The adaptive maintenance of phenotypic polymorphism

Phenotypic polymorphisms have fascinated evolutionary biologists since the field’s inception, providing easily observable and quantifiable variation amenable to both empirical and theoretical study. However, a clear method for developing alternative hypotheses for the adaptive processes by which multiple morphs are maintained remains elusive. Here we review hypotheses for the maintenance of polymorphisms and propose a framework in which polymorphisms can be described by (i) a type of balancing selection and (ii) the ecological and social context through which balancing selection acts. We argue that a complete hypothesis for the maintenance of polymorphism should describe both variables, offering a simple method for generating testable alternatives. To demonstrate this, we explore the case of female-limited polymorphism, a class of polymorphisms with diverse explanations, yet little unifying theory across taxa. We show that, in most cases, social competition drives the maintenance of female-limited polymorphism. Applying this framework to both within-sex and species-wide polymorphism reveals distinctions and commonalities across disparate taxa, provides a clear structure for developing hypotheses, and reveals new paths for future research in the adaptive maintenance of polymorphism.