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Homosexuality Is a Spandrel 20 June 2007

Posted by Todd in Biology, Evolution, Homosexuality, Science.

There’s a great redux of the research about the differences between gay and straight people in last week’s New York Magazine, “The Science of Gaydar.” It seems that the differences are actually mounting the more we study these things, and the evidence is just piling on that homosexuality is biological. Of course, in my own experience, I already knew this, as I don’t remember ever not being gay (although as a child, I didn’t have a language or a way to understand how I was different, I just knew that during kissing tag, I really wanted to kiss Trent, not Jenny.)

Anyway, it’s a great article, if it makes the typical journalistic mistake of simplifying biological issues and misrepresenting others (although only slightly) to create a “controversy.” This article, however, does a good job of just guiding you through the evidence.

The one repeated argument in the article that really irritates me, which I have talked about here, is the idea that homosexuality is maladaptive evolutionarily. Since I’m only an armchair evolutionary biologist (I’m actually a sociologist), I could be misunderstanding things, but there are actually three measures of survivability in a trait: maladaptive, neutral (a “spandrel”), and adaptive. The idea that homosexuality is maladaptive relies on a narrow reading of Richard Dawkins notion of the selfish genes, that it is maladaptive trait because it creates a reproductive dead end for the individual genes. But evolution works population-wide in sexually reproducing species; that is, it’s about the spread of adaptable traits in the population. A maladaptive trait decreases survivability of the species, and in worst case scenarios lead to extinction. In any case, maladaptive traits are selected against. Yet homosexuality appears in all mammal species and most bird species. So there seems to be something else going on.

For a trait to be adaptive, it must increase survivability (and fitness, or differential reproduction) of the species as it spreads through the population. Although the hypotheses are intriguing explaining homosexuality as adaptive (since it appears universally present in human populations), I actually find the evidence to be lacking. It doesn’t appear, with the evidence I’ve seen, to enhance the survivability of the species.

That leaves homosexuality as a spandrel, or an accidental side effect. Because evolution is stochastic, arising out of multiple simultaneous causes, and because traits work in conjunction or interaction with all other traits, the mathematics of determining adaptability require seeing how traits arise from or interact with other traits. Spandrels are unintended consequences of evolution, traits that arise out of other traits. To me, it seems clear that sexual reproduction (with its massive advantage of mixing gene pools) produces the conditions underwhich some individuals can be born with their sexual desires “misdirected” (biologically speaking), but that the advantages of sexual reproduction far outweigh the disadvantages of having some individuals sexually unreproductive. In otherwords, homosexuality seems to me to be an unintended by-product of other mechanisms that are incredibly adaptive. Homosexuality is a spandrel. It is neither maladaptive (it has no negative effects on survivability of the species, or it would have been selected against thousands of years ago; nor is it particularly adaptive. It is, evolutionarily speaking, neutral.

I’m speaking here only of human homosexuality. In other species, I think it could be more easily argued that it is adaptive (for example, bonobos’ pansexuality; dolphins life-long same-sex partnerships). But I think in most species where it exists regularly, it is not maladaptive, or it would have been selected against.

At the end of the day, however, this question only concerns the origins of homosexuality in our evolutionary history. It tells us nothing of the value or meaning of homosexuality.



1. MoHoHawaii - 21 June 2007

I buy this argument (even though the conjectures about how homosexuality might be adaptive are so much fun that they ought to be true).

Whether it’s adaptive or not, we wouldn’t be people without homosexuality. Cultures would have evolved in very different ways without their gender atypical populations.

2. liseysmom - 21 June 2007

I’ve always viewed it a bit like being left-handed. People used to think it was evil and sinister, and society must change any child wanting to use their left hand! But now we realize, well… some people are just lefties.

3. gm - 27 August 2010

im pretty sure the word ‘sinistral’ or ‘sinister’ has been medically used to descrive left-handed people

4. bkyu - 27 August 2010

Incidentally, since I first posted this piece, I’ve had a chance to look closely at the hair-whorl item in the article linked above. That is terrible research, with no methodological rigor at all (i.e., the researcher sat on a ‘gay’ beach and counted whorls). It might turn out to be true, but this so-called “study” should be disregarded, or at least held in extreme provision, until verified with some real research.

Per gm’s point, I’m not sure if ‘sinister’ was ever used medically, but it is definitely used in latinate languages to denote left-handedness.

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