Sexual Selection

THE PUZZLE OF THE PEACOCK’S TAIL

“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!”2

Charles Darwin penned this line in a letter to a friend in 1860. It had been one year since he had published On the Origin of Species, and he was now immersed in a heated international debate over his theory of evolution. He was only half joking about feeling sick at the sight of a peacock’s tail. Those feathers, beautiful though they were, seemed to pose a direct threat to the theory he had spent decades developing and on which he had staked his professional reputation.

Darwin’s theory of evolution by natural selection held that features that helped an animal survive would get passed down to subsequent generations, and features that didn’t help with survival would gradually get weeded out of existence. The peacock’s tail was gaudy and enormous, reaching heights of up to five feet. Such a tail would only weigh the bird down and make it harder to escape predators, so why would it have evolved?

Darwin didn’t consider himself a quick or highly analytical thinker. His memory was poor, and he couldn’t follow long mathematical arguments. Nevertheless, Darwin felt that he made up for those shortcomings with a crucial strength: his urge to figure out how reality worked. Ever since he could remember, he had been driven to make sense of the world around him. He followed what he called a “golden rule” to fight against motivated reasoning:

. . . whenever a published fact, a new observation or thought came across me, which was opposed to my general results, to make a memorandum of it without fail and at once; for I had found by experience that such facts and thoughts were far more apt to escape from the memory than favourable ones.3

Therefore, even though the peacock’s tail made him anxious, Darwin couldn’t stop puzzling over it. How could it possibly be consistent with natural selection?

Within a few years, he had figured out the beginnings of a compelling answer. Natural selection wasn’t the only force shaping evolution. Sexual selection was just as important. Some features, like a big, gaudy tail, were especially attractive to members of the opposite sex. Those features could therefore become common in a species over time, because even though they might hurt the animal’s odds of surviving, they helped the animal’s odds of reproducing. The latter could outweigh the former.

Ironically, the feathers that made Darwin sick with worry only wound up making his theory stronger in the end. It wasn’t the first time. As Darwin was researching On the Origin of Species, he followed up on every observation he could find that contradicted his theory, puzzling over them and revising his theory in response. By the time he was done, his account of natural selection was so solid and well evidenced that despite the fierce resistance it initially sparked, within a decade the majority of the scientific establishment had become convinced Darwin was right.