A Most Private Evolution: Dumb Designs for Sex: Evolutionary Biology Walks on the Weird Side

By Milius, Susan | Science News, February 14, 2009 | Go to article overview

A Most Private Evolution: Dumb Designs for Sex: Evolutionary Biology Walks on the Weird Side


Milius, Susan, Science News


Maybe female seed beetles have their own what-the-bleep exclamation. Even for insects, it's difficult to imagine any other reaction to a male Callosobruchus maculatus beetle's sex organ, which has spikes.

"It jumps to mind as something quite dumb," says Goran Arnqvist, an evolutionary biologist at Uppsala University in Sweden, who for much of the past eight years has studied seed beetle sex.

Male beetles of several Callosobruchus species have sharp edges on their sperm-delivery organs. The females' ducts grow a bit of extra toughening but not enough to make sex safe from the risk of injury. After many tests, Arnqvist has concluded that the genital excesses aren't good for the species as a whole. These seed beetles would have less-damaging sex--and would produce more babies--if males lost their edges.

Discussions of evolution often glorify the beautifully apt forms: orchids with nectar recesses just the right length for the tonguelike structure of a certain moth, or harmless butterflies with the same wing colors as a poisonous neighbor. Yet the most dramatic examples of the power of evolutionary theory may come from the strange and ugly stuff--biology that seems too dumb to have been designed.

Trying to understand counterintuitive sexual parts and habits follows in the best of scientific traditions. As Charles Darwin worked on evolution, he pondered male phenomena that looked useless, or even harmful, for surviving. Outsized horns on male beetles puzzled him, as did male birds with gorgeous plumage.

Out of this consternation came his insight into a process he called sexual selection, which he distinguished from natural selection. There may be survival of the fittest, but there's also survival of the sexiest.

Today the sex-related selection process doesn't get much attention outside scientific circles, but it's a powerful tool for making sense of downright peculiar stuff. Arnqvist and other biologists are expanding Darwin's framework, exploring the counterintuitive aspects of sex from flirtation to family life. And theorists are discussing female behavior that Darwin never recognized, or perhaps just didn't care to discuss in print.

Not-so-natural selection

When Darwin first put his full idea of natural selection into print, he knew it wasn't enough. In 1859, he argued in On the Origin of Species that organisms best adapted to their environment survive in greater numbers and leave more offspring than do their less fit neighbors. Thus more suitable traits gradually replace clunkier versions.

Yet antlers on stags and tails on peacocks could hardly be adaptations to the environment. Both antlers and tails may be so familiar that it takes a minute to summon a sense of their absurdity. They're huge. They must drain energy to produce. There's no way they improve agility in locomotion or foraging.

"The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" Darwin wrote in a letter to the botanist Asa Gray, albeit in a whimsical paragraph. Nauseated or not, Darwin was willing to step beyond survival of the fittest.

He devoted a few pages in the Origin to introduce sexual selection as a sort of wild-oats younger brother of natural selection. Sexual selection, as Darwin formulated it in the sixth edition of Origin, depends "not on the struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for possession of the other."

Antlers evolved as stag-on-stag weaponry for fights over a female, he argued. Males also compete in contests "of a more peaceful character," he wrote. Extravagant plumage, singing and what he called "strange antics," such as bird acrobatic displays, bedazzle a female into choosing one male over his rivals.

What's good for bedazzling can be bad for survival, of course.

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