Hypotheses for the Adaptive Maintenance of Phenotypic Polymorphisms

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, identifying the selective mechanisms by which these polymorphisms are maintained can be challenging, in part because alternative hypotheses can be difficult to distinguish. Here we review hypotheses for the maintenance of phenotypic polymorphisms, integrating them into a simple framework in which hypotheses can be described by both of two dimensions: (1) a type of maintenance selection that favors a balanced polymorphism; and (2) a selective context (e.g., ecological, social). Taken together, a phenotypic polymorphism persists through a type of maintenance selection which acts through a selective context. Within each dimension, we provide a path for categorization of alternative hypotheses and their respective predictions. To demonstrate, we explore the case of female-limited polymorphism, a class of polymorphisms with diverse explanations, yet little unifying theory across taxa. We suggest that, in most cases, negative frequency-dependence and social competition drive the maintenance of female-limited polymorphism. Applying this framework to both within-sex and species-wide polymorphism reveals distinctions and commonalities across disparate taxa and provides a clear structure for developing and testing hypotheses.