Genital coevolution is the process by which the reproductive structures of males and females evolve in response to each other over generations. It is often driven by sexual selection and reproductive competition. This dynamic is particularly evident in species with complex mating strategies or sexual conflict, in which the reproductive interests of males and females do not fully align. These opposing pressures can result in an evolutionary "arms race," with each sex developing adaptations to counter the other's strategies.
For example, in some insect and spider species, males evolve structures to increase their chances of successful mating, while females develop traits to regulate or avoid unwanted reproduction. A well-known case is genital coevolution in ducks. In certain species, like mallards, males have evolved long, corkscrew-shaped penises capable of rapid extension, often during nonconsensual mating attempts. In turn, female ducks have developed spiral-shaped vaginal tracts with twists and dead-end pockets that give them greater control over reproduction, making fertilization more difficult without cooperation.
Genitalia are among the fastest-evolving body parts in animals, often changing significantly within just a few hundred or thousand generations. This rapid evolution reflects the strong influence of factors such as sexual selection, mating systems, and reproductive competition on the development of these traits.
