The evolution of animal genitalia: distinguishing between hypotheses by single species studies

Rapid evolution of genitalia is one of the most general patterns of morphological diversification in animals. Despite its generality, the causes of this evolutionary trend remain obscure. Several alternative hypotheses have been suggested to account for the evolution of genitalia (notably the lock-and-key, pleiotropism, and sexual selection hypotheses). Here, I argue that thorough intraspecific studies are the key to gaining insight into the patterns and processes of genitalic evolution. Critical assumptions and predictions that may be used to distinguish between the different hypotheses are identified and discussed. However, current knowledge of selection on genitalia, or even of the degree of phenotypic and genotypic variability of genital morphology, is highly limited, allowing only a very tentative assessment of the various hypotheses. In-depth single species studies of current patterns and processes of selection on genitalia are badly needed, and a single species research program is briefly outlined.