Newly married couple Richard Dowling and Cormac Gollogly kiss after the first same-sex marriage takes place on November 17, 2015, in Clonmel, Ireland. (Photo: Charles McQuillan/Getty Images)

Why are there homosexuals? At first glance, it's a silly, even offensive question—some people are gay, and you'll just have to deal with it. But for biologists, it's a real puzzle. How, after all, could evolution create people genetically predisposed not to breed (at least, not without third-party assistance)? Perhaps, according to a new perspective, nature just wanted a little variety.

"Same-sex sexual behavior is ubiquitous across the animal kingdom, and has been cataloged in hundreds of animal species in ways that range from same-sex courtship and copulation to long-term pair bonding and parenting," writes MIT postdoctoral researcher Brian Skinner. Homosexuality's ubiquity, furthermore, suggests that it has some evolutionary advantage. The puzzle is this: Historically, same-sex couples couldn't reproduce on their own. Without that, there's no way to pass one's traits, including homosexuality, on to the next generation, so same-sex attraction should have died out.

In fact, nature does need variety, in the form of genetic mutations, without which species can't change, organisms can't get stronger, and evolution doesn't go anywhere.

Actually, biology has an answer to that question, called kin or group selection. In one version, homosexuality actually does provide a genetic advantage, in the form of, say, a gay uncle who helps with the kids. In particular, he helps them survive long enough to pass their genes on to the next generation, and by virtue of an uncle's genetic connection to his nieces and nephews, some of their kids might be genetically more likely to be gay too.

Kin/group selection still assumes, however, that homosexuality has an inherent evolutionary benefit, and here's where Skinner's proposal takes off in an entirely different direction. What if, instead of same-sex relations bringing about some evolutionary benefit, it was the other way around? What if nature needed a little variety, and the result was homosexuality?

In fact, nature does need variety, in the form of genetic mutations, without which species can't change, organisms can't get stronger, and evolution doesn't go anywhere. That's Skinner's first idea: Nature is prone to genetic variety.

His second idea is the real departure: Attraction isn't based on genetic sex, he proposes, but rather on traits that are typically, but not necessarily, associated with men or women. Then, Skinner works out, same-sex attraction is inevitable.

Here's an example: Suppose men tend to have beards and also tend to prefer partners without beards, with some exceptions (nature's need for variety ensures there are always exceptions). Same thing for women, in reverse—they tend not to have beards and tend to prefer partners with them. Now, because attraction is about beards, not males and females, some same-sex pairs will form—say, two men who prefer partners with beards, or two women who prefer partners without beards.

Skinner's ideas aren't anywhere near conclusive—they form what scientists call a toy model, one meant to elucidate the concepts but not ready for rigorous experimental tests. Yet even Skinner's toy raises a point often overlooked in the scientific discussion of homosexuality: There doesn't need to be some grand evolutionary reward to explain the existence of gay people. Some people just are. .

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