The Evolution of Beauty

The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World—and Us is a 2017 work of non-fiction by American ornithologist and museum curator Richard O. Prum. Based on Prum’s lifelong fascination with the elaborate beauty of certain tropical birds, The Evolution of Beauty argues that Darwin’s overlooked theory of sexual selection may have as much explanatory power as the more-favored theory of natural selection. The theory of sexual selection holds that the aesthetic preferences of females determine the selection of inherited traits. Prum argues that this theory has been under-valued due to deeply ingrained patriarchal assumptions in the scientific establishment. The Evolution of Beauty was a finalist for the 2018 Pulitzer Prize.

Prum begins his account with a problem. In his long career as an ornithologist, he has seen many examples of species traits that cannot be satisfactorily explained by the theory of natural selection. This theory—dominant in the field of evolutionary biology—holds that traits that encourage survival are the traits that are passed on and develop from one generation to the next. However, a peacock’s tail does not seem to encourage survival. It may even be a hindrance.

To solve this problem, Prum returns to Darwin. He finds that alongside the theory of natural selection, the great Victorian scientist also developed a theory of sexual selection, arguing that the development of traits in a species can also be determined by “the taste for the beautiful,” particularly the tastes of females.



However, most biologists in Darwin’s day—and since—have rejected this theory. Prum suggests that in part, Darwin’s contemporaries preferred to elevate natural selection as the sole force behind evolution because it gave them a more powerful counterweight to the religious narrative in which God was the sole force behind creation. Other biologists, Prum argues, were simply too misogynistic to accept the role of female choice in the development of human beings. One early reviewer of Darwin’s Descent of Man (in which the theory of sexual selection was proposed) rejected the idea that “the instability of vicious feminine caprice” could possibly play a role in something as noble as human evolution. Instead, Darwin’s contemporaries developed an alternative account, in which male ornaments such as a peacock’s tail are understood by females as markers of greater “fitness.”

To argue for a restoration of the theory of sexual selection, Prum turns to his specialist subject, tropical birds. He lovingly describes the dazzling plumage of the male Great Argus Pheasant—a cone of feathers decorated with golden spheres—and the moonwalking mating dance of the Red-capped Manakin. He describes the elaborate artistic productions of male Bowerbirds, who build decorated grottos for females to inspect. Prum points out that Bowerbirds mastered the trick of perspective long before human artists discovered it during the Renaissance.

A particular specialist in Manakins, Prum is especially interested in the Club-winged Manakin, whose wings produce a singing noise to attract mates. Prum notes that this is a “decadent” trait: in selecting mates with “singing” wings, females choose partners who are less able to fly.



Prum argues that natural selection simply cannot explain these traits. Instead, Prum advances what he calls the “Beauty Happens” hypothesis, first outlined by Ronald Fisher. A female’s arbitrary preference for a particular trait decides her mate-selection, and their offspring inherit both the trait and the preference, resulting in a “runaway co-evolution,” in which both trait and preference become increasingly pronounced in each generation. Evolution does not produce only “fitness,” but also “arbitrary and useless beauty.”

In some cases, selection for beauty may have unexpected benefits. Prum even argues that feathers may have initially evolved not for flight but as decoration. However, just as often, traits selected for beauty may detract from fitness. Prum argues that this problem is neutralized by the advantage of “aesthetic” courtship. This form of courtship, Prum argues, creates an environment in which females can exercise sexual autonomy and enjoy safety from male aggression.

The alternative is sexual conflict, in which male and female traits evolve as females seek to avoid male sexual control. Prum advances the examples of ducks, showing how duck vaginas have evolved to protect female ducks against forced sex.



In light of these two alternative evolutionary paths, Prum considers human evolution. He argues that the development of human social behavior, and also developments in male human physical characteristics, suggest that humans have evolved through a female-choice process, and not a sexual conflict one. Prum argues that this hypothesis can explain a wide range of human traits that escape the explanatory range of natural selection, including: female orgasms, homosexuality, humans’ capacity for sex outside the window of female fertility, the gradual reduction over evolutionary time in the relative physical strength of male humans, and the development of monogamy and paternal care as widespread social structures. Aside from the constant sexual interest of bonobos, no other ape displays any of these traits.

Prum ultimately concludes that sex for pleasure is humans’ courtship rite, our equivalent of the Bowerbird’s bower. He closes by advocating that we embrace our evolutionary relationship with aesthetic beauty and female autonomy.