Dietary shifts have played a major role in the evolution of many vertebrates. The idea that the evolution of herbivory is physiologically constrained in squamates is challenged by a number of observations that suggest that at least some lizards can overcome the putative physiological difficulties of herbivory on evolutionary and even ecological timescales. We compared a number of morphological and physiological traits purportedly associated with plant consumption between two island populations of the lacertid lizard Podarcis sicula. Previous studies revealed considerable differences in the amount of plant material consumed between those populations. We continued the investigation of this study system and explored the degree of divergence in morphology (dentition, gut morphology), digestive performance (gut passage time, digestive efficiency), and ecology (endosymbiont density). In addition, we also performed a preliminary analysis of the plasticity of some of these modifications. Our results confirm and expand earlier findings concerning divergence in the morphology of feeding structures between two island populations of P. sicula lizards. In addition to the differences in skull dimensions and the prevalence of cecal valves previously reported, these two recently diverged populations also differ in aspects of their dentition (teeth width) and the lengths of the stomach and small intestine. The plasticity experiment suggests that at least some of the changes associated with a dietary shift toward a higher proportion of plant material may be plastic. Our results also show that these morphological changes effectively translate into differences in digestive performance: the population with the longer digestive tract exhibits longer gut passage time and improved digestive efficiency. © 2010 by The University of Chicago.

BACKGROUND:Patterns of spatial variation in discrete phenotypic traits can be used to draw inferences about the adaptive significance of traits and evolutionary processes, especially when compared to patterns of neutral genetic variation. Population divergence in adaptive traits such as color morphs can be influenced by both local ecology and stochastic factors such as genetic drift or founder events. Here, we use quantitative color measurements of males and females of Skyros wall lizard, Podarcis gaigeae, to demonstrate that this species is polymorphic with respect to throat color, and the morphs form discrete phenotypic clusters with limited overlap between categories. We use divergence in throat color morph frequencies and compare that to neutral genetic variation to infer the evolutionary processes acting on islet- and mainland populations.RESULTS:Geographically close islet- and mainland populations of the Skyros wall lizard exhibit strong divergence in throat color morph frequencies. Population variation in throat color morph frequencies between islets was higher than that between mainland populations, and the effective population sizes on the islets were small (Ne:s < 100). Population divergence (FST) for throat color morph frequencies fell within the neutral FST-distribution estimated from microsatellite markers, and genetic drift could thus not be rejected as an explanation for the pattern. Moreover, for both comparisons among mainland-mainland population pairs and between mainland-islet population pairs, morph frequency divergence was significantly correlated with neutral divergence, further pointing to some role for genetic drift in divergence also at the phenotypic level of throat color morphs.CONCLUSIONS:Genetic drift could not be rejected as an explanation for the pattern of population divergence in morph frequencies. In spite of an expected stabilising selection, throat color frequencies diverged in the islet populations. These results suggest that there is an interaction between selection and genetic drift causing divergence even at a phenotypic level in these small, subdivided populations.