The gay gene & other considerations

Jonah over at The Frontal Cortex has some commentary up on the gay sheep story. A reader pointed out that this controversy started off with some wild claims made by PETA. Nevertheless (more at Andrew Sullivan's), no matter the details of the claim, there are a few points I'd like to pick up on....

Jonah says:

So here's my hypothesis: if you select against homosexuality in a biological community, you will also be selecting against our instinct for solidarity. The same genes that give rise to gayness might also give rise to cooperation. When scientists create a population of all heterosexual sheep - this would be a boon to ranchers, since a high percentage of male sheep are gay - they will find that their sheep are now more violent as well.

Assume that Jonah is correct and that some of the loci with a range of alleles which allow for human predisposition to sociality also result in small number of obligate homosexuals. It seems that the evolutionary logic demands that modifier genes emerge which mask this fitness killing trait. What Roughgarden (and Jonah) would have us believe is that there are structural-biophysical reasons why this can not happen, because if it could, it should. I can believe and accept some level of homosexual behavior emerging facultatively (e.g., bonobo females) as part of conditional behavioral strategies (some human populations have also engaged in facultative ritual homosexuality). But that does not entail obligate homosexuality as a necessity. The evolution of selfish genetic elements shows us that genomic dynamics can result in deleterious consequences for individuals, and these consequences can persist in a metastable form because of the long term ubiquity of various parasitic elements. Not only does this apply intragenomically, but there are also parasites which modify human behavior. The paradox of worker sterility in eusocial insects forced us to come to grips with inclusive fitness and Hamilton's Rule, but the mathematics doesn't pan out for obligate homosexuality in such a fashion. We have a range of somewhat baroque and peculiar choices before us to explain the biological root of this behavior, and I don't place Roughgarden's hypothesis very high on the chain of parsimony. So when Jonah says:

Selecting against homosexuality isn't just immoral and unethical: it's also just a terrible idea, driven by bad biology.

I say hold up brother! Let's not put the cart before the horse here. Roughgarden's thesis is speculative, to be charitable, so the inference that more obligate homosexuality leads toward more sociality is itself stretching the foundations of established biology. Fundamentally the evolutionary logic is pretty uncharted here. I suspect that the evolutionary rationale won't be adaptive at all, but some sort of selfish element, whether it be intragenomic, or extra-genomic (e.g., a parasite). I also wouldn't be surprised if medicine does get advanced enough that the proximate biological processes which underpin obligate homosexuality are nailed down (and many other psychological predispositions). As for morality, I think it is all about choice, and let's get real, in a world where white parents make sure that their kids don't go to school with blacks (no matter the test scores of the school!) you'll not be hearing many of the silent screams of any fetuses diagnosed with a high likelihood of obligate homosexuality.

i-7600f5df62a814176fff7aa7137f9eb0-homogay.jpgBut let's take the thesis that homosexuality emerges as a byproduct for selection at numerous loci for particular traits like sociality seriously. In other words, Jonah is proposing that sexual orientation is due to a quantitative trait which results in a threshold characteristic, obligate attraction to the same sex. Various mental "illneses" might be well defined in this manner, "autism" might be the "tail" of the quantitative distribution, just as "retardation" describes all of those below an IQ of 70, even though a great number are part of the normal range of human variation (i.e., the "gifted" above 130 are also part of the normal range).1 Now, my point about modifying alleles is that obligate homosexuality, the extreme end of the tail, reduces fitness a great deal (by definition, since obligate homosexuals among humans can not reproduce without medical technology) and it seems that even if balancing selection is generating this range it seems that soon enough other alleles would arise to suppress the fitness killing tendencies (obligate homosexuality is not evolutionarily neutral). Of course, one could say the same of autism & Asperger's in regards to its tendency toward fitness reduction, though I do not think it is nearly as extreme, and I tend toward acceptance of mixed-strategies in human behavior as the norm in regards to personality. In contrast, I do not accept the possibility of mixed sexual reproductive strategies, seeing as how we are rather constrained functionally in this regard. But as I said, let us move forward with the hypothesis that homosexuality arises from the quantitative range of the prosocial tendencies. Jonah suggests that aborting gay fetuses would be abominal insofar as it would reduce the alleles for sociality. This is truncation selection, the problem with Jonah's concern is that only a miniscule proportion of the population would be subject to selection (those who exhibit a tendency beyond the 2% threshold which drives them to obligate homosexuality). In other words, the change in allele frequencies would be rather trivial on a one generation basis, and, if one supposed that optimal fitness was in the middle of the distribution then one can infer that a metastable situation would emerge. An x number of homosexual fetuses would be aborted per generation, but the higher fitness of those with prosocial tendencies would result in the natural generation of more homosexual fetuses in generation t + 1 . Now, when you consider that the fitness of obligate homosexuals is rather low to begin with, what this is doing is reducing the life expectancy of that particular vessel of the germ line from 70 years to a few months. Selection is a very minor force (if at all) unless there is a major bias toward false positives (i.e., those with a small likelihood of being homosexual, just in case, being aborted) also being eliminated from the gene pool.2

As I said, I'm skeptical of this adaptive narrative because quantitative traits tend to exhibit moderate fitness implications over the long term (to maintain variation it seems likely that mutation and various forms of balancing selection must be at work unless the phenotype is utterly neutral). If the fitness implication is powerful (i.e., trait x implies strong increment or decrement of fitness) then directional selection quickly homogenizes the polymorphism and the variance needed for a quantitative distribution disappears. James F. Crow has offered the hope that we could perhaps study humans like Drosophila. Alas, that isn't so, but homosexuality is an interesting biological characteristic and I wish the phobic and friendly sides would decouple their normative prescriptions from the scientific data and let it be....

1 - A significant fraction of those with sub-70 IQs have diseases such as Down Syndrome which have wide ranging effects. But some simply have low IQs, just as some humans are rather short without being subject to dwarfism.

2 - What I'm saying here is that the abortion of homosexual fetuses wouldn't remove much of the genetic variation which results in homosexuality. The implication is that individuals who were non-homosexual, but high prosocial, would remain, and a Hobbesian future is not likely.

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"It seems that the evolutionary logic demands that modifier genes emerge which mask this fitness killing trait"

Either that or the genetic effect is highly regulated by environmental conditions- maybe even more parsimonious..

Also, there is no connection between homnosexuality and sociality. This is an attempt by a few homosexual scientists to rationalize their own lifestyles using science.. Always a bad practice.

Schizophrenia and bi-polar disorder come to mind as model traits for homosexuality. There are clearly strong negative fitness effects, yet these are highly heritable (broad sense). In addition to all the normal candidates, mutation selection balance has been suggested to explain these traits. I wonder if that's a feasible model to explain homosexuality as well.

By Rikurzhen (not verified) on 11 Jan 2007 #permalink

Either that or the genetic effect is highly regulated by environmental conditions- maybe even more parsimonious..

can you elaborate? are you talking about environmental stochasticity resulting in differential fitness for differential alleles and the maintenance of polymorphism?

Schizophrenia and bi-polar disorder come to mind as model traits for homosexuality.

yes. i thought of that. though schizophrenia doesn't always manifest until later in life.

In addition to all the normal candidates, mutation selection balance has been suggested to explain these traits.

you mean a model where these quantitative traits are always weighed down by the background genetic load induced by loss of function mutations? in this model homosexuality could be loss of function for the tendencies which result in proheterosexual behavior/orientation. a good analogy might be with infertility though, obviously it is universal in a small subset of a given species but it clearly isn't adaptive.

heading out the door as i type, but ledgerwood, ewald & cochran (2003) wrote a review of the evidence that schizophernia is infectious. iirc bi-polar shows a strong seasonality effect.

you mean a model where these quantitative traits are always weighed down by the background genetic load induced by loss of function mutations?

yes. infertility is a good example. what percentage of couples experience infertility to some degree? like 10%?

i think that theory depends on a large mutational target space, which works well for schizophrenia or fertility, but maybe less well for sexuality.

By Rikurzhen (not verified) on 11 Jan 2007 #permalink

I'm not so much squicked by this sort of research as just a curious as to why it often seems a bit reductive, i.e. either it's obligatively gay or it ain't, instead of an interplay of obligatively gay<----->straight on a spectrum informed by a facultative element that's often culture loaded (when we study anthropology and historical accounts).

Given the evidence that homosexuality of both types is on a pretty complex spectrum, I tend to land on the side of in virto environmental factors tied with genetic ones, still though, even if we had a medical technique to remove obligate homosexuality, I tend to think that even the facultative will survive to a great level. After all, what we are discussing is sexual attraction to the same gender (as in wanting to have sex and forming an emotional bond) and not sexual arousal by the same gender (as in, I'm not innately disgusted by the idea, why not try it for fun?)

In other words, in our culture today, as long as there are boys reading skin mags at a sleepovers, and sorority pledges wanting to get the attention of football team captains, there will be some form of homosexuality.

By Spike Gomes (not verified) on 11 Jan 2007 #permalink

I'm sure that a lot of 'homosexuality' among animals is akin to 'homosexuality' in prisons. Male animals who can't get a female will shag anything.

...for example, in African wildlife reserves, a significant problem for rhinos is being crushed to death by young male elephants getting frisky.

"When scientists create a population of all heterosexual sheep - this would be a boon to ranchers, since a high percentage of male sheep are gay"

I would have to question this "boon". There is no reason to need anything approaching a 1:1 ratio of male:female breeding stock; in fact, quite a few male livestock are neutered for easier handling, which makes their orientation rather moot. If half of your male sheep are gay, the other half of your male sheep will happily take up the slack. Since your average livestock breeder doesn't really care about which particular ram impregnates which particular ewe, it all works out pretty much the same.

By MJ Memphis (not verified) on 16 Jan 2007 #permalink

"I suspect that the evolutionary rationale won't be adaptive at all, but some sort of selfish element, whether it be intragenomic, or extra-genomic (e.g., a parasite)."

I think you are on the right here. Have any studies been done on the prevelence of homosexual offspring of women who have conceived while they were menstrating?
Is it possible that the "unclean" shedding blood would create an unfit environment for a developing fetus?