This story is from The Pulse, a weekly health and science podcast.
In the months following the release of his ground-breaking theory of natural selection, “On The Origin of Species,” Charles Darwin lamented the existence of an animal that flamboyantly challenged his thesis.
“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” Darwin wrote in a letter to botanist Asa Gray.
The giant feathers lugged around by male peacocks called into question the most basic assumptions of his theory that, over the course of generations, species change and evolve in ways that favor their survival.
The tail was like a heavy, colorful target on their backs.
“If anything, these traits should decrease survivorship, not increase it,” said Michael Ryan, a professor of integrative biology at the University of Texas at Austin and a senior research associate at the Smithsonian Tropical Research Institute in Panama.
In “A Taste for the Beautiful: The Evolution of Attraction,” Ryan traces how similar contradictions in natural phenomena led Darwin to believe another parallel force must be at play, too – not just natural selection, but also sexual selection.
“Natural selection favors traits that enhance survivorship,” said Ryan. “Sexual selection favors traits that enhance an individual’s ability to acquire mates.”
For Darwin, this distinction explained why so many other males across the animal kingdom possess seemingly useless characteristics and abilities.
Generally speaking, because females often produce far fewer eggs than males produce sperm, so females have to be more choosy about their partners.
Over time, these traits increasingly conform to female preferences, sometimes even when they fly in the face of natural selection.