Species-Specific Spatial Patterns of Variation in Sexual Dimorphism by Two Lizards Settled in the Same Geographic Context

SIMPLE SUMMARY: Understanding how sexual dimorphism responds to natural and sexual selection is essential to figuring out how intraspecific phenotypic diversity is produced. By comparing the response of two species of lizards inhabiting the same archipelago, we show that sexual dimorphism is a complex phenomenon resulting from the interaction between sexual and natural selection. These two forces act simultaneously, not necessarily in the same direction, and may generate species-specific spatial pattern of morphological variability even in species settled in the same geographic context. ABSTRACT: The evolution of sexual dimorphism (SD) results from intricate interactions between sexual and natural selections. Sexually selected traits are expected to depend on individual condition, while natural selected traits should not be. Islands offer an ideal context to test how these drivers interact with one another, as the size is a reliable proxy for resource availability. Here, we analysed SD in body size (snout-vent length) and head shape (assessed by geometric morphometric) in two species of lizards (Podarcis muralis and P. siculus) inhabiting the Tuscan archipelago (Central Italy). We found a strong SD variation among islands in both species. Furthermore, in P. muralis emerged some significant correlations between SD and island size, supporting the occurrence of possible effects of individual condition on SD. By contrast, SD in P. siculus followed opposite trajectories than in P. muralis, suggesting that in this species, natural selection could play a major role as a driver of SD. Our findings show that natural and sexual selection can interact in complex ways, and the responses are species-specific. Therefore, spatial patterns of variation in SD may strongly differ among species, even when they settle in the same geographic contest.