Evolution and homosexuality

Seed has an interview with Joan Roughgarden, somewhat controversial evolutionary biologist and author of Evolution’s Rainbow : Diversity, Gender, and Sexuality in Nature and People(amzn/b&n/abe/pwll). Here’s the short summary of her basic thesis:

Joan Roughgarden thinks Charles Darwin made a terrible mistake. Not about natural selection–she’s no bible-toting creationist—but about his other great theory of evolution: sexual selection. According to Roughgarden, sexual selection can’t explain the homosexuality that’s been documented in over 450 different vertebrate species. This means that same-sex sexuality—long disparaged as a quirk of human culture—is a normal, and probably necessary, fact of life. By neglecting all those gay animals, she says, Darwin misunderstood the basic nature of heterosexuality.

Roughgarden is an awkward case that provokes a difficult split in people’s opinions. She is 100% right that homosexuality is common and that its prevalence ought to be regarded more seriously as an indication of an interesting and enlightening phenomenon in evolution. However, she’s completely wrong in rejecting sexual selection: in rejecting a simplistically heterosexual view of nature she swings too far the other way, adopting a simplistically homosexual view instead of a messy, complex, and almost certainly more correct mixed view. She’s rather superficial in her treatment of Darwin. And most annoyingly, she has a bad habit of playing the transgender card and accusing her critics of disagreeing with her because of some LGBT bias.

Let’s consider Darwin and his view of sexual selection first. In chapter 4 of the Origin, he summarizes the case for sexual selection in this way:

Thus it is, as I believe, that when the males and females of any animal have the same general habits of life, but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection: that is, by individual males having had, in successive generations, some slight advantage over other males, in their weapons, means of defence, or charms, which they have transmitted to their male offspring alone. Yet, I would not wish to attribute all sexual differences to this agency: for we see in our domestic animals peculiarities arising and becoming attached to the male sex, which apparently have not been augmented through selection by man. The tuft of hair on the breast of the wild turkey-cock cannot be of any use, and it is doubtful whether it can be ornamental in the eyes of the female bird; indeed, had the tuft appeared under domestication, it would have been called a monstrosity.

Of course, in his typical fashion, he also recounts examples of the phenomenon. He also has the biases of his time, and he emphasizes that the product of this mechanism is that it makes males more beautiful; he does not discuss homosexuality, and I’m sure he would have recoiled in distaste (for cultural reasons) at the very idea. However, Darwin is also a pluralist. Note how careful he is to avoid attributing all sexual differences to sexual selection. That he is documenting and supporting one novel mechanism does not mean that he is arguing that it is the sole mechanism; if he were alive today and were able to overcome a certain Victorian squeamishness about the subject, there’s nothing in his writing to suggest that he wouldn’t welcome Roughgarden’s catalog of homosexual natural history as a worthwhile addition to the field. Finding other mechanisms does not negate his mechanism, however; we could have a productive argument about relative contributions of each.

For instance, I can think of quite a few hypothetical mechanisms that would drive the prevalence of homosexuality. They could every one be true, and just postulating or even providing conclusive evidence for a mechanism doesn’t mean all the other mechanisms are false. I’ll list a few ideas: none are contrary to evolutionary thinking.

Homosexuality is selectively neutral. This is an idea I favor, and Roughgarden herself notes that it is counterintuitive.

Roughgarden’s first order of business was proving that homosexuality isn’t a maladaptive trait. At first glance, this seems like a futile endeavor. Being gay clearly makes individuals less likely to pass on their genes, a major biological faux pas. From the perspective of evolution, homosexual behavior has always been a genetic dead end, something that has to be explained away.

(I really dislike that sweeping comment, “From the perspective of evolution…” I consider myself to be writing from the perspective of evolution, and I have long disagreed with that oversimplified view.)

Unless you are a very strict religious fundamentalist, which most biologists are not, it’s obvious that most sexual activity does not have a procreative purpose. We human beings, to pick an obvious example, will typically have between 0 and a dozen babies over our lifetimes; we have sex rather more often than that, and engage in a host of other sex related behaviors, from girl-watching and flirting and dating to masturbation and sexual relations other than vaginal intercourse. I guarantee you that virtually all the heterosexual teenagers watching submarine races at the Point are not interested in reproduction. Sure, it’s a direct genetic dead-end, but it’s a bit unfair to claim that biologists universally dismiss it, or that they all think teenagers petting represent a loss of reproductive potential.

The key point, though, is that heterosexuality is not a guarantee that an individual will have children, nor is homosexuality a guarantee that an individual will not. Many heterosexual couples elect not to have children, and many homosexuals elect to have them. This shouldn’t be a surprise; all it takes to start a baby is a few pokes and a spurt, and it really doesn’t take much effort to overcome an inclination for such a brief event. We are sex-obsessed animals, so redirecting an ejaculation to a particular orifice isn’t that astonishing.

Unfortunately, Roughgarden decides that homosexuality matters for all the wrong reasons.

But Roughgarden believes that biologists have it backwards. Given the pervasive presence of homosexuality throughout the animal kingdom, same-sex partnering must be an adaptive trait that’s been carefully preserved by natural selection. As Roughgarden points out, “a ‘common genetic disease’ is a contradiction in terms, and homosexuality is three to four orders of magnitude more common than true genetic diseases such as Huntington’s disease.”

Homosexuality, if it is a “disease”, is one that doesn’t kill you, that doesn’t diminish your reproductive ability (even if it does reduce your interest in a specific set of procreative sexual acts), and that may not reduce your effective rates of reproduction relative to heterosexuals. Roughgarden is an evolutionary biologist, so she should know full well that even deleterious alleles can rise in frequency in a population, and that arguing from frequency to a necessary phenotypic advantage is an error…especially when the “disease” is so innocuous. For all of her complaints about Darwin, Roughgarden is falling into the fallacious ultra-Darwinian trap here of assuming an advantage from simple existence.

Homosexuality promotes community bonding. This is the idea that Roughgarden favors: that some degree of homosexuality confers a direct advantage to individuals living in a community, because it facilitates bonding between same-sex individuals. It’s a tool for avoiding expensive, wasteful conflicts.

Go ahead and read the article; I think Roughgarden makes a good case that this has considerable utility in social groups, and she’s done some modeling that shows that this is, theoretically, a valid path to stable communities. There are objections that this requires group selection, which always puts an idea on shaky ground, but it seems to me that a willingness to settle problems erotically rather than in risky combat would also have possibilities of direct advantage to the individual.

Where I have reservations, though, is that it is a conclusion drawn from the premise that a common feature must have a genetic component. I don’t see that. Show me that homosexuality is genetically heritable, and then this will be a more likely hypothesis.

Still, I generally agree with the idea that sexual activity is about far more than just reproduction. I think you’d be very hard pressed to find any biologist who finds that in the least controversial.

Homosexuality is coupled to other advantageous traits. We could call this one the “boy, there sure are a lot of gay people on Broadway” hypothesis. It also assumes that homosexuality has a genetic component, and further, that there are other higher-level properties of the organism that are associated with it. In this case, it is assumed that while homosexuality may reduce fecundity in one way (diminished preference for the opposite sex), it is compensated by other correlated features that are linked to or induced by homosexuality, such as greater creativity or sociability…and success breeds greater opportunities for breeding, or better ability to care for any offspring.

This is a common explanation, but it compounds the weakness of the previous mechanism: now we not only need evidence for a genetic component to homosexuality, but we must also have a genetic component to something like “creativity” or “musical ability” or “extroversion”, and we need some kind of biological mechanism tying them all together. None of this is in evidence. In principle, though, it’s a perfectly reasonable explanation.

It’s not one I favor in practice. In theory, though, it’s compatible with ordinary evolutionary biology.

Homosexuality is a product of weak genetic specification. I like this one better. I don’t believe in the idea that we contain a lot of detailed, hardwired preferences in our brains—some, maybe, but there is a tendency in a lot of grossly reductionist science to blithely postulate genetic triggers for all kinds of semi-random activities. Even something as fundamental as an urge to have sex, while biological and nearly universal, isn’t reducible to something as simple as a few genes dedicated to the function. It’s a product of anatomy and neural and vascular physiology, it’s tied in to the structure of the hypothalamus, it’s a response to endocrine function, it’s basically an emergent property of a great many genes interacting during development. And even at that, it’s capable of being overridden by exposing someone to just the right weird doctrine at an impressionable age. The totality of the organism biases it in a particular way.

Similarly, I don’t think any predisposition towards a particular sex is simply defined, and any hardwiring is very broadly based and relatively easily redirected. We have brains that impel us to have sex (but not irresistibly), and our brains also focus that interest in a general way towards a particular sex (but again, not irresistibly). I tend to think of nature as dictating that we will have a preference by providing a neural substrate that supports the selection of a preference…but that that preference, whatever it may be, is shaped by experience and training and culture.

Brains are plastic. Whatever hardwiring is present is weaker than many people assume, and is easily sculpted in different and unspecified directions by developmental events. That doesn’t mean it can’t become strong: I think my sexual interests are rather strongly fixated on the female form now, even if I don’t think it’s because I have genes that somehow programmed in a fascination with breasts and hips and softer features.

Homosexuality is a byproduct. This is my favorite explanation, because ultimately it’s about development. Why do men have nipples? Because women need them. Both men and women have the same set of genes (more or less), and follow very similar developmental pathways, and the nipple represents a developmental constraint or byproduct: mutations that knock out the male nipple might also knock out the female nipple, so the structure is retained in both sexes. Male nipples are a byproduct of a function needed by the other sex.

We might also ask, why do some men love other men? The answer: because women need to love men. (We could also propose the complement, that lesbians exist because men need to love women.) If there are pathways that can predispose an individual to find males sexually attractive, the base structure is present in both men and women, and what we have are additional mechanisms to modulate the expression of the trait in men vs. women. Just as we guys have an echo of a female attribute in our nipples, why not assume that we also bear echoes of female mate preferences in our brains—echoes that can’t be expunged without also eliminating women’s desire for men (and oh, no, we mustn’t have that, I know)?

Some people are prone to argue that the byproduct explanation diminishes the importance of a phenomenon—Roughgarden seems to do that, herself—but really, it’s a mistaken notion. Pleiotropy and polygenic interactions are the rules in genetics, so everything is a byproduct of something else, and it doesn’t diminish their importance to the whole in the slightest. I rather like my nipples, and I’m sure women are as attached to their clitorises as I am to my penis—if we all carry some trace of a homoerotic impulse as a consequence of the common humanity of men and women, that’s no detriment.

It’s a weird thing. I actually like Roughgarden’s general ideas, and I think a lot of what she says has this nice, clean core of correctness…but then she goes off in some strange direction that ignores biology. For instance, here’s a perfect example:

Being gay or straight seems to be an intrinsic and implacable part of our identity. Roughgarden disagrees. “In our culture, we assume that there is a straight-gay binary, and that you are either one or the other. But if you look at vertebrates, that just isn’t the case. You will almost never find animals or primates that are exclusively gay. Other human cultures show the same thing.” Since Roughgarden believes that the hetero/homo distinction is a purely cultural creation, and not a fact of biology, she thinks it is only a matter of time before we return to the standard primate model. “I’m convinced that in 50 years, the gay-straight dichotomy will dissolve. I think it just takes too much social energy to preserve. All this campy, flamboyant behavior: It’s just such hard work.”

I think she’s right that sexuality is much more fluid than the usual gay/straight stereotype, that there’s a continuum, that all of us contain the potential for a range of sexual behaviors. But then those last few sentences…they don’t make sense. All sexual behaviors consume a substantial amount of an organism’s effort. Even heterosexuality is a major drain—just think of all the wasted calories burned by high heels alone, or all the dead boys determined to prove their machismo by doing stupid stunts. I also think it’s a mistake to pretend there is some standard primate, and that we’re currently drifting towards an imaginary mean. Human primates have been spending millennia slowly shifting around various diverse patterns of sexual behavior; I think she’s indulging in wishful thinking to believe that we’re going towards some mysterious low-cost gender paradise where human beings don’t sit around dreaming up ways to accentuate their sexuality.

Lastly, I’ve got to mention one thing about Roughgarden that annoys me and many other people: her assumption of victimhood.

“I think many scientists discount me because of who I am. They assume that I can’t be objective, that I’ve got some bias or hidden LGBT agenda.

I simply don’t think this is true, although I’m sure there are some few reactionary scientists whose knees jerk at the thought of a transgendered person. But look, my biases run the other way: I tend to be for greater gay rights, I am untroubled by documentation of homosexual behavior in nature, and I also stray far from Darwinian orthodoxy in science. If my opposition to some of her ideas has any basis in bias, it’s not against her “hidden LGBT agenda”; I find her doctrinaire panadaptationism more bothersome.


  1. #1 Sastra
    June 13, 2006

    A question from a non-scientist: could kin selection be another possible hypothetical mechanism driving homosexual prevalence?

    I can’t remember where I read it, but I think someone once proposed a kind of “benevolent uncle” theory — that having a persistent but recessive trait for homosexuality in a family could be advantageous for that family, and would therefore spread through generations and populations. The idea was that “uncles” without families of their own would be able to devote more time and energy to gathering wealth, becoming warriors, or gaining status in other ways — and this would benefit the reproductive fitness of their genetically-related kin.

    Would this make sense?

  2. #2 Kapitano
    June 14, 2006

    I suspect this relates to the question of ‘at what level’ evolution takes place – the genes mutating (a la Dawkins), the individual procreating (Darwin), or the species surviving (Gould).

    Making the big assumptions that homosexual orientation is objectively real, and is adaptive – that it is or was useful in some way – we can ask: on what level might it have been useful?

    Does it help a strand of DNA if the person it codes for is gay? I don’t see how it could be.

    Is it useful to an individual if they’re homosexual? I suppose it might be correllated with greater creativity or sensitivity – but IMO that’s just another pleasant but false stereotype.

    Might it offer an advantage to the species if some individuals don’t reproduce themselves, but assist in taking care of the children, helping out in group work, and maybe even making other people’s lives more enjoyable? Maybe.

  3. #3 Laura
    November 20, 2006

    (cross-posted from Raising Kaine blog)

    Recent research has indicated that at least in humans, homosexuality in males is correlated with birth order – that is, the farther down you are in birth order, the more likely you are to be gay.

    I wondered when I read this what possible adaptive value this could have, but after reading Richard Dawkins’ _The Selfish Gene_, I came up with a hypothesis.

    Dawkins argues that any “genetic” characteristic which tends to be more successful in reproducing itself will be favored. He uses this argument to great effect, concluding that protection/assistance to siblings (or other family members) is favored because of the shared genetic inheritance – if my brother survives, (approximately) half of my genes will also survive. I looked at this analysis and wondered if children with more adult resources devoted to them were more likely to survive – especially in the context of perhaps 1,000 or more years ago – obviously, a child with two parents was more likely to prosper than a child with one. Similarly, a child with a larger extended family might be expected to do better. What if some genetic trait, likely in the female and in pregnancy, changed the hormone balance of the fetus towards homosexuality for subsequent children? What does the childless person do with his/her resources? Mightn’t this result in more adults caring for the children of the oldest (or older) siblings and therefore positively influence survival and/or reproductive success of those children?

    I have absolutely no evidence relating to this hypothesis, it was just something I thought of because otherwise I cannot account for the persistence of homosexuality in a population. While I know many gay people with children, on average they have (many) fewer than heterosexual people – so how does the trait persist? It must somehow positively influence the survival of others with the shared gene(s).

    This is a testable hypothesis, given the evidence above. I hope somebody does the research to indicate whether particular family lines with higher than average numbers of homosexuals are more successful (in any species)!

    And to Denny, thanks for your comments. You say your gay friends have more children than your straight friends, but my experience is completely opposite. In fact, of my gay friends, biological reproduction is running at less than 10%.

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