Sexual dimorphism in body size and the origin of sex-determination systems

Diverse sex-determining systems occur in vertebrates, including environmental sex determination (ESD), genetic sex determination (GSD) of type XX/XY (heterogametic males), and GSD of type ZZ/ZW (heterogametic females). The origins of the two genetic types are poorly understood. We use protected invasion theory to derive a model that generates testable predictions about the origins of the two genetic types from ESD. Protected invasion theory predicts biases in the evolutionary origins of new traits by focusing on the probability that a selectively favored trait will avoid loss by genetic drift when rare. We show that the theory makes predictions about the conditions under which XY or ZW systems are more likely to arise from an ancestral state of ESD. In particular, assuming that there is an average trend toward increasing body size in lineages, the origins of XY systems are predicted to be accompanied by increases in male∶female body size ratio. In contrast, ZW systems are predicted to be accompanied by decreases in male∶female body size ratio. We find support for these predictions in the form of a marked association among vertebrates between sex-determining system and body size dimorphism in paired comparisons independent of shared phylogeny.