Why Is Evolutionary Innovation Obsessed with Male Genitals

If many innovations originate through sexual selection, we would expect most micro-innovations that distinguish one species from another to be sexual ornaments. This contradicts some traditional views of how species split apart, but, surprisingly, this is pretty much what biologists see. The vast majority of species-defining innovations seem inconsequential for survival. Francis Bacon, father of the scientific method, disparaged the seemingly pointless variety of plants and animals, calling them "the mere Sport of Nature." Darwin was equally perplexed, often wondering why there was so much variety but so little real novelty. If innovations spread through populations because of their survival benefits, why do so few innovations show the survival improvements associated with major innovations and adaptive radiations?

One clue comes from the criteria that taxonomists use to classify specimens into species. Male sexual ornaments and male genitals are the most useful traits for distinguishing most animal species from closely related members of the same genus. If you can't tell whether a beetle is one species or another, look at its color pattern, its weaponry, and its genitals. In his book Sexual Selection and Animal Genitalia, William Eberhard emphasized that male genitals are often the first things to diverge when one species splits off from another. Evolutionary innovation seems focused on the details of penis shape. In Eberhard's view, this is because female choice focuses on the details of penis shape, and female choice apparently drives most micro-innovation. In plant taxonomy, the analogous sexually selected traits are the flowers, and they are often most useful in making species identifications. It is often harder to tell what species a female animal is, because the appearance of females diverges much less between species. Bird watchers know this: given a female, you can often only identify the genus, but given a male, you can zero in on the exact species.

The micro-innovations that distinguish species often evolve through sexual selection, as sexual ornaments (or genitals) shaped by mate choice. At one level, this fact simply restates the modern definition of a biological species: a reproductively isolated group of individuals. The commonest kinds of traits that distinguish species must be traits that can work as sexual isolators to keep one group from interbreeding with other groups. Sexual choice is a very efficient sexual isolator for keeping species distinct. As the biologist Hugh Paterson pointed out in the 1970s, species are basically consensual systems of mate choice. The result is that human taxonomists end up using the same traits to distinguish species that species members themselves use: sexual ornaments. This is why most micro-innovations are concentrated in genitals, ornaments, and courtship behaviors.

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