Tuesday, December 15, 2009

On the Evolution of Homosexuality

 
A reader alerted me to a posting by Greta Christina on The Blowfish Blog.1 She discusses Why Did Gayness Evolve?.

This is not your ordinary posting on the topic. For one thing, she acknowledges that you can only discuss the evolution of homosexuality if there's a significant genetic component. She makes the assumption that there is a genetic component but it is not proven.

From that point on, her analysis of the possible reasons for evolving homosexuality is as good as it gets. Greta Christina avoids the obvious clichés and concentrates on real biology. The words "spandrels" is used a lot.

This is someone who understands evolution. Speaking of understanding evolution, in order to discuss whether homosexuality evolved you need to have a workable definition of evolution. My definition restricts evolution to heritable changes in a population and that's why the discussion of a possible genetic component is relevant.

Some people prefer a different definition of evolution—something like "descent with modification." I wonder how you can discuss the possible evolution of homosexuality using such a definition of evolution? Would it mean that the increased prevalence (and acceptance) of homosexual behavior in ancient societies was an example of evolution?


1. Thank-you Fred.

13 comments :

  1. By far the most interesting and convincing hypothesis about origin of homosexuality can be found here. IMHO, it 1) makes perfect sense, 2) neatly accounts for almost all known facts, 3) makes a number of clear predictions that are, as far as modern genetics and medicine goes, are very easy to test. (That's not to say that it is all there can be).

    The fact that non-biologist came up with this hypothesis is all the more remarkable. Highly recommended! I once tried to interest a "leading expert" in it but never got a reply. The guy is probably busy trying to find "homosexuality gene". Good luck with that.

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  2. From what I remember, there are at least 3 evolutionary (or maybe I should say selectionist) "reasons" homosexuality exists in most mammalian species. Off the top of my head:

    1) Homosexuality is a multi-genic trait, and only arises when specific and somewhat rare combinations of alleles occur.

    2) Homosexuality is the "cost" associated with expressing genes in males that boost fertility of females.

    3) That kin selection thing - if you believe in that stuff

    The evidence that homosexuality is a genetic trait is quite strong; even OMIM's got an entry on it.

    And lest we forget, being homosexual is not the same as being sterile; there is no physical barrier that prevents homosexuals from reproducing...

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  3. I have read an excellent article about the evolution of human male homosexuality here: http://www.scientificblogging.com/news_releases/sexually_antagonistic_selection_a_darwinian_evolution_model_for_homosexuality

    It uses a known phenomenon in nature called sexual antagonism to explain male homosexuality. Sexual antagonism is reasonably well known in nature although we didn't know if it worked in nature.

    Since female homosexuality doesn't fit that model (the numbers don't work out), it's believed that female homosexuality is a completely different phenomenon.

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  4. Bah! Typo! "Sexual antagonism is reasonably well known in nature although we didn't know if it worked in HUMANS."

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  5. I don't even really see that homosexuality itself is some "thing" that needs to be explained. Sure, we observe that there are people who prefer (in some cases exclusively) sexual partners of the same sex. But, as Greta points out, there is essentially a spectrum of sexual preference between the (according to her, relatively rare) extremes of 100% heterosexual and 100% homosexual.

    In other words, within human populations there exists a wide range of variation in a genetically-influenced characteristic. Why, this unprecedented observation requires an immediate evolutionary explanation! Call Rudyard Kipling!

    Seriously though, the spandrel explanation seems more reasonable to me than trying to find some untestable adaptive explanation for an apparently maladaptive trait. But I say, why seek an explanation at all? We're all different in different ways, and that's exactly what you'd expect. I'm entirely satisfied to consider gayness as a normal and entirely expected part of the spectrum.

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  6. Tim,

    I'm sure I could phrase any sensitive topic in exactly the same way and get out of actually studying it.

    We don't even know if homosexuality if genetic or environmental or both yet. We don't know if it's one big phenomenon or several smaller ones.

    You seem to want to take it an not study it. How uninteresting.

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  7. Determinist,

    Studying human sexuality, and working out the interplay between genetic and environmental influences on sexual preferences is a fascinating and worthwhile field of study. What I'm suggesting is that approaching homosexuality as some discrete thing whose existence demands a special explanation may be the wrong way to go about it.

    If we eventually work out a thorough understanding of how genes and environmental factors interact to produce variations in sexual preference (and no, I don't think we have achieved this yet), I'm not convinced that we'll need a separate explanation for why some males prefer other males as sexual partners. If it can be demonstrated that homosexuality is not just a subset of the spectrum of sexual preference that includes heterosexuality, then it will require a special explanation. Until then, in my opinion there's no good reason to think that such an explanation is required.

    Let me also be clear that I'm talking here about evolutionary explanations (that is what both original posts were about). I'm not suggesting that we should not seek neurological or physiological explanations for how different sexual preferences are specified. So no, I am not (and never was) suggesting that we should not study sexuality in general, and homosexuality in particular.

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  8. Tim,

    If evolution is to remain a viable theory, it must be able to explain everything. So, there has to be an evolutionary explanation for why certain people prefer same sex partners (through the entire continuum of exclusively gay to bisexual to heterosexual).

    Why, evolutionarily, do humans have the sexual preferences that they do? The heterosexual preferences seem self explanatory and the homosexual ones do not. That's about all there is to it.

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  9. Determinist makes a fool of itself by saying,

    If evolution is to remain a viable theory, it must be able to explain everything.

    That's not correct. Evolutionary theory has to be consistent with what we know but it doesn't have to explain things where we don't have enough information.

    So, there has to be an evolutionary explanation for why certain people prefer same sex partners (through the entire continuum of exclusively gay to bisexual to heterosexual).

    Homosexuality is one of those things where we don't have enough information. We don't know whether homosexuality has a significant genetic component. If it doesn't, then no evolutionary explanation is necessary to explain it. As a matter of fact, evolutionary explanations would be wrong if there's no genetic component to homosexuality.

    If there is a genetic component then we still don't know whether it's maladaptive, adaptive, or neutral. In that case, we can't propose a reasonable evolutionary explanation until we have more information.
     

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  10. "That's not correct. Evolutionary theory has to be consistent with what we know but it doesn't have to explain things where we don't have enough information."

    Of course. Maybe I should have said "eventually explain everything" - no one and no theory can explain something that we don't have enough information on unless they get lucky.

    Fair point - sorry for the mistake.

    "We don't know whether homosexuality has a significant genetic component. If it doesn't, then no evolutionary explanation is necessary to explain it."

    Evolution isn't limited to things with purely genetic components. That's silly.

    Genetic factors can influence purely non-genetic phenomenon in lots of different ways.

    Looking for non-genetic evolution - we can see meme theory/cultural evolution, although to say that it is purely non-genetic is probably misleading.

    This is somewhat moot with homosexuality though, since we're pretty much positive that there is a significant genetic factor.

    In cases where one twin is gay, the other twin is gay just over half the time. This is far higher than with fraternal twins, so that pretty much closes the book on whether there is a genetic factor or not.

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  11. In cases where one twin is gay, the other twin is gay just over half the time. This is far higher than with fraternal twins, so that pretty much closes the book on whether there is a genetic factor or not.

    Not really! This would likely be true if in both cases you are talking about twins separated short after birth.

    And even if it turns out to be true, it is still incredibly difficult to rule out environmental influences during pregnancy. Gene-environment interactions can neatly explain higher frequencies among MZ twins.

    So be careful with those closed books :-)

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  12. So, spandrels and expatations are only one step away from a truly adaptationist explanation, huh.
    What shall we call this? "Ultimate" adaptationism?

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  13. DK,

    you say:
    "'Not really! This would likely be true if in both cases you are talking about twins separated short after birth. "

    Apparently, this number also holds with twins separated at birth.

    The whole environmental influence in the womb, I thought, was eliminated by a comparison with fraternal twins.

    Do you disagree?

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