The penis is an incredibly diverse and rapidly evolving structure, such that even in closely related species that otherwise differ very little in their morphology, penis form can be highly differentiated. Penises are also much more complex than their fundamental function — sperm transfer — would seem to require. The rapid divergent evolution of male structures is typically the signature of traits under sexual selection and the current evidence suggests the penis is no different in this regard. Despite the general agreement that sexual selection is the main driver of penis evolution, many questions about penis evolution remain unresolved. Furthermore, the penis might be an ideal characteristic on which to focus in the drive to link phenotype with genotype.
Across animals with internal fertilization, the penis and associated structures evolve rapidly and divergently.
Penises are also much more complicated than their simple primary function, sperm transfer, would seem to require, and can sometimes even damage the female during copulation.
Sexual selection, and particularly female choice, is largely responsible for the diversity of the penis, although natural selection might occasionally act on penis form.
Despite rapid evolution, in most instances, selection on the penis does not seem to be particularly strong.
Evidence suggests the human penis is sexually selected, with increased penis size preferred by women in some studies, perhaps explaining why the human penis is large compared with most great apes.
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We thank Michelle Taylor for the box 2 analysis and many colleagues for discussion of genital evolution.
Nature Reviews Urology thanks P. Brennan, Y. Kamimura, and the other anonymous reviewer(s) for their help with the peer review of this manuscript.
The authors declare no competing interests.
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Hosken, D.J., Archer, C.R., House, C.M. et al. Penis evolution across species: divergence and diversity. Nat Rev Urol 16, 98–106 (2019). https://doi.org/10.1038/s41585-018-0112-z