"Survival of the Fittest." Although Darwin didn't come up with that phrase, it has come to define his theory of evolution: that traits that improve a species' odds of staying alive are specifically favored to be passed on. (Consider the Galápagos finches, whose beaks have changed over time to crack open different kinds of seeds.) But in The Evolution of Beauty, ornithologist Richard O. Prum argues that superficial desires play a role as well.
Take, for instance, the male club-winged manakin, a bird whose ornate feathers have evolved to produce a "song" while they fly (thanks to a series of bumps that vibrate in the wind) that attracts females. Far from making them fitter, these feathers — and the bigger bones required to support them — actually slow the manakins down, suggesting that, as Prum concludes, animal beauty standards "can be as irrational, unpredictable and dynamic" as our own.