Darwin got sexual selection backwards, research suggests

Charles Darwin was a careful scientist. In the middle of the 19th century, while he was collecting evidence for his theory that species evolve by natural selection, he noticed it didn’t explain the fancy tails of male peacocks, the antlers paraded by male deer, or why some the males of some species are far larger then their female counterparts.

For these quirks, Darwin proposed a secondary theory: the sexual selection of traits that increase an animal’s chance of securing a mate and reproducing. He carefully distinguished between weapons such as horns, spurs, fangs and sheer size that are used to subdue competing rivals, and ornaments that are aimed at charming the opposite sex.

Darwin thought that sexually selected traits could be explained by uneven sex ratios – when there are more males than females in a population, or vice versa. He reasoned that a male with fewer available females would have to work harder to secure one of them as a mate, and that this competition would drive sexual selection.

In a new study, my colleagues and I have confirmed a link between sexual selection and sex ratios, as Darwin suspected. But surprisingly, our findings suggest Darwin got things the wrong way round. We found that sexual selection is most pronounced not when potential mates are scarce, but when they’re abundant – and this means looking again at the selection pressures at play in animal populations that feature uneven sex ratios.

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