Thread: Evolution and homosexuality?

I am wondering if you biologists could help me out.

Correct me if I am wrong, but does natural selection not work in such a way, that mutations that help a species survive are carried on to the next generation, while those which are detrimental are not?

If so, why has homosexuality gone unchecked by natural selection? It's not as though it helps us to reproduce does it?

Does homosexuality still exist because it isn't hereditary? I have heard that external factors while the baby is in the womb can affect the infants sexuality? Perhaps someone could shed some light on that point?


I've got some ideas on these points, but was hoping a biologist might give me thier scientific reasoning or hypothesis for a comparison.

Cheers, sox 8)

I am not going to talk about homosexuality, as that is politically loaded, but here are some thoughts on how evolution can favour, or allow, seemingly deleterious alleles to survive.

Take the example of sickle cell anaemia. People with this condition have it because they have inherited the same allele at that locus from both parents. This creates a blood condition that is extremely painful, not really treatable (though some of its effects may be ameliorated) and almost invariably painful. With these symptoms, one might perhaps expect evolution to have weeded out the allele.

But...

If a person inherits only one copy of the allele, and the other one is 'normal' (ceteris paribus) then the effect is a condition that can, on occasion, be painful (I have some friends who have this, frequently called "Sickle Cell Trait", and they occasionally suffer severe joint and muscular pains), but what we do know about it is that the peculiarities it lends to the blood cells reduce greatly the ability of the malaria plasmodium from thriving within their bloodstreams. In effect, they achieve a great deal of protection against one of the tropics' biggest killers.

As a result, people with sickle cell trait are often at an evolutionary advantage compared to those with 'normal' alleles in both those loci. This means that the trait (and the allele upon which it depends) will actually persist in the population. Overwhelmingly, I think, you will find that the sickle cell allele is found in populations originally (for a given value of originally) from tropical and equatorial Africa.

I won't go into the maths and percentages of how this happens (I don't have time or energy enough), but it is something that is quite easy to research on the web; so you can, if you wish, do the work for yourself. If you arrive at these conclusions, then you will have an understanding, or even an appreciation, of why evolution can sometimes favour the presence of what seem like harmful genes (harmful to survival or to reproduction, both being affected by natural selection).

Remains only then for you to consider your individual case and see what evidence there may be that the trait in your case may often be favoured by evolution, depending upon the circumstances (environment, including social environment, and not forgetting kin selection).

Hope this helps

Thanks that's food for thought...

However im still curious as to what causes homosexuality...

I fully agree with sunshine warrior on the genetics example he gave, and I'm glad he did because he saved me from having to do it. Here is a quote from a previous thread (and there have been several) on homosexuality.

Originally Posted by paralith

Originally Posted by jacketate

My experties in evolution and genetics is very limited but i was interested in looking at natural selection when it comes to homosexuals.

If there was a genetic coding for homosexuality then because the carrier would have been ill suited and not reproduced ( due to being homosexual ) wouldn't the gene have become extinct by now.

I am personally undecided thats why i am doing this research project. So i can understand a concept which i can't comprehend.

As Jeremyhfht says:

Originally Posted by Jeremyhfht

There IS no "homosexual gene". This retarded idea is the result of politics. What there ARE, are a number of genes that COULD lead to homosexuality depending on your experiences in life. This means they could allow the user to reproduce.

From an evolutionary standpoint, intra-gender sexual interactions are used by bonobo females to strengthen their alliances with each other. This is important for them as females are the dominant gender in bonobos. They still, however, reproduce with males. I would expect that human males in particular would be most likely to use sexual interactions to strengthen their relationships as well, as males were the dominant gender during the evolution of humans. And the males that form the best alliances can dominate their social group, and reserve the majority of mating opportunities, with females, for themselves.

This is how a degree of sexual attraction for members of your own gender can be adaptive and selected for. And once the gene suite that influences this behavior is in place, as jeremy says, a higher than average dose of these genes and/or environmental stimuli can push some individuals to the extreme of actual repulsion for members of the opposite gender.

There is also a theory that it has something to do with kin selection.

Studies have shown that the youngest male child is 28-48% more likely to be a homosexual. Along with the theory that non-reproductive males could contribute to the survival of their siblings and their children, without being competitive for mates, suggest that it could be beneficial for a mechanism that produces homosexuals once the family gets too large.

So, in this case a family of 5 boys, 1 is homosexual, but they likely all have the genes that make homosexuality possible. The 1 homosexual increases the likelyhood of the other 4 surviving to reproduce and that of their children. So, the genes that promote homosexuality are passed on. It could be a matter of socialisation or changing hormone levels in the aging mother that make the youngest child more likely to be gay.

Edit: Also, as a homosexual I'd like to attest that I am still capable of reproduction, I prefer those of my same gender, however under the right social pressures whose to say I wouldn't hide it and still reproduce. Many people get married and have kids, before they finally can't take it anymore and come out. Look at Oscar Wilde, he married and had two kids, as well as a slew of male lovers.

Originally Posted by sox

However im still curious as to what causes homosexuality...

In addition to the mechanisms proposed earlier I think we should consider the possibility that excessive watching of Emmerdale can have an effect.

Originally Posted by Ophiolite

Originally Posted by sox

However im still curious as to what causes homosexuality...

In addition to the mechanisms proposed earlier I think we should consider the possibility that excessive watching of Emmerdale can have an effect.

Haha, I had to look up what that was, can't stand soap operas

Homosexuality is considered a birth defect. The mind ends up looking more similar to a female than a male, while the body remains that of a male.

Originally Posted by verzen

Homosexuality is considered a birth defect. The mind ends up looking more similar to a female than a male, while the body remains that of a male.

And yet, certain integral male behavioral patterns persist, such as a marked preference for good looks and other physical attributes, and a higher degree of multiple mating. Homosexual men act feminine in part because that is still the archetype of what men find attractive. It is not nearly so simple as you imply.

Also, does it not depend on which parent the child is more close to as it grows up? With respect i_feel_tiredsleepy, were you perhaps closer to your mother than your father? I realise that if this were true, that it could rather be an effect of the already homosexual traits coming to the fore, rather than the cause, but could it not be the other way around (as perhaps an additional contributing factor)? I mean, as i_feel_tiredsleepy's stats might suggest, that a youngest child might get more attention from his mother than a first child due to the "novelty" wearing off? I am then interested if anyone has stats on children raised by single parents, along with the level of involvement of boyfriends or uncles in such a scenario.

Originally Posted by paralith

Originally Posted by verzen

Homosexuality is considered a birth defect. The mind ends up looking more similar to a female than a male, while the body remains that of a male.

And yet, certain integral male behavioral patterns persist, such as a marked preference for good looks and other physical attributes, and a higher degree of multiple mating. Homosexual men act feminine in part because that is still the archetype of what men find attractive. It is not nearly so simple as you imply.

There has been, you know, scientific research on the subject and I am pointing out the studies that were found.
Homosexuality is a birth defect. No, you can not wake up one day and decide that you want to be a homosexual. If you are or were ever a homosexual, than it's because of what happened at birth. It's like autism, you can't turn it off whenever you feel like it. =P

Originally Posted by verzen

Originally Posted by paralith

Originally Posted by verzen

Homosexuality is considered a birth defect. The mind ends up looking more similar to a female than a male, while the body remains that of a male.

And yet, certain integral male behavioral patterns persist, such as a marked preference for good looks and other physical attributes, and a higher degree of multiple mating. Homosexual men act feminine in part because that is still the archetype of what men find attractive. It is not nearly so simple as you imply.

There has been, you know, scientific research on the subject and I am pointing out the studies that were found.
Homosexuality is a birth defect. No, you can not wake up one day and decide that you want to be a homosexual. If you are or were ever a homosexual, than it's because of what happened at birth. It's like autism, you can't turn it off whenever you feel like it. =P

I aware of the research you are talking about. They compare firing patterns of men, women, and homosexual men in the brain under MRI in response to certain stimuli. It is by no means all inclusive. It suggests that there have been some structural changes in the brains of homosexual men that result in similar firing patterns as those seen in women, but this does not mean that they simply have a 100% female brain. Consider what makes a man's brain different than a woman's brain: growth and development under the influence of testosterone. If you have a man's body, you developed under the influence of testosterone. The undifferentiated urogenital ridge in the fetus was triggered to develop into testes, which then pumped out testosterone, which in turn was responsible for directing the development rest of the sex specific traits of that fetus. And as I said previously, many typical male behaviors are still clearly seen among homosexual men.

Nor do I think you have a conscious choice about being homosexual, but I do believe that it is a combination of genetic and environmental effects that result in a homosexual phenotype. Remember what tired_sleepy posted about younger siblings being more likely to be homosexual. If it were purely up to genetics, then the chances that a child will become homosexual should be equal no matter what your birth order.

"Birth defect" is a rather emotive phrase and not one I've ever heard a scientist use with respect to homosexuality. It's a gross over-simplification as well as being quite offensive.

Originally Posted by TheBiologista

"Birth defect" is a rather emotive phrase and not one I've ever heard a scientist use with respect to homosexuality. It's a gross over-simplification as well as being quite offensive.

Yes, it's like calling blue eyes a birth defect, because they have melanin levels different from the norm in their iris. Words like "defect" show a marked bias towards the subject, assuming that homosexuality is somehow to the detriment of the child.

Moreover, it is true many homosexuals display feminine characteristics, however many are almost indistinguishable from heterosexual males. Despite what the media portrays, we are not all flamers. People also confuse the concepts of gender identity with sexual orientation, some homosexual men have issues with gender identity, while many are firmly assured of their male identity.

I don't think I was any closer to my mother than any normal child, in fact I think I spent more time around my brother than either of my parents.

Edit: Also, comparing homosexuality to autism is just absurd.

Edit2: There is also a lot of anthropological evidence that suggest that homosexuality relies largely on environmental factors. There is a marked absence of homosexuality in matriarchal societies, it seems that something about societies that promote heavy male male competition promote homosexuality.

There is also the concept of pederasty, which is a form of homosexuality that is rarely practiced today, but could be observed amongst ancient Greeks, feudal Japanese, and many tribal societies. Where often older males will engage in sexual activities with teenage boys as part of a right of passage. From ceremonial insemination of boys at puberty in Guinea, to even the reported instances of commonly practiced pederasty in private boys schools of the 19th century. I doubt somehow that these cases of homosexual behavior becoming common were the results in shifts of the frequency of certain alleles.

Originally Posted by i_feel_tiredsleepy

Studies have shown that the youngest male child is 28-48% more likely to be a homosexual. Along with the theory that non-reproductive males could contribute to the survival of their siblings and their children, without being competitive for mates, suggest that it could be beneficial for a mechanism that produces homosexuals once the family gets too large.

So, in this case a family of 5 boys, 1 is homosexual, but they likely all have the genes that make homosexuality possible. The 1 homosexual increases the likelyhood of the other 4 surviving to reproduce and that of their children. So, the genes that promote homosexuality are passed on. It could be a matter of socialisation or changing hormone levels in the aging mother that make the youngest child more likely to be gay.

Woo hoo! That's just as I suspected, having noticed an unusual number of homosexuals in addition to being gay, also being somebody's "bachelor uncle" or "queer maid aunt", and yeah, younger sibling too.

Plain example of "non-breeder" genes doing quite well in social insects, also a very few "hive" mammals like the naked mole rat. If this works for hive species, then why not humans, as we are much like hive animals..?

Anecdotally. Both my sets of grandparents had many children, and in both cases the youngest sibling turned out non-breeding. My lesbian aunt took some load off her breeder siblings by babysitting and so forth, also shouldered the task of caring for the aging parents when that time came. So, I breeder, owe her something for my "success": my son, and so on. My bachelor uncle lives with his mom and naturally looks after her, so other family members need not. Again, that frees me, breeder, to invest more into my offspring, who may carry grand-uncle's unmarriageable genes.

Anybody care to speculate about how "non-breeder" genes could play a starker role in human "hive" populations?

I think that the roles/implications of older generations (e.g. grandparents) should not be overlooked. For example in some cultures young children are cared for by the previous generation. We rely on grandparents being healthy and able. Would "non-breeder" strategy work differently with this arrangement?

Who says homosexuality is a genetic 'target'? It's most likely a byproduct of a well functioning genetic system. And since it doesn't really seem to be harmful in a way or related to a simple genetic background it will always be around.

especially since the human species has a great cultural emphasis.

Originally Posted by Pong

Originally Posted by i_feel_tiredsleepy

Studies have shown that the youngest male child is 28-48% more likely to be a homosexual. Along with the theory that non-reproductive males could contribute to the survival of their siblings and their children, without being competitive for mates, suggest that it could be beneficial for a mechanism that produces homosexuals once the family gets too large.

So, in this case a family of 5 boys, 1 is homosexual, but they likely all have the genes that make homosexuality possible. The 1 homosexual increases the likelyhood of the other 4 surviving to reproduce and that of their children. So, the genes that promote homosexuality are passed on. It could be a matter of socialisation or changing hormone levels in the aging mother that make the youngest child more likely to be gay.

Woo hoo! That's just as I suspected, having noticed an unusual number of homosexuals in addition to being gay, also being somebody's "bachelor uncle" or "queer maid aunt", and yeah, younger sibling too.

Plain example of "non-breeder" genes doing quite well in social insects, also a very few "hive" mammals like the naked mole rat. If this works for hive species, then why not humans, as we are much like hive animals..?

Anecdotally. Both my sets of grandparents had many children, and in both cases the youngest sibling turned out non-breeding. My lesbian aunt took some load off her breeder siblings by babysitting and so forth, also shouldered the task of caring for the aging parents when that time came. So, I breeder, owe her something for my "success": my son, and so on. My bachelor uncle lives with his mom and naturally looks after her, so other family members need not. Again, that frees me, breeder, to invest more into my offspring, who may carry grand-uncle's unmarriageable genes.

Anybody care to speculate about how "non-breeder" genes could play a starker role in human "hive" populations?

I think that the roles/implications of older generations (e.g. grandparents) should not be overlooked. For example in some cultures young children are cared for by the previous generation. We rely on grandparents being healthy and able. Would "non-breeder" strategy work differently with this arrangement?

Some good points there. Given how social structures are a major part of what has made humans quite a successful species, we can see how there might be an evolutionary advantage to homosexuality and related orientations. The big question would be how natural selection would act positively on the trait, given that it is a trait that, by very simple analysis, we would expect to see selection against. Perhaps selection acts favourably not upon homosexuality itself but on sexual flexibility.

Interestingly it has been suggested by several studies that such flexibility is greater in females than in males. That might tie into Pong's hypothesis if we consider the traditional gender role of females as carers.