When Sex Sucks

Are you hoping that breeding with somebody with "good genes" will help you have a child who is somehow better then you are? So are a lot of creatures. Unfortunately, it looks like some good genes can't be passed on. In fact, the very genes that make your mate seem spicy might actually hinder your kids' success in the mating game later on.

A couple of Canadian biologists at Queens University in Ontario published a study in PLoS Biology (a Public Library of Science journal) a couple of weeks ago that suggests women who pick mates "fitter" than themselves have very little chance of passing that fitness on to their daughters. Same goes for men who mate with women fitter than themselves: sons born from such a union are actually less fit than sons born to low-fitness ladies. In the genetic war between the sexes, genes that are good for one sex aren't necessarily good for the opposite-sex children who inherit them.

Biologists Alison Pischedda and Adam K. Chippindale discovered this by forcing a bunch of fruit flies to have sex in various combinations of fit and unfit. Fitness wasn't measured in sexiness or success in fly politics -- the scientists measured it by how many offspring a fly could have. In other words, fitness equals how much influence a fly will have over the gene pool.

When flies choose mates, they're engaging in a gene crapshoot called sexual selection, the Darwinian process by which the quest for perfect mates influences evolution. Conventional wisdom holds that sexual selection is usually good for a species: it creates babies that are stronger, prettier, fitter. The idea is that sexual creatures tend to be attracted to mates who are fit in one way or another. Maybe that mate is appealing because she's particularly good at surviving in the desert with a bunch of drugged-out hippies, or maybe he's shaped so nicely that he's obviously healthy. If the possible mate is human, it's possible she'll come across as attractive because she's a good problem-solver or skilled at telling jokes. All of these characteristics mean that the creature in question has a higher probability of surviving and spreading his or her genes far and wide by creating fit babies. So sexual selection is the process of picking a mate who will help you in the quest for genetic domination.

But Pischedda and Chippindale wondered if seeking out the perfect mate could ever be detrimental to offspring. The answer is yes.

It turns out that certain fitness genes shared by male and female flies on the X chromosome express themselves differently depending on sex. So a gene on a male's X chromosome might make him an incredibly prolific father, but that same gene expressed in his daughter would prevent her from reproducing in large numbers. Because males only pass along their Y chromosome to male babies, they never pass along their beneficial X genes to sons either.

Why would genes behave like this if they are selfish, as pop geneticist Richard Dawkins puts it? The answer, Pischedda and Chippindale speculate, is that these genes are acting selflessly.

They're keeping the population diverse. Imagine if fit parents bred only fit children. Translated into human terms, let's assume that Britney Spears and K-Fed are fit parents because they keep shooting out babies. If their children inherited the fitness gene from Britney or K-Fed, they would also spawn lots of children. And so would those children. Pretty soon, you'd have a nation of aimless pop stars whose talents lie mostly in the area of gyration.

By cutting off fitness after one generation, we're guaranteed a population whose genes come from a wide variety of sources. That's why we have nerdy kids, sporty kids, and freaky kids, as well as eroticized teenyboppers who sing. If Pischedda and Chippindale are right, their experiment could undermine the idea that sexual selection is purely a selfish process. Sometimes genes work for the good of the species rather than the good of individuals.