Non-adaptive processes explain chemical signal divergence in island lizards

Understanding the processes that drive divergence in animal signals is central to behavioral and evolutionary ecology. While adaptive explanations have traditionally dominated, especially for traits involved in sexual signaling, the role of non-adaptive processes remains underexplored ? particularly for vertebrate chemical signals. Here, we investigated chemical signal variation across multiple island populations of the Italian wall lizard Podarcis siculus. Using lipid and protein profiling of glandular secretions, microsatellite-based genetic distances, and socio-ecological variables, we tested competing hypotheses for signal divergence. Genetic and geographic distances were strong predictors of variation in chemical signal profiles, whereas environmental conditions (i.e. climate, habitat and food availability) and sexual size dimorphism offered little explanatory power. These findings suggest that stochastic mechanisms, such as genetic drift driven by historical isolation and restricted gene flow, have likely played a prominent role in shaping chemical signal diversity in this system. Our results underscore the importance of considering both non-adaptive and adaptive processes in signal evolution, particularly in geographically isolated systems with limited dispersal, such as island archipelagos.