Natural selection plays an important role in the evolution of sexual communication systems. Two of the more important selection agents are predation and the optical environment. However, few studies have attempted to tease apart the effects of these important factors on the evolution of sexual signals at the same time. In this study, we employ a model system of Bahamian mosquitofish populations that vary widely in water color and predation risk to uncover the individual and interactive effects of these factors on interpopulation divergence in a colorful sexual trait, fin color.
Results/Conclusions
Our results show that the optical environment was clearly the most important driver of divergent sexual coloration in our study system. However, the most notable result was a unimodal pattern of phenotypic divergence along an optical gradient generated by spatially variable dissolved organic carbon (DOC) concentrations. While these results clearly indicate an important role for DOC-driven optical variation in sexual selection and evolutionary divergence, the mechanisms underlying our observations are unclear. To explore possible mechanisms, we proffer three hypotheses (contrast maximization, signal costs, and carotenoid limitation) that may explain unimodal divergence and briefly discuss the evolutionary implications of nonmonotonic clines.