The seductive nature of selection

I’ve been puttering away on my talk for CFI LA on 15 February — it’s all about how bad adapationist thinking corrupts good science. This is going to be easy, because examples just keep falling into my lap, all the time. Here’s one from just yesterday.

Note that I’m not against selection — it really is an important and powerful component of evolutionary theory. Perhaps too powerful, conceptually; there are a lot of people who are rightfully thrilled with the idea, and start thinking everything is a product of selection, down to the last detail, and then they think the science is a matter of just filling in the gaps with your imagination. One subject that always brings omniadaptationists up short is homosexuality. I think it might literally short-circuit their brains. So I got asked about it on Twitter.

I am a physician writing a book. I have a question about homosexuality in evolutionary sense. Shouldnt it be considered a pathology

I am already on my guard. I can tell he’s about to spout some erroneous non sequiturs about homosexuality and evolution. I’m also wondering why someone who is writing a book would do research on it by asking questions of random people on Twitter…shouldn’t he start by memorizing a wikipedia article?

But no, why would you consider homosexuality a pathology “in evolutionary sense”?

As any adaptation which leads to less offsprings is a deleterious adaptation and homosexuality in animals leads to no offspring…

I saw it coming: the limited adaptationist response. If something is, it must have a function, and it must have been selected for. For some reason, they always zoom in on homosexuality. Why not watching football? There’s a stereotypical heterosexual male activity, yet it does not lead to the siring of offspring. But…but…selection must be responsible for everything, so if people do it, it must have had consequences in the past for those who did not do it — it’s tragic, imagine all the neolithic hunter-gatherers who died or failed to have children because they weren’t interested in watching someone else run around with a ball.

Or don’t.

Try this instead. Imagine that there’s no sexual or selective advantage to watching football, but no real harm, either. Generations of Homo sapiens were not filtered at all for their fondness for watching football. But just by chance, some people enjoy it, and they teach their children to enjoy it, and it spreads entirely by cultural influence. That’s one possible explanation that doesn’t involve biological adaptation at all, and it fits the facts better than postulating a history of unfit non-football-watchers getting culled from the population.

Or how about this: as social animals, there is actually a fitness advantage to preferring the company of your fellow species members — there has been selection against loner humans, and selection for those who like to gather together for work or pleasure. As a byproduct of that behavior, we engage in social activities…like getting together with friends, a platter of hot wings, and a ball game on the television. There has been no special selection for the specific activity — some might prefer to watch cricket, I’ve heard, or even ballroom dancing.

That’s a believable explanation, but even here I’d say plausibility is not sufficient. More convincing is that human beings seem to have a greater tolerance for close-up social gatherings of large numbers of other people that our closest relatives lack, which suggests that yes, it is quite likely that we do have a significant difference in large-scale sociability from chimpanzees or gorillas.

Apply that reasoning to homosexuality. Forget trying to find an adaptive explanation for the specific behavior, instead focus on the bigger picture. Evolution has produced people who like sexual intimacy, and given them drives and rewards for sex, because if you lack those things, you are unlikely to leave offspring for the next generation. But it hasn’t been so tightly constrained that it has fine-tuned everyone’s brains to only like one kind of sex. So we end up with a plethora of preferences, and people stumbling about seeking gratification with other people, and it generally works well enough that we get more than enough babies for the next generation.

Notice that I’m not even trying to argue, as many do, that there is a reproductive advantage to the population to have non-reproducing relatives who assist with child care. I don’t find that convincing, either. I find it sufficient to postulate that Nature and Evolution have placed in our genes and culture a desire to find love with one another, and that does the trick.

Note also that one of my questioner’s assumptions is false. Homosexuals do have offspring. It doesn’t matter if you’re gay or straight: some of us want children, some of us don’t. Barring medical issues, it’s really, really easy to get pregnant — it happens all the time, entirely by accident. There are plenty of men who readily make donations to sperm banks, and although there are fewer women willing to participate — pregnancy has great costs not found in sperm donation — there are some who will happily bear children for others. There are also lots of children who need adoption, so even if you don’t want to engage in the messy exchange of fluids involved in intercourse, there are ways to have babies. See? A general imperative to love people is sufficient. And if many people choose not to have children? No problem. It’s not as if we have fallen below the replacement rate (or even that it would be bad if we did: we are not fruit flies, spawning madly as quickly as we can, we are a species that favors a qualitatively better life, preferring fewer offspring raised well and with care).

I also like to point out that there are bacteria that can reproduce every half hour, while we humans have 2 or 3 offspring over the course of decades. Clearly, it was a bad strategy to go all multicellular and set up complex patterns of reproduction. Look at all the energy we expend on brains, when we should instead be using it to spew out more gametes.

Shouldn’t the fact that you’re thinking and engaging in such reproductively pointless activities as writing books tell you that perhaps your gonads aren’t the entire focus of your existence?

Can you explain why homosexuality is considered a “variation in norm” as it clearly does not serve the same purpose of mating.

Mating doesn’t serve the same purpose as mating. I haven’t had sex for reproductive purposes for years, and in fact, for most of my life I’ve taken action to actively prevent reproduction during mating. Was that pathological of me?

Think about your own life. How much of it do you think serves the purpose of mating?

He threw a few more painfully clueless queries at me, but I ignored them. The problem is that he’s building on false premises and isn’t at all willing to question them…and why? Because adaptation is a powerfully seductive explanation. But resist! Think about all the other ways a property can spread through a population!


  1. cicely says

    And how does pestering PZ with inane questions further Time Traveler A.D. serve the purpose of mating? It must…or be considered a pathology, yes? After all, it leads to no offspring, which, as we all know, is the sole reason for all behaviors.

  2. says

    To me that reasoning sounds like a “one trait, one gene”-fallacy. Once you realize that single traits can be the result of complex interactions between genes (and environmental factors) it’s easy to see how these things can happen. Even if gays are less likely to reproduce the mechanisms that produce this behavior can be beneficial to a population. It could even be that a percentage of gays in a groups has a benefit we haven’t considered.

  3. says

    This is one of the reasons I like this blog. Sometimes I catch myself in the midst of a “just-so-story” when considering some biological characteristic. PZ reminds me to ground myself in what we can know vs. what appeals to our sense of “ooh, what if this is an adaptation for ….whatever.” Imagination is great for stirring up new and interesting ideas, but don’t take them as facts until you have some evidence. Thanks.

  4. moarscienceplz says

    the limited adaptationist response. If something is, it must have a function, and it must have been selected for.

    I think Time Traveler should try to determine the function of male nipples before worrying about homosexuality.

  5. footface says

    Do I have this right? It’s not that, say, football watching is adaptive; it’s that football watching is one… expression or outcome of other, more general adaptive traits (such as sociability). So in this view, adaptation is still fundamental. It’s just not the kind of “targeted” just-so-story adaptation many of us might have in mind?

  6. Grewgills says

    @Erland 2
    I think that is a big part of it. If something as simple as the color of your skin isn’t controlled by a single gene, then how can something as complex as social behavior be controlled by a single gene? It is ridiculous on its face. It also ignores that evolution is not something that is reducible to the level of the individual and that complex behaviors like sexuality exist on a continuum rather than as simple bimodal states.

  7. says

    Great response to the ignoramus from twitter. I will just add, if I may, that some anthropologists believe same-sex sexual behavior may have been under some selection for conflict resolution and/or alliance cementing reasons. Bonobos for example frequently engage in homosexual behavior after a conflict, in an apparent effort to honestly signal to conspecifics that the conflict is over. I’m not sure the claim is that specific genes promoting gay sex were under positive selection, or if selection has allowed plasticity in our sexual behavior and this lead to advantages, but either way, at least some research has allowed for an adaptive value to homosexuality.

  8. azhael says

    The selected for phenomenon is sexuality, not necessarily exclusive heterosexuality (which by the way, is likely not normative). You don’t have to be exclusively heterosexual to succeed reproductively, and as PZ pointed out, you can be even if you are 100% homosexual. Being homosexual is not the same as being esterile, and while exclusive homosexuality might be a barrier towards reproduction in other species, it doesn’t have to be so in our species…at all….

    Since sexual attraction is a very complicated phenomenon, controlled and modulated by a large number of genes, it’s certainly predictable that we should see plenty of variation and a diversity of phenotypes, particularly since as i pointed out earlier, there is plenty of room for variation without compromissing reproductive success. Of course, when we actually look out the fucking window and pay attention, we do see that diversity, all over the place…Only if you begin with the ridiculous preconception that exclusive heterosexuality is both normative and necessary for the survival of a species, and a staggering ignorance about the natural world (including the biology of your own species), could you possibly think that there is something odd, unnatural or maladaptive about homosexuality and homosexual behaviour.

  9. says

    I think that the prevalence of homosexuality (IIRC as high as 5-10% depending on the source and definition) suggests that it might even be beneficial to a group. Wasn’t there something about menopause providing extra caregivers within a group? Wouldn’t the same apply to gays? Besides as PZ pointed out homosexuality doesn’t preclude procreation.

  10. azhael says

    The fa’afafine in samoan societies have been suggested as an example of “third genders” being adaptive for family groups, but the problem with these speculative hypothesis is that you can’t test for them, and if you can’t test for them, they are worthless. When i say you can test for them i mean in the past, i have no problem with saying that having LGBTQA individuals in your societal group can definitely be beneficial and even adaptive, today, but you can’t make that claim for positive selection in ancient populations…

  11. jodyp says

    I had to stop lurking for a moment just to tell you what a pleasure it is to read you discussing evolution, Mr. Myers. It’s always informative and dare I say enlightening.

  12. dysomniak "They are unanimous in their hate for me, and I welcome their hatred!" says

    He claims to be a physician but thinks anything without an adaptive explanation is a “pathology?” Does he consider red hair a pathology? How about “outie” bellybuttons?

  13. AMM says

    Fuzzy thinking.

    First of all, being “homosexual” doesn’t in practice mean that one _never_ has sex with someone of the opposite sex. Most of the gay and lesbian people my age (and, for that matter, trans people) have (bio-)children. Whether it’s because they only figured out their sexuality after they’d procreated or because their culture made procreation mandatory doesn’t matter. Their genes, including any gene-specified or gene-enabled homosexuality, get passed on just as effectively as any pure vanilla heterosexual’s.

    Second of all, even in less complicated species than humans, there are plenty of species where the mating mechanism doesn’t insure that creatures only ever try to mate with con-specifics of the opposite sex (or even necessarily only ever con-specifics.) I believe fruit flies (some common kind of fly, at least) were one example where males frequently try to mate with other males. All that’s needed is that _enough_ Fs mate with enough Ms to insure another generation. How that’s accomplished is secondary. Homosexuality in humans _could_ simply be an example of a general mating urge combined with the sort of plasticity of behavior that made humans such a success. Hard-wiring humans to only ever mate with the opposite sex might end up hard-wiring other things which would make them less successful at overrunning the planet. _Heterosexuality_ could simply be a product of socialization — the evolution here would be on the society level, i.e., that societies which don’t promote heterosexuality enough to reproduce die off.

    IMHO, the problem is that most of us have been raised on fairy tales about how nature works, intended to make our social norms seem like laws of nature. Real live nature is a lot more complicated and often quite different from what we’re raised to believe.

  14. unclefrogy says

    as PZ suggested we need to take a wider view. Looking at watching football at the more basic level as a social activity and ignoring the specifics shows that it has much in common with other social activities. If we look at the majority of our sexual activity as social activity since very little of it has any direct connection with reproduction. We see in the Bonobos that much of their sexual activity has nothing to do with reproduction. Taking the wider view we see that for us sexual activity has more to do with forming very close intimate relationships clearly a social function. If it has a function in reproduction outside of the exchange of gametes it is in providing or helping to provide a stable peaceful community for raising our slowly developing off spring to maturity.
    I would venture to say that much of our activity grows out of the desire for that stable environment and many of our problems grow out of the failure to really analyze our situation with all the facts and not our “traditional practice” of beliefs and preconceptions.
    uncle frogy

  15. opposablethumbs says

    NOT …….. cricket!?!?!?!!? Ballroom dancing if you must, but cricket?

    What? Makes more sense than Time Traveler A.D anyway (who not only spells “traveller” wrong :-p but has also failed to note that it’s CE (come on, old chap, really) not A.D (eh, make that D. with its own full stop why don’t you) (yes, I am being absurdly persnickety. Time Traveler deserves no other flavour of persnicketyness)).

  16. drst says

    I wonder sometimes if all the homophobes (acknowledged or not) just don’t believe asexuals exist at all, or they just don’t want to consider our existence because it messes up their worldview too much. If they think it’s “pathological” to only want to have sex with people of the same gender, never wanting to have sex with anyone would be either a myth or mental illness (of course there are not people who are not homophobic who dismiss asexuality as being an illness too. Sigh).

    (Some asexuals have reproduced, of course, for a variety of reasons. Somehow I suspect even if every asexual person were to stop having sex and reproducing entirely, we would not disappear from the population, because diversity is a thing and humans are individuals, not the mere sum of traits handed down by their parents.)

  17. mithrandir says

    This may also be a job for the “nearly neutral” model for evolution. Suppose, for example, homosexuality is the result of multiple genes, and only one of the many, many permutations of alleles result in exclusively same-sex desire; the resulting selection pressure on any one allele just isn’t enough to eliminate it.

    That’s just one of many possibilities, of course, and just another further reinforcement of the point that naive selectionism isn’t a good model for evolution.

  18. says

    I wouldn’t make any predictions about the effects of same sex attraction until we got all sex and gender and sexual pigeonholing out of the system so we could actually observe how many people with body parts x exclusively fucked other people with body part x.
    Because I think we’re in for a lot more variation than what we see today.

  19. Crimson Clupeidae says

    I’m also wondering why someone who is writing a book would do research on it by asking questions of random people on Twitter…shouldn’t he start by memorizing a wikipedia article?

    What you did there. See it, I did.

  20. Thomathy, Such A 'Mo says

    footface @ #5, you sort of have it right. Except that natural selection (which I assume you mean by ‘adaptation’) is not the only mechanism of evolution. It is a key mechanism, but it may not necessarily be the mechanism by which any given trait is evolved (unless that trait is adaptive or is assumed to be adaptive).

    A problem with assuming that homosexuality is an oddity from an adaptive sense is simplistically assuming that sexuality is a single trait when it is not and further assuming that it must be adaptive. It is much more complex than that and is rather the culmination of many different traits (it’s emergent, if you like) and those traits may or may not all be adaptive or the product of some combination of evolutionary mechanisms or a single one of those mechanisms other than natural selection.

    The football-liking analogy works really quite well and not all the traits that lead to football-liking are necessarily adaptive.

    There are exceptions, robust exceptions, but I think a general rule when looking at human behaviour in particular that’s quite safe to stick by is that it’s probably evolutionarily complex rather than simple and that a given behaviour is probably more complex than not.

  21. azhael says


    Yes, absolutely, and this is a subject in which the cultural pigeonholing is HUGE. So much so that even though it seems almost certain that exclusive hetero/homosexuality are not normative, most people think in black and white terms of exclusively hetero and exclusively homo. The cultural effects are so enormous that they often completely obscure the biological reality, which really seems to be that most individuals fall somewhere in the bisexual spectrum (which biologically would be utterly unsurprising).

  22. Grewgills says

    @drst 17
    Another good point. This isn’t even as simple as a continuum along a single axis. It is several continua along multiple axes dealing with sexuality and gender and, well too many particulars to point out in a blog comment. Trying to attribute this to a single gene or small suite of genes is a scarily simplistic view for someone as presumably educated as an MD.
    @azhael 11
    There is a very similar concept in Hawaiian culture and I think it is pretty widespread in Polynesia. It does deal, so far as I have seen and read, almost exclusively with people born with male plumbing taking on a third role within the culture. Unfortunately the traditional Hawaiian term has largely lost its traditional meaning in common usage and is used more often as a slur for homosexuals (thus me choosing not to use it here). I have done some study in Hawaiian culture and a much smaller amount of reading on broader Polynesian culture and haven’t run across an accepted role in the culture for the inverse situation. Hawaiian culture at least did place a lot of limits on women, so that might be why.

  23. The Mellow Monkey says

    Giliell @ 19

    I wouldn’t make any predictions about the effects of same sex attraction until we got all sex and gender and sexual pigeonholing out of the system so we could actually observe how many people with body parts x exclusively fucked other people with body part x.
    Because I think we’re in for a lot more variation than what we see today.

    This. Which genders are identified as existing, how gender is performed, how sex is defined, ideas about “appropriate” sexual behavior and attractions, etc, etc, all have variations across cultures and time.

    It’s wonderfully tempting to say that some modern western conception of gender/sex/orientation is The One Truth and spin explanations for why that is, but that’s not giving any answers. It’s just shutting down questions and erasing the existence of every culture and person outside that worldview.

  24. Amphiox says

    It is interesting how when the hyper-adaptive arguments are trotted out, it is so often with respect to a trait that varies in a population. Since the effect of natural selection is to drive advantageous traits towards fixation, the first thing anyone who postulates some of kind super-general advantage for a trait that is variable in the population has to be why isn’t the trait fixed.

    And when a trait is variable, that de facto means that whatever selection pressures are acting on said trait can’t be strong enough to drive it to fixation…. Something else must be acting to counteract selection, or the trait isn’t that advantageous to begin with.

  25. Suido says

    Watching cricket is an adaptive sexual trait because there’s time to raise a family during a single game.

  26. Pierce R. Butler says

    And then adaptationists must face the flip side of the coin: why don’t we all look like movie stars, when you consider the selective nature of seduction?

  27. Amphiox says

    It seems to me that, in the course of human history (and prehistory), attraction is only one of several factors that individuals take into account when selecting who to mate with and when, and, when it comes to reproduction, one need only produce two offspring to replace oneself genetically in the next generation. And even if the very thought of engaging in said act repulses you to the point of nausea, with sufficient inducement, whether social, cultural, or material, one may still be willing to do it twice in one’s lifetime!

  28. weatherwax says

    “I am a physician writing a book. I have a question about homosexuality in evolutionary sense.”

    I’m a theoretically highly educated individual writing a book. So will you please answer these complicated questions in 180 characters or less so I don’t have to read anything myself?

    Yeah, I’m not buying it.

  29. raven says

    AFAICT, very little of human behavior is genetically encoded or serves a function in maximizing our reproductive success. We are thinking beings who do a lot of things because we like to or want to.

    (Or maybe I’m missing the point.)

    1. Watching TV? If anything that would be evolutionarily disadvantageous.

    2. Surfing the net? While you can learn just about anything, I doubt if that is where most people spend their time. It’s more likely Twitter and Facebook or porn.

    3. Keeping dogs and cats as pets?

    4. Backpacking, hiking, hangliding, surfing and all the other recreational activities.

    5. Hobbies. Collectings stamps, coins, rocks, fossils, spoons, dolls and so on.

    6. Reading fiction.

    7. Whatever you do for fun that doesn’t maximize your reproductive output.

    The rather dubious Time Traveller (sic) has made a major mistake. Implicit in his nonreasoning is the idea that humans are meat puppets being operating by their genes all the time. We’ve transcended that.

  30. raven says

    wikipedia childfree:

    In 2003, a U.S. Census study found that a record 19% of U.S. women age 40–44 did not have children (compared with 10% in 1976).

    If time traveler wants to worry about evolutionarily maladaptive behavior, around 20% of all US women won’t even have children. It’s similar for males I assume, meaning 40% of the US population won’t..have children.

    You can’t minimize your reproductive output more than that i.e. zero.

    So either the multi-ethnic and multi-racial USA has come down with a genetic defect or; time traveler is an idiot who can’t think his way out of a paper bag.

    We don’t show much evidence of being ruled by evolutionary considerations. It’s a theory about biology, not society, life style, culture, or life choices.

  31. raven says

    Time Traveler A.D. I am a physician writing a book. I have a question about homosexuality in evolutionary sense. Shouldnt it be considered a pathology

    I’m going to turn this around.

    Raven: I’m a normal person reading a book right now (It’s Steampunk FWIW). I have a question about idiots like you in an evolutionary sense. Shouldn’t being an idiot like you be considered a pathology and shouldn’t it disqualify you from being a breeder?

  32. Grewgills says

    @Raven 31

    around 20% of all US women won’t even have children. It’s similar for males I assume, meaning 40% of the US population won’t..have children.

    I agree with your larger point, but if 20% of women don’t have children and 20% of men don’t have children that is 20% of people not having children not 40%.

  33. se habla espol says

    Note also that one of my questioner’s assumptions is false. Homosexuals do have offspring.

    It only takes one anecdote to demonstrate falsity of an overgeneralizaton.

    About fifty years ago, my first wife left me and our daughter for another woman — so much for “homosexuality in animals leads to no offspring” idea, even withoiut sperm-banking. The mom was apparently happy in the lesbian life: we had no contact for many years. During that time, rumor has it that she had another daughter, of whom I know nothing interesting.

    Daughter has been married to the same guy for 30 years. They have produced five offspring (plus one miscarriage). None of their offspring have any known homosexual leanings, although son-in-law’s religious-right culture muddies that aspect. If there is a ‘homosexual gene’, it shows itself not to be very strong.

  34. says

    How come these people always focus on homosexuality and not other ‘maladaptive’ traits? I have no biological children. My daughter is adopted. So I’m worse off evolutionarily than many gay people. And what about couples that can’t have children. One spouse is letting their love for their partner override their biological imperative to produce offspring. Or couples with a mix of biological and adopted kids – all those resources wasted in other people’s children when they could have had more direct offspring. Why don’t they talk more about the maladaptiveness of adoption, which has far more direct effects on your reproductive success than who you spend most of your time with?

  35. timetravelerad says

    I am not a hater. I study Homo sapiens and everyone is equally human in my eyes. Its important to speak the truth no matter how painful or emotionally unpleasant it may be. We are all educated people here I assume so we should never stop asking questions and be willing to accept the truth no matter how close to our heart the issue is. Same sex mating happens for many reason and yes many homosexual animals are in fact bisexuals and they have children. I am talking about strictly genetic form of homosexuality where an animal will refuse to mate with the opposite sex such as rams. About 8% of rams will only mate with other rams. Not many of them will leave offsprings. When they die so will their genomes which had been alive for over a billion years in one form or the other. Homosexuality is a complex issue with politics and ethics preventing many Biologist and physicians to openly talk about it. I have many questions for example if 100% of a certain population becomes homosexual then obviously it will wipe out that population and hence it should be considered a pathology. Whether this will happen or not is a second question. If 100% homosexuality is not compatible with continuation of life then what percentage should be acceptable and how do we ensure that we dont cross that threshold? How does it maintain itself in a population at a number not explained by rate of homosexuals having children? Its genetic (there is data to support it) but genetic doesnt mean always hereditary. Genetic only means something involving genes for example all cancers are genetic mutations but not all of them are hereditary. They are spontaneous mutations. So high homosexuality rate could be explained by new spontaneous genetic mutations and not necessarily hereditary transfer of such genes suggesting any evolutionary advantage. There are other many such questions but even biologist find it difficult to detach their own emotions and political views from the issue and the truth is being covered up for sake of being polite. Reason I asked this on twitter is that I wanted a public debate about it from all the leading biologists of the World with no political/religious pressure of any kind. Is it too much to ask?