Yesterday, PZ linked to a short list of leading evolutionary explanations for homosexuality. On this subject, PZ is an ardent non-adaptationist:
There are really just two classes of explanation [for homosexuality], the adaptationist strategy of trying to find a necessary enhancement to fitness, and the correct strategy of recognizing that not all attributes of an individual organism are going to be optimal for that individual’s reproduction, so don’t even try. Love isn’t hardwired by biology, and it can go in all kinds of different directions.
PZ’s point is well taken, and if humans were the only species to reliably engage in same-sex sexual behavior, his spandrelesque theory would undoubtedly be the only valid explanation. But I think the “gayness is a side-effect of the stochastic nature of love” concept runs into some trouble when confronted with the long list of other animals that also engage in homosexual behavior. Invoking the mystery of love doesn’t exactly explain gay mountain rams, or lesbian oystercatchers.
Last year, I profiled the work of Joan Roughgarden, an interesting if controversial evolutionary biologist at Stanford. She is best known for her book Evolution’s Rainbow, which uses the widespread nature of homosexuality in the animal kingdom to critique Darwinian social selection:
To make her case, Roughgarden filled the text with a staggering collection of animal perversities, from the penises of female spotted hyenas to the mènage à trois tactics of bluegill sunfish. As Roughgarden put it, “What’s coming out [in the past 10-15 years] is to the rest of the species what the Kinsey Report was to humans.”
According to Roughgarden, classic sexual selection can’t account for these strange carnal habits. After all, Darwin imagined sex as a relatively straightforward transaction. Males compete for females. Evolutionary success is defined by the quantity of offspring. Thus, any distractions from the business of making babies–distractions like homosexuality, masturbation, etc.–are precious wastes of fluids. You’d think by now, several hundred million years after sex began, nature would have done away with such inefficiencies, and males and females would only act to maximize rates of sexual reproduction.
But the opposite has happened. Instead of copulation becoming more functional and straightforward, it has only gotten weirder as species have evolved–more sodomy and other frivolous pleasures that are useless for propagating the species. The more socially complex the animal, the more sexual “deviance” it exhibits. Look at primates: Compared to our closest relatives, contemporary, Westernized Homo sapiens are the staid ones.
In other words, if homosexuality is an “accident,” then it’s an accident that has been endlessly repeated (and tolerated) by natural selection. Gayness has occurred in all sorts of distant phyla, from birds to primates. Given this ubiquity, I think it makes sense to look for adaptive explanations.
Here is Roughgarden’s explanation:
So how might homosexuality be good for us? Any concept of sexual selection that emphasizes the selfish propagation of genes and sperm won’t be able to account for the abundance of non-heterosexual sex. All those gay penguins and persons will remain inexplicable. However, if one looks at homosexuality from the perspective of a community, one can begin to see why nature might foster a variety of sexual interactions.
According to Roughgarden, gayness is a necessary side effect of getting along. Homosexuality evolved in tandem with vertebrate societies, in which a motley group of individuals has to either live together or die alone. In fact, Roughgarden even argues that homosexuality is a defining feature of advanced animal communities, which require communal bonds in order to function. “The more complex and sophisticated a social system is,” she writes, “the more likely it is to have homosexuality intermixed with heterosexuality.”