Sexes Are Polymorphisms: Describing and Understanding Sexual Diversity as Phenotypic Variation

Recent discussion has emphasized the need for more precise frameworks for describing sex. Sexes are a type of within-species phenotypic variation, and treating them as such offers a valuable perspective across levels of scientific inquiry. I propose that phenotypic polymorphism-the presence of distinct morphs within a species-provides an especially relevant framework for studying sexes as a form of variation. Although sexes are themselves a type of polymorphism, they are rarely described in these terms. Drawing on examples from diverse taxa, I demonstrate that the mechanisms underlying the production and adaptive maintenance of polymorphisms are the same as those that produce sexes. No genetic, developmental, or evolutionary principle distinguishes sexes from other polymorphisms. I further highlight cases in which polymorphisms play as central a role to reproduction as sexes, underscoring how a polymorphism-based approach can capture the diversity of reproductive strategies while improving conceptual frameworks. By treating sexes as a form of phenotypic variation, the polymorphism perspective clarifies that sexes are a population-level trait, and that existing tools for analyzing variation within and across species can be used for a more descriptive and empirical understanding of reproductive types and their evolution.