Sex genes ‘multi-task’

Finding changes how we look at our sexuality‚ and how it makes us who we are

Why do we find certain things sexually attractive? Why might you be drawn to tall men‚ large breasts‚ strong chins‚ or big eyes? A graduate student at U of T may have found a piece of the complex puzzle that is human sexuality.

Ph.D. student Mark Fitzpatrick has shown that genes that underlie our sexual traits‚ both physical and behavioural‚ have effects that stretch beyond the sexual. Genes that affect body shape‚ fertility‚ libido‚ and a huge number of other sex characteristics‚ are also involved in totally unrelated processes‚ such as how well you fight disease. A woman’s large breasts‚ for example‚ may be partially caused by the same genes that give her a heightened immune system. Like many of our sexual traits‚ we still do not know why human breasts evolved—no animal has mammary glands like ours‚ and large breasts do not produce more milk than smaller ones. So why did large breasts evolve‚ and why do some men seem to prefer them?

Fitzpatrick seems to have found part of the answer: something that makes you sexually appealing may have evolved for totally different reasons‚ as a side effect of another function. Moreover‚ your sexual preferences may also have been shaped for reasons unrelated to sex.

This finding may help us enormously towards understanding our seemingly irrational desires. If the gene that gives a man a strong chin (something women are known to like) also makes him‚ say‚ longer–lived‚ then a woman’s desire may not be so irrational after all. Evolution may simply have shaped her to have desires that result in her having the healthiest children possible‚ because women in history who chose strong–chinned men had more children that lived to adulthood.

Evolution‚ through the mechanism of natural selection‚ is usually understood as “the survival of the fittest.” But “fittest” really just means whoever leaves behind the most offspring. An animal may be unusually fast or strong‚ but if it never gets to mate then its athletic genes will never spread.

So Darwin denoted another type of selection‚ which he called “sexual selection.” He thought that any trait that increased your chances of reproducing‚ say a bright bushy tail that females find attractive‚ will spread into the rest of the population. He said this would explain the tails of peacocks: although the weight of the tails prevents male peacocks from flying‚ and the bright feathers make them more vulnerable to predators‚ peacocks have evolved to look that way simply because females like it.

In almost every species with sexual preferences‚ it is the males who do the courting and the females who do the choosing. A female will only produce a small number of eggs in her lifetime‚ maybe even just a few hundred (human women make about 400). A female has to make an enormous investment when she chooses whom to mate with. She doesn’t have many eggs to spare‚ and pregnancy can be incredibly taxing.

Males on the other hand can produce an almost infinite number of sperm in their lives‚ and in most species their responsibility ends after coitus. So a male’s goal is to mate with as many females as he can‚ while a female’s aim is to mate only with the best males.

Modern biologists have found that Darwin was in fact right on the button—sexual preferences have a lot to do with how life on earth evolved. “It accounts for so much of the diversity we see” in living things‚ says U of T’s Dr. Locke Rowe.

Sexual selection is one of the most intensively studied topics in evolution. Scientists have catalogued an immense number of traits whose evolution was affected by sex‚ such as songs‚ colours‚ smells‚ shapes‚ sizes‚ and behaviours.

Moreover‚ biologists have shown that there is a genetic basis for many of these traits. Males have bushier tails because their genes create bushy tails. Some females are choosier than others because they have genes that make them so.

Scientists have assumed therefore that if you have a trait that the opposite sex is known to like‚ then you have it because the opposite sex likes it.

But geneticists have found that almost all genes have multiple functions. A gene that affects the colour of your hair will also affect another‚ totally unrelated aspect of your body‚ such as your heart rate. “Every geneticist will tell you that they believe every gene is [multi–functional]‚” says Fitzpatrick. “But none of the sex scientists were considering this.”

Fitzpatrick investigated this problem using the genetic information now available with the genome projects. A genome is simply all the genes in a particular species. The genomes of the fly‚ mouse‚ rat‚ and human have all been completed.

But just because you know how many genes are in an animal doesn’t mean that you know what each gene does—finding that out is much more difficult. Biologists know a fair bit about what the genes in flies do‚ but comparatively little about human genes.

So Fitzpatrick looked at fruit flies‚ an animal that has been intensively studied by both sex scientists and geneticists. He found that roughly 75 per cent of all fruit fly genes are multifunctional‚ and that the same percentage of the genes involved in sexual selection are multifunctional.

“These genes are multi–tasking‚” he said‚ “which is a really neat idea because when we were first trying to figure out how many genes humans should have‚ we expected them to have well over 100‚000 and fruit flies to have about 30‚000. But it turns out that flies have about 13‚000 and humans 30‚000. One way you can get an animal as complex as a human with fewer genes is if you multi–task those genes.

“It’s really amazing‚” he added.

So due to this multi–tasking Fitzpatrick argues that the genes that underlie our sexual traits may have evolved for reasons other than sex. A gene that gives a bird a bushy tail may have evolved because that gene also gives that bird a more efficient digestion system. Conversely‚ females may have evolved to like bushy tails because their preference would result in them having healthier young.

Fitzpatrick’s study has huge implications: we may like what we like for reasons we are completely oblivious to.

There are many examples of this in the animal kingdom. Male uakari monkeys have red faces‚ and the females are known to like redder faces. But it is now known that the same gene that makes their faces red also makes them resistant to malaria. So the red faces‚ and the female preference in turn‚ may have evolved primarily because of disease.

It is difficult to expand Fitzpatrick’s study to us because sexual selection is not as well studied in humans. Scientists have been able to find a few traits that people subconsciously prefer‚ such as symmetry‚ or pronounced chins on men. But the real way to prove something scientifically is with experiments‚ and it is ethically problematic to experiment on human sexual tastes.

“Say you take a really short guy who has been striking out every night at the bar. Now you put him on a couple of stilts and [see how women’s reaction changes]. Well‚ people don’t really go for this type of thing‚” said Fitzpatrick.

“It’s also very difficult to think about these problems in a clear way‚” adds Rowe.

But the underlying principle of Fitzpatrick’s study no doubt applies to humans as well—genetically speaking‚ we are extremely similar to other animals. We share 98.8 percent of our DNA with chimpanzees‚ and we even share 60 percent of our genes with flies. So his findings may have a huge impact on our understanding of sex.

And all he had to do was examine the information in an online database. “If you’ve got an idea it’s all there‚” he said. “You don’t have to be a scientist to get a hold of this [information]‚ you don’t need special passwords. The only limitations now are your ideas.”

“I think it’s really cool that an early graduate student‚ by surveying a public database was able to answer a pretty fundamental question‚” added Rowe.