaction that would result if differences between populations were genetically based. The growth rates of Nebraska lizards, normally twice that of New Jersey lizards, were reduced by half when the animals were transplanted to New Jersey. However, New Jersey lizards showed no increase in growth rates when transplanted to Nebraska. Comparisons of relative food availability (arthropod abundance) indicated that more food was available in New Jersey during the experiment. Estimates of potential activity day length using hollow copper models were from 2 to 2.5 h longer in Nebraska. This result suggests that the thermal biophysical environment may have reduced growth rates of Nebraska lizards transplanted to New Jersey, perhaps by reducing foraging time or by limiting physiological processes supporting growth. Our results are consistent with previous studies of S. undulatus that assumed genetic differences in life histories between populations and implicated the thermal biophysical environment as an ecological source of variation.
