Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Neural basis of homosexuality

  1. Jun 22, 2011 #1
    I have been contemplating this question for a few days now and I am interested to know if anyone here has any input on the matter, and to critique my reasoning.
    There are other threads on this, none of which, from what I could find, made a distinction between homosexual behaviour and homosexuality.

    I would make the distinction between the two by arguing that homosexuality is a preference, in terms of sexual attractiveness, to the same sex, whereas homosexual behaviour is merely interaction between the same sex for various reasons - such as diffusing a tense situation, reinforce hierarchy - but not interaction because of a sexual attractiveness. For example, Bonobo chimps engage in genital rubbing, apparently to reconcile after aggression (from wiki), this however, is a behaviour that has evolved to maintain social relations within a group, and the individuals will engage in heterosexual relationships for the purposes of reproduction.

    There is plenty of evidence of homosexual behaviour in non-human species, but does anyone know of any evidence for homosexuality?

    I believe that there may be a continuum in terms of sexual attractiveness. Where individuals exhibit absolute preference for a particular sex at one extreme, and no preference (bisexuality) at the other extreme. For example, this is from The Atlantic Wire, claiming to quote Ed Yong (the science writer) saying:

    ' "Really, it's a bad idea to use terms like "gay" and "straight" when talking about mice at all. "When mice with normal levels of serotonin are given a choice between males and females, they will mount the male at least 20% of the time," points out Ed Yong. "This, and the widespread nature of homosexual behaviour in animals, supports the idea that sexual preference is more of a continuum." '

    I disagree that homosexual behaviour supports a continuum, as I believe it evolved as a mechanism for communication within groups - to regulate groups, etc. - and is not to be considered when contemplating homosexuality. Essentially, it's a behaviour outside of the realm of sexuality. It should be analysed independently; such that the sexual preference of the individual has no bearing on the behaviour, although this may not be true for all behaviours, courtship behaviours for example, but other behaviours, like same sex genital rubbing, have evolved independently as a behaviours to communicate and regulate groups -it merely makes use of the physiological responses/motivations for sexual reproduction.
    The point about 20% of the time engaging in coitus with other male mice is the bit that is consistent with the continuum; the important point for homosexuality is if the reverse pattern has been observed: a male mouse which attempts to mate with male mice much more frequently than with female mice. This can of course be any animal, and I was wondering if anyone knows of any studies that make this distinction and have found this type of preference?

    I suspect the extremes in preference that are observed in many human societies is more a product of the culture. Anthropological studies seem to suggest many cultures involve homosexual behaviours amongst heterosexuals, and homosexuality is quite obviously, based on the testimonies of homosexuals - a natural preference for the same sex and not a product of culture. So perhaps bisexuality, to varying degrees dictated by genes, should be the 'default' for humans, and not this skewed absolute preference for a particular sex which is often observed. I believe this may be so as it is consistent with other species, in which homosexual coitus commonly occurs.
    Any input appreciated.
  2. jcsd
  3. Jun 22, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Re: Homosexuality

    EDIT: While I was typing this response the thread title changed from "homosexuality" to "does anyone know of any evidence for homosexuality". Regarding the latter evidence for homosexuality in humans is widespread and obvious, in animals the wikipedia article linked below features some links of homosexual preference rather than behaviour.

    I would disagree that homosexuality is a 'preference' purely because that word implies to me that all options are acceptable with some being more desired than others rather than not wanting some options at all. Just looking at the wikipedia article on http://en.wikipedia.org/wiki/Homosexual_behavior_in_animals" [Broken] whilst it is commented that it is rare for animals to display homosexuality rather than homosexual behaviour it does happen in some species, the end of the first paragraph notes that some Rams only engage in sexual activity with other Rams and refuse to mate with Ewes.

    I would agree with this, unfortunately due to human language we have a predisposition to put people in distinct categories (gay, bi, straight) rather than understand that there can be huge variation.

    I'm not quite sure what you mean by homosexual behaviour not belonging to a continuum? There is a clear continuum in terms of how explicit the act is from same-sex kissing to sodomy/tribadism. Whether these things are homosexual behaviour is a matter of opinion and culture, some cultures deem two men kissing on the lips to be homosexual whilst others can consider it to be a typically manly thing to do when greeting friends, same goes for the behaviour that some sports have for congratulatory groping of the buttocks.

    I would be wary about proposing evolutionary explanations for behaviour without firm data. Such things are hugely multifactoral and delve deeply into the murky depths of nature/nuture interactions.

    As with above this would be very hard to examine as we have great difficulty teasing apart the effects of nature and nurture. I would personally agree that to some extent what we deem to be homosexual behaviour is governed by our culture but I am not so certain that this is true about our sexuality.
    Last edited by a moderator: May 5, 2017
  4. Jun 22, 2011 #3
    Re: does anyone know of any evidence for homosexuality

    Edit: I just looked up the word coitus, and it appears to refer specifically to male-female intercourse, so by coitus I mean intercourse for purposes of reproduction between same sex or opposite sex pairings.

    Thanks for the response Ryan. I actually popped back because I had a change of mind about something, but I can't decide whether its plausible or not.

    I didn't change the title, and I disagree with what it's changed to - I think its misleading. I state that I believe homosexuality exists, which is obvious from, well, homosexuals themselves. I left the title ambiguous as I think it is a very general inquiry in to homosexuality. Concerning wiki, I overlooked that in the article, and yes I see that they do make a distinction. The quote from Levay is a book that there isn't access to online, and he states it's a rarity. It's not exactly 'common' in humans, and considering population sizes in some cases and the probable difficulty of identifying it, I think the use of "rarity" may be misleading.

    By preference I mean that the animal's nervous system is 'programmed' such that it is motivated to mate with either opposite sex or same sex, or to exhibit no preference, with a continuum. This is (one of the many places) where I begin to get confused:
    Some form of external stimuli is required to inform the animal to mate. I can't imagine there is anything particularly 'special' about it. For example, a mouse might see another mouse, and internal signals may prompt the mouse to mate. Pheromones may increase or decrease this probability, a female mouse might emit pheromones which increases the probability of the 'prompt' occurring. Instead, the male mouse's nervous system may be programmed such that male pheromones encourage him further to mate, and may not be particularly responsive to female pheromones, in which case the male mouse is more likely to engage in coitus with a male mouse. This mouse may then be considered homosexual, engaging more frequently in same sex coitus. The relative sensitivity of the mouse to either 'male stimuli' or 'female stimuli' will dictate the frequency with which the mouse engages in coitus with the respective sex. Does this make sense?
    Having said that, I am finding it difficult to apply to humans, since we have more sophisticated nervous systems, and could distinguish more readily, the above sounds more like its prone to 'mis firings' occasionally (say, for mice who engage in same-sex coitus 20% of the time as previously quoted, these may be considered mis firings in which the mouse was told to mate, when it is clearly an unsuccessful attempt). Perhaps our better ability to distinguish sexes (if true) accounts for the apparent absolute preference that often seems to occur. Homosexual practices in some societies does not suggest that bisexuality exists in these cultures, it may simply be a 'ritual' which is engaged in, but not necessarily pleasurable, although due to physiological reasons, any participant who climaxes may find it pleasurable to some extent. (I don't want this to become to crude!). There's an anthropologist whose name I can't recall who studies sex-relations in various cultures, and I recall him stating that participants in some sexual practices were not necessarily 'keen' to partake in them. I'll find the name if requested.

    I avoided Rams because it refers to domesticated, and may be used as an argument to suggest it is unnatural for that reason, which seems to implied by Jane Goodall here:

    I meant it doesn't support the idea of a continuum in reference to absolute attraction to one sex through to absolutely no preference for either sex.

    I agree that the research is probably difficult, there are books on animal homosexuality and human homosexuality but I didn't want to commit myself to reading books on the subject, yet (plus some do not make the point I mentioned above, that care has to be taken when making inferences from human cultural practices) as I have a whole load already lined up to read! I was hoping someone might know of some studies that shed light on this topic.

    Thanks again for the response.
    Last edited by a moderator: May 5, 2017
  5. Jun 22, 2011 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    Re: does anyone know of any evidence for homosexuality

    I think the key point is that in some species homosexuality can occur, I think it is a weak argument to suggest that domestication of animals could lead to homosexuality that could not happen in the wild. One of the biggest problems is how to ascertain if an animal is homosexual or just engaging exclusively in homosexual activity.

    Hmm the problem I have with this is our lack of ability to investigate and quantify this "programming". Especially as to the origin of these traits (nature vs nurture again)

    Again I think the largest problem with this is quantifying what biological processes underlie sexual attraction. We've not got a good understanding of why animals indulge in homosexual behaviour; whether it is social, misfiring or bisexuality.

    There's also the problem of trying to do all this research on the millions of different species! I'm not sure there's much available data currently to work through these questions.
  6. Jun 22, 2011 #5
    Re: does anyone know of any evidence for homosexuality

    Yes, I’m not sure about the thread title either but this is a topic I have engaged with on a forum before and met with a viewpoint that I had not expected. Perhaps the question should be about evidence for the genetic programming of homosexuality. And Ryan has already pointed out that the scientifically rigorous answer is – there isn’t enough data to form any opinion with a scientific basis.

    It always seems such an obvious truth to me that there are aspects of human behaviour that are not founded on our genetic programming. Beyond the simple matter of the nature / nurture debate, surely some parts of our behaviour have their basis in our emergent intelligence rather than in our genetically programmed responses. It also seems an obvious point to me that there is a great deal more than just homosexuality in human sexual behaviour that has nothing whatever to do with reproduction. So why then is it only homosexuality that needs to have some evolutionary purpose, and a genetic root?

    The cited evidence of homosexual behaviour in other species does lend powerful weight to the idea of some aspect of the behaviour having a basis in the genes, I accept. But I am not at all convinced that the behaviours in other species that are described really have that much connection with the complex behaviours that constitute human homosexuality. I know I have no scientific basis for the assertion, but it seems very strongly to me that if any kind of ‘explanation’ of human homosexuality exists, then it is a psychological one, not a genetic one.
  7. Jun 23, 2011 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    Re: does anyone know of any evidence for homosexuality

    This is the problem, our understanding of how the genome relates to the phenome (especially in terms of behaviour) is inadequate to tackle these questions. Perhaps if the Human Brain Project is awarded funding then in a few decades time we will have an interesting map of the connectome for cognitive neuroscientists and psychologists to work with (perhaps the two disciplines will finally be on their way to bridging the mind/brain gap that separates them). But even then we would require hellishly difficult and long research from multiple fields (genomics, proteomics, metabolics, development, neuroscience etc) to finally start empirically working out which psychological traits are nature, which are nurture and which are emergent results of a synergy of the two.
  8. Jun 23, 2011 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    Re: does anyone know of any evidence for homosexuality

    I definitely think that this is an interesting topic and do have a lot to say about it however I feel it does diverge from the thread too far. I would invite you to repost your question elsewhere either in the biology thread or perhaps in the philosophy section under the heading of "http://en.wikipedia.org/wiki/Determinism" [Broken]", there I would gladly discuss the issue with you :smile:
    Last edited by a moderator: May 5, 2017
  9. Jun 24, 2011 #8
    Re: does anyone know of any evidence for homosexuality

    I agree that the domestication argument is a ridiculous one, at least as Jane Goodall seems to interpret it.

    I think it depends on what is meant by homosexual activity. I think that the definition of homosexual immediately elicits the interpretation as we apply it to humans. Homosexual and heterosexual implies an absolute preference; I think this applies more readily to animals with a more sophisticated nervous system, since they can better ‘regulate’ these impulses. To take an extreme difference as an example, pseudocopulation is used by orchids to pollinate, this induces the insect's nervous system to attempt to mate with the orchid but pseudocopulation would, I assume, need to be more sophisticated to fool a human. This is an exaggerated example, but the point I’m trying to make is that with a more sophisticated nervous system, more externally derived information and reasoning can be applied in determining whether to attempt copulation. A mouse cannot undertake the kind of reasoning a human can, and will differentiate between male and female less often; this can apply to a male that responds to opposite sex or same sex signals (this can be attractants or repellants, as is the case with male bedbugs). If it is an attractant for the animal, a homosexual animal will responds to same sex stimuli, a heterosexual one to opposite sex stimuli. As the stimuli and input become increasingly complicated, then differentiation may become more difficult, but this can be offset by increased intelligence. For example, an hypothetical single celled organism may be attracted by a chemical signal released by the ‘opposite’ sexed cell (donor and recipient relationship), this binds to receptors on the cell and prompts, for example, conjugation. For a mouse, a number of external signals may come into play, pheromones, visual stimuli, etc., all computed by complex neural networks; this can be subject to developmental alterations that are subtle and more prone to misfire when more information has to be processed, integrated, etc. So a mouse may occasionally make an error: e.g. 20% of the time, even though the other male mouse does not presumably emit the female pheromone, he will nonetheless find himself the object of the first mouse’s affections. The first mouse has to identify a mouse is present using scent and visual information, and has been ‘told’ by its nervous system to attempt to mate. A human’s nervous system is more complex, but it allows for reasoning that allows a better distinction to be made between male and female, better control of impulses, etc, that means engaging in copulation with the ‘wrong’ sex is less likey (wrong being either male or female depending on whether the individual is male or female and homosexual or heterosexual). I think I am just covering the same ground as before, and as you (Ryan) point out “our lack of ability to investigate and quantify this "programming"” means its purely speculation – although I think it’s interesting, but that might just be me!

    We apply the label homosexual behaviour to certain behaviours and it seems to conjure the idea of homosexuality; but as I stated previously, I think they have to be considered separately, and do not reflect attractiveness for purposes of copulation or indeed anything of a sexual nature (Female Bonobos that engage in homosexual behaviour copulate with males). They may use the physiological responses which occur in sexual intercourse, but these same physical responses are probably not unique to that either (are there any unique responses?).

    Damn, I wasn’t sure if there was. If my random musing are anything to go by, then comparing a small subset of, say, mammals, and looking at intelligence versus absoluteness in sex of parner for copulation would show a positive correlation: the more intelligent, the better the ability to ‘regulate’ responses and therefore to show more preference. I don’t know how to compare this across species, mouse<human is easy, mouse<cat<human? Domestication may be an issue… (but not in the way Jane Goodall considers it). And bisexuality may confuse this, also.

    I don’t know either! That’s what I was talking about earlier, although I am the first to recognise that my posts are often rambling, tortuous and unclear, I generally write them as I think about them, rather than ‘plan’ them. In my opinion, sexual behaviours often should be considered separately; and as Ryan pointed out, a distinction is made in the wiki article.

    What do you mean by psychological one? In that homosexuals have adopted to be homosexual and heterosexuals have adopted to be heterosexual? Apologies if I am wrong. It seems to me that the complete opposite is true. It’s programmed into the nervous system. Copulation is an absolute necessity for genes for them to be propagated (in some organisms), so the need for sexual drive is an important one. There is nothing ‘outside’ of the organism to encourage this, it has to be programmed into the nervous system. An animal can’t just ‘know’, if that makes sense. Also, there is nothing overseeing the programming, its random alterations that are subject to natural selection. For a simplified model, an organism programmed by a single gene to mate with the opposite sex will pass on that gene, one programmed to mate with the same sex will not pass on that gene. I am NOT arguing for a single gene theory obviously; even with a single gene theory a gene for homosexuality can still be fixed in the population! So an organism has to be programmed to mate and to mate with a particular sex, although they could be programmed to mate with either; as I say, there is nothing ‘outside’ of the organism dictating these rules, it has to be programmed into the nervous system, surely?

    Some great new words and projects for me to learn about in there, thanks! (Why have I never heard of phenome?).
    Last edited: Jun 24, 2011
  10. Jun 25, 2011 #9


    User Avatar
    Gold Member

    Re: does anyone know of any evidence for homosexuality

    I don't really know that many people at all (heterosexual or homosexual) that participate in intercourse for the purpose of reproduction.... in fact, there's plenty of popular methods and products whose purposes are to avoid reproduction, while still allowing the participation.
  11. Jun 25, 2011 #10
    Re: does anyone know of any evidence for homosexuality

    True, but the point is not whether or not that takes place but is meant to refer to intercourse distinct from something which may have evolved as a behaviour merely to, for example, regulate heirarchy in a social group. Its supposed to reflect whether the individual finds males or females sexually attractive. They may not be intending to produce offspring, they may even know that it will not produce offspring (e.g. homosexual humans), but is nonetheless rooted in the programming by genes to pursue this end. That is the reason the nervous system of the animal is programmed to find the opposuite sex attractive and programmed to 'want' to reproduce, to propagate genes. Just because humans use intercourse for enjoyment is not important, there's a reason it is enjoyable.

    I found this from an article in Brain: A Journal of Neurology (2008) Who do we think we are? The Brain and Gender Identity. 131 (12), pp. 3115-3117 by Joe Herbert:

    "It is curious that structures in the hypothalamus, a part of the brain closely associated
    with internal events in the body rather than those outside it, could determine sexual preference or gender identity. Previous studies by the same lab (Zhou et al., 1995), as well
    as others, show structural differences associated with varieties of sexual behaviour and attitude in other areas of the brain, for example the bed nucleus of the stria terminalis (BST), which is linked both with the hypothalamus and the amygdala. The latter has more direct access to external stimuli, such as those determining sexual attractiveness. It seems that there may be a neural ‘system’, rather than a single nodal area, that determines
    or influences the different parameters of human sexuality. Add to this the evident contribution of the cerebral cortex, involved in social awareness, attitudes, decision-making and the use of sex as a social instrument and we can see that the boundaries of the ‘sexual’ brain are as indistinct as the definition of sex itself." (pg. 3116.)
  12. Jun 26, 2011 #11
    Re: does anyone know of any evidence for homosexuality

    The fundamental sexual instinct is something that comes from our genes, or perhaps we should say from the phenotype that is an expression of our genes. (Is that a better way to put it Ryan?) I don’t think anyone doubts that or needs to work out its evolutionary purpose. Sometimes there are much more subtle and complex behaviours that stem from the phenotype and have an explanation based in evolutionary selective advantage. Again, I might cite altruism as a good example, but will resist the temptation to wander off the point by expanding on why. The problem is, I think, sometimes some people take that fundamental idea and extrapolate it in directions that aren’t necessarily warranted. Do we need an evolutionary explanation for why some people like going to a nightclub and others like walking in the countryside? Or why some people prefer Indian cuisine and others prefer Italian? Is there anything founded in their genotype that explains why someone is Manchester United supporter? Or a Denver Broncos supporter, or whatever. Some human behaviours – it seems to me – do not necessarily have, or need, some deeper evolutionary explanation, they are just aspects of our emergent intelligence.

    With the matter of homosexuality, we perhaps need to be clear what we are talking about. There is a whole range of behaviours – usually associated with homosexual men – that are referred to as ‘camp’. Camp is really just a way of interfacing with the outside world, and the truth is that all of us construct some kind of outer persona as a way of dealing with the world. So camp, I am sure, is founded not in the genes but in the psyche. If by homosexuality, we simply mean attracted to other members of the same sex and not attracted to members of the opposite sex, then yes, I accept the possibility that there might be some genetic basis and evolutionary explanation for it, but I am much less than convinced that there necessarily is. And even if there is, I still feel strongly that the nurture component is much heavier than the nature component.

    There is a British pop musician of years gone by who tells a great story. After having a hit song that openly flaunted the fact that he was gay, one of the British tabloid newspapers published a story which reflected much more about the newspaper's editorial confusion over this issue – and perhaps that of its readership – than it did about the musician himself. The scandalous story was about the fact that he was now in a happy and stable relationship with a woman. Doubtless some, and perhaps you, will explain that by classifying him as bi-sexual. I would suggest to you that it was more a question of him being homosexual at one point in his life and heterosexual at another. That doesn’t suggest to me that his sexuality has an explanation founded in his genes.
  13. Jun 28, 2011 #12
    Okay, I’m not sure if nobahar is going to return to this thread – I understand that some users leave longer time gaps between visits than others. Apologies to anyone who was hoping this thread would sink, but I have had a thought about this that I wanted to raise. The truth is that thus far, both sides of this discussion have been largely speculative. But the point that has occurred to me should have a fairly rigorous scientific answer and it might cast light on this discussion. Again, apologies if this seems an obvious point to others, it has only just occurred to me.

    If a phenotypic change offers an organism a selective advantage, then usually what happens is that, after a certain number of generations, it propagates right throughout the population. Evolutionary scientists talk about it becoming ‘fixed’ in the species – it becomes an identifiable feature of that species. If a phenotypic change is the source of a selective disadvantage to the organism, it tends to be selected against pretty quickly, perhaps even immediately. The same change may keep cropping up every so often, but because it is a source of disadvantage, it generally doesn’t last long. If a phenotypic change offers neither advantage nor disadvantage, then it may drift, and at any snapshot in time it may exist at a certain proportion in the population, but no mechanism operates to maintain it at that proportion, and there is no real evolutionary ‘explanation’ for the existence of the feature. Its existence at whatever proportion it currently happens to be is just a matter of chance.

    The concept of group selection is very controversial. There is some powerful evidence in support of the idea and some experienced biologists are convinced that it is real, but there are others who are just as convinced that it is not real. But, my understanding is, even if it is real, it just operates at group level rather than at the level of the individual organism. It does not operate to maintain a particular phenotypic feature at a certain proportion in the population. Advantageous features still propagate throughout the species given enough generations, disadvantageous features are still selected against and disappear and neutral changes drift without control and without any particular explanation.

    So it seems to me that there is no mechanism to maintain a phenotypic based homosexuality at a certain proportion in the population. Neither is there any good reason to argue that homosexuality exists because of some evolutionary purpose such as homosexuals being non-reproducing carers. Am I right, or am I missing something?
  14. Jun 28, 2011 #13
    I don't know why people would want it to sink; if they are not interested, then they don't have to read the thread!, its hardly an offensive topic and has no implications.

    I'm not convinced by group-level selection, etc. nor do I think "carer" or "auxiliary" roles as a means of selection is a particularly convincing argument (I can't say as I was familiar with it, either).
    As far as I am aware, multi-gene traits which would be selected against through natural selection would not necessarily disappear. There may be many genes that contribute to the phenotype and therefore, when not present together, may happily proceed through the generations; furthermore, the genes may well be pleiotropic, and contribute to a number of phenotypes which may be selected for through natural selection. As such, the genes contributing to homosexuality may well be maintained in the population through their contributions to other phenotypes, and that they will only fail to be propagated in to the next generation when present together; even then, the body in which the genes reside together may well produce progeny.
    This is my understanding of multi-gene traits and the propagation of individual genes. The effects of the gene have to be averaged: the gene may in some bodies produce a phenotype that is not favoured but may be necessary or favoured in other combinations. If the gene is almost always desirable, then its occasional residing in a body in which it is selected against will be of little importance for its continuation.
  15. Jun 29, 2011 #14
    You are right nobahar, I should not guess what others want. I wasn’t thinking about the subject matter, my concern was about the speculative nature of the discussion thus far when it is clear that something with a little more scientific rigour is what is preferred around here. Anyway, whatever, lets see where we go from here.

    Hesitating to go over basics that everyone here understands, just as a point of reference, it is vital to this point to consider exactly how selection operates. For those of us who struggle, as I did, to see mutation as something that can lead to a phenotypic feature like an eagle’s wing – which to an engineer like myself is so strikingly superbly engineered for purpose – it is important to grasp this separation of mutation as the source of species change and selection as the mechanism that tailors species to their environment. There is also, it seems to me, another vital point to grasp about exactly how an advantageous phenotypic change propagates through the population. If you take a hypothetical species that exists at reasonably stable numbers – lets say half a million – and we take an advantageous phenotypic change that appeared at first in just one organism from the population, but over a certain number of generations propagated throughout the species, the key point is that at the end of the time period, every one of the half a million surviving organisms are direct descendants of the one organism in which the change appeared, and the overwhelming majority of the rest of the population at the start of the time period have no surviving descendants at the end of it.

    That is my understanding, at least, of how the mechanism of selection works and it resolves for me this struggle to understand how species become so tailored to their environment. I see no scope within that mechanism to control a particular phenotypic feature at a certain proportion within the population. Neutral phenotypic features that are not subject to selection, or phenotypic features that survive by riding on the back of another advantageous feature, as you seemed to be suggesting homosexuality might, have no mechanism to maintain them at a specific proportion of the population and have no particular evolutionary explanation for their existence beyond the matter of chance. If this doesn’t completely eliminate the possibility of homosexuality having a foundation in the genes, then at the very least it renders it completely pointless to attempt to offer an evolutionary explanation for it.
  16. Jun 29, 2011 #15


    User Avatar
    Staff Emeritus
    Science Advisor

    Nearly but not quite, some of those original .5m won't have any descendants but the majority will. The individual with the advantageous mutation will breed and their offspring will interbreed with individuals descended from one of the other .5m The final generation (where all have the gene) could all trace their ancestry back to almost everyone of those original .5m (bar those whose lineage died out)

    To be honest wikipedia is a great place to start if you want to learn about evolution (everything from the maths of allele frequency change to the molecular biology). Homosexuality isn't going to be a simple matter of "here's the gay gene, in most people is's mutated to the straight gene". We will need a better understanding of how genes and the environment both influence sexuality before we can tackle this question further.
  17. Jun 29, 2011 #16
    Yes Ryan, I take your point, this is all speculative. And I whole heartedly agree that, for all its detractors, Wikipedia is often a great resource. Believe me, I do often pursue the links you provide as I did recently on metabolic pathways, on classifications of types of mutation and on signal transduction. I do love some of the diagrams such as the one on signal transduction that I linked to on that thread, though I do not, for one moment, pretend to understand them. I suppose that what I like about them is what they show about just how much life scientists have achieved, just how deeply they understand some of life’s more complex processes and just how preposterous is the notion that they are all pursuing some path of delusion and heresy that will lead them into the company of Beelzebub. Anyway, I am sure you get the point, rather than a general referral to Wikipedia, it is helpful for the likes of me if those of you with greater expertise direct us to specific pages relevant to the particular matter under discussion.

    In any case, I have to say Ryan that you do seem to be avoiding the key point that I have been pursuing in the last couple of posts. Ignore the specific issue of homosexuality for the moment. Is there any mechanism by which a phenotypic feature can be maintained within a population at or around a certain proportion of that population because there is some selective advantage that works only at or around that proportion?

    I’m thinking of another specific example that might cast a different light. Sickle red blood cells are essentially the result of mutation that might be seen as disadvantageous because it leads to certain genetic conditions that are damaging to health. Yet they have become very common in malarial areas of the world because they happen to offer some resistance to malaria. They are not at all common in non-malarial areas of the world. So, essentially they are disadvantageous and in non-malarial areas have a tendency to be selected against. In malarial areas, the selective advantage of malarial resistance outweighs the selective disadvantage of the other conditions they cause. So the feature is currently in the process of being propagated throughout the populations of malarial areas. It exists then as a proportion of the whole population of the earth as a reflection of the proportion of malarial areas to non-malarial areas. But this is really just a reflection of non-homogeneity among the world population rather than a mechanism to maintain a feature at a specific proportion. Among an interbreeding, homogenous population I am still struggling for a possible mechanism that could maintain a specific phenotypic feature at or around a specific proportion just because it offers some advantage only at that proportion.
  18. Jun 29, 2011 #17


    User Avatar
    Staff Emeritus
    Science Advisor

    Ah I am sorry, I've been quite busy over the past few days and didn't pick up on this strongly enough. It is a good and interesting question. My first thought on the matter would be to tentatively say no, because by definition an advantageous mutation would be one that increases the reproductive capacity of the individual. The only thing I can think of is that the extent to which a mutation is advantageous decreases over time because as more and more of the population hold the mutation it becomes less competitive to have it, though this doesn't maintain the mutation at a certain proportion, merely slows down it's adoption. However I will think on this more and do some digging and see what I can come up with :smile:

    EDIT: With regards to your sickle-cell/malaria example you are correct in saying it doesn't entirely fit the bill because whether or not a mutation is an advantage is contextual based on the environment. Your very interesting question properly phrased would read something along the lines of "within one population under the same environmental conditions is there a mechanisms by which a mutation is only advantageous at a certain proportion of the population." With regards to sickle-cell/malaria and any situation whereby a mutation detrimental to health is advantageous against disease I wonder if predator prey dynamics could be a mechanism to provide this? (Essentially when there are lots of prey predators eat more and grow more populous, this wipes out the prey and so predator numbers decrease, thanks to the decrease prey numbers grow again. Thus we get a cycle).
    Last edited: Jun 29, 2011
  19. Jun 29, 2011 #18
    There is not much to understand in that diagram itself, as it is just a visual representation of some signal transduction pathways inside a cell.

    Organisms in a population can freely breed with each other and this suggests that they live in close geographical proximity and are subject to similar selective pressures. Thus if some trait is advantageous or disadvantageous to a particular group within a population, there has to be some intrinsic property that differentiates that group from the rest of the population. At the moment, the only such thing I can think of is the sex of the organism. If suppose a particular disease affected only males (and it is not a heritable disease), traits which increase resistance would be selected for in males. However the same gene (or group of genes) that causes this trait might have many possible effects on the females. It may be neutral or it may harm them in some way. And then that would decide how it works for females.

    What is a homogeneous population?
  20. Jun 29, 2011 #19
    I retract my previous conviction to denying the possible role of carer or auxiliary. Mainly in light of this discussion on the same topic from elsewhere, from a few years ago: http://forums.xkcd.com/viewtopic.php?f=18&t=62971; I stumbled across it after I recalled Evolutionarily Stable Strategies as discussed by Professor Dawkins in The Selfish Gene, and ESSs are proposed as a possible mechanism in the thread.
    Also, here's someone from the same thread who agrees with my previous post, but probably explains it better:

    "..Whatever the genetics of homosexuality, it's quite clear that it involves multiple alleles and multiple genes. There's no single gene that makes you gay. Rather, certain combinations of multiple alleles (probably) increase the chance that one will be homosexual. When those alleles come together in those combinations and result in a homosexual organism, it's quite possible (likely, IMO) that the result is disfavored from an evolutionary perspective. In other words, to the extent that homosexuals are less likely to reproduce, that combination of alleles may be disfavored, with no compensatory benefit at all from (e.g.) kin selection.

    However, those alleles are not under selection only in homosexuals. They also exist in other individuals, where they may have significantly larger positive benefits. Making up a highly simplified example, imagine three different alleles, each in a different gene. Suppose that inheriting any one or two of these alleles increases your reproductive fitness by 3 or 6 fold, respectively, compared to individuals who carry none. However, if you happen to inherit all three alleles, you'll be homosexual and your reproductive fitness will be 0. In such a case, there's no benefit at all to being homosexual. Homosexuality as a trait is not being selected for. Yet the alleles that contribute to homosexuality are still under a net positive selection. (At least, as long as they don't become so predominant in the population that most offspring are born homosexual.)..." posted by qetzal
  21. Jun 30, 2011 #20
    Okay, there is a message that I am getting loud and clear that the selection mechanism as I outlined it in post #12 is hopelessly simplistic. There is a far more complex issue of genes that get a free ride out of nothing more than the good fortune to be in the middle of a sequence that is selected for. Or perhaps there are some genes that are involved in multiple phenotypic features only one of which is actively selected for but the rest of which are also consequently preserved. A proper analysis of all of this requires some complex statistical calculations around allele frequencies, all of this sort of thing. You can’t just say positive is selected for and propagates through the population, negative is selected against and disappears and neutral just drifts. That’s far too simplistic. I get it, I hear you all.

    But. It still seems to me that phenotypic features for which there are evolutionary explanations – such as an eagle’s wing that is the very thing that gives the eagle the facility to fill the particular niche it has found – must have been positively selected for. And yes, I know, an eagle’s wing doesn’t develop in one step, nor even in a small handful of them. It has to have developed in a long series of tiny changes, each one of which has to have offered a selective advantage, in the manner of dinosaur scales to bird feathers. When it comes to behaviours, perhaps the explanations can be more direct. I seem to remember reading a piece that said that there was some evidence that hedgehogs are modifying their behaviour. Rolling into a spiky ball might have been effective when faced with the threat of a predator, but it is not so effective against the wheels of a 24 ton tuck. There is evidence, according to the piece I read, that hedgehog’s are beginning to learn to run instead of curling into a ball. Now, if there is anything in that it is clearly a pretty rapid development in evolutionary terms. I’m not sure what an average hedgehog generation would be but we certainly can’t be talking more than fifty generations and probably a good deal less. That’s not to say that it is a behaviour that has, by any judgement, become fixed, but it has become prevalent enough to attract notice. In any case, if there is anything to that story, it is a behaviour for which there is an evolutionary explanation and whatever genotypic features lie at its root are being positively selected for among the hedgehog population by 24 ton trucks.

    But if a phenotypic feature exists because it is not subject to selection and has simply drifted to whatever prevalence it has among the population, or if it exists because of one of these complex relationships between genotypic features that are subject to selection and others that are not, then there is no particular evolutionary explanation for the phenotypic feature beyond that happenstance. So, if you are going to argue for any evolutionary explanation for homosexuality, it does seem to follow that you are arguing not only that it was actively selected for, but that it was actively selected for at or around a certain proportion of the population. And I am unconvinced that any mechanism exists for that to happen. The nearest anyone has come to a credible explanation for that is Ryan with the suggestion that you could have a situation where the growing prevalence of a phenotypic feature among the population actually affects the very environmental pressure that is leading to its selection, in the manner of a closed loop control system, so that, like a closed loop control system, the whole system finds its own balance point when the phenotypic feature exists at a certain proportion of the population. I’m not for a moment suggesting that as the actual explanation for homosexuality existing at a certain proportion of the population, I remain convinced that there is sufficient evidence in the realities around us to believe that it is society rather than the genotype that is driving that particular behaviour. But at least it is a credible possibility. I’m still very sceptical about an idea of it being non-reproducing carers in a hunter gatherer society being an evolutionary explanation for it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook