homosexuality vs. evolution

Great Debates
1

Well, the fundies are going to love this one, their two biggest rivals going head to head:
[aside: Let me make perfectly clear that I have no ill intentions with this thread. I’m not “gay bashing”, this is just a problem that I have been wrestling with in my head for some time – I don’t claim to have an answer, and I’m not judging any values here.]
In Darwinistic evolution, only those traits that contribute to the survival of a species will continue to be seen in that species. How then, has homosexuality managed to survive so many generations (some even say it’s frequency has increased–not sure on this point)? It has no reproductive value to the human species, and cannot be passed down to offspring.
By this reasoning, I must conclude that homosexuality is in fact caused (I know ‘caused’ seems to imply something negative, I just can’t think of a better word.) by environmental factors.
The problem (one of many) I can’t figure out, is that every homosexual I’ve asked says that they were born that way, that they didn’t just wake up one day and decide they were gay. I can’t simply discount the firsthand testimony of so many. I can’t fathom why someone would want to be gay, voluntarily facing so many stereotypes and social bigotry (though that doesn’t mean people can’t voluntarily make that decision).
How can evolution support a trait that is not hereditary?

-eggo, who’s brain hurts.

2

OK, lets dissect your question.
In Darwinistic evolution, only those traits that contribute to the survival of a species will continue to be seen in that species.
Not true. Neutral traits are very widespread in every species.

It has no reproductive value to the human species,
Two problems with that statement!
Firstly evolution doesn’t work on a species level, it works on an individual level. If something has an individual survival advantage that is all that counts.
Secondly I can think of several possible theories that would explain the evolutionary advantage to homosexuality. Some of the more obvious are:
Non-reproducing family members could help care for related young as is seen in many species of birds and mammals.
Homosexuality could allow for deeper bonds between males, as appears to be the case in the homosexual relationships of Bonobo. This makes for less tension, less intra-tribe fighting and a more cohesive hunting/fighting force.

** and cannot be passed down to offspring.**
If it were genetic then it would be passed down. I’m assuming that you are implying homosexuals don’t have children, which simply isn’t the case. Added to this genes that are expressed in only some individuals can easily be passed down even if they effectively ‘sterilise’ those who express them. So long as they provide a large enough advantage to the relatives of those who express them they will even come to dominate. There are presumably genes for altruism in birds and many mammals that follow this rule, and the Naked Mole-rat actually has a gene that sterilises all but one female in the nest.

By this reasoning, I must conclude that homosexuality is in fact caused
Which would be fair enough if the reasoning were a little more sound.

I know ‘caused’ seems to imply something negative, I just can’t think of a better word
Could I suggest ‘Is the result of environment rather than genetics’ or ‘is a choice’

every homosexual I’ve asked says that they were born that way

  1. Many homosexuals are very assertive about their lifestyle being a choice. You just haven’t met any.
  2. This of course doesn’t in any way mean that homosexuality isn’t the result of environment. Height is affected by diet. If you had asked someone 100 years ago why they were short they’d probably respond that they were born that way. Just because we can’t identify the environmental factors that cause something doesn’t mean it’s not environmental.

** I can’t simply discount the firsthand testimony of so many**
Nor should you, but you should probably treat anecdotal evidence as what it is. Suffering from epilepsy doesn’t make someone an expert on its causes, nor does being a brilliant musician make someone an expert on what caused that.

I can’t fathom why someone would want to be gay
I can’t fathom why someone would voluntarily listen to Britney Spears, but they do.

How can evolution support a trait that is not hereditary?
It can’t.

3

I tend to think that a certain amount of homosexual behavior is normal, and that can be seen in the animal kingdom and in situations where people who normally consider themselves heterosexual have no access to people of the opposite sex. Since the tendency to engage in same-sex intercourse would of course vary within the species, it stands to reason there are some who are almost exclusively homosexual (yet many people who have no doubt that they are gay can perform with a member of the opposite sex).

4

Also, though I consider myself heterosexual and have never wanted to have sex with someone of the same sex, I also know that certain physical stimuli that could lead to my getting an erection could be provided by a member of the same sex, and if I had a reason to I’m pretty sure I could have sex with a male. I just lack the motivation. 8^)

5

I think a likely explanation is that the mutation rate exceeds the “weeding out rate”. If homosexuality is recessive, then a gentotypical homosexuality rate of 3% would represent a 17% rate of the homosexuality gene (of course, the genotypical rate is almost certainly higher than the phenotypical rate, and there’s probably not a single “gay gene”, but simplifications must be made). So ignoring the fact societal pressures would probably cause most homosexuals to get married anyway, a carrier would have 17% chance of marrying another carrier. If that happened, there would be a 25% chance that a particular child would be homosexual. Total effect: about 4% of a carrier’s children would be homosexual. Now, 4% of 17% is .7%, so there would have to be a mutation rate of .7% to compensate. High, but not impossible. Of course, the preceding was a gross simplification.

6

Didn’t we dance this to death last month or the month before? Someone go bump that ol’ bitch up here. I’m willing to bet no new information has been released on this topic.

7

Well, do midgets have any benefit from not being able to grow? They are still around.

8

[hijack]
I’ll just point out that achondroplasia, probably the most common form of dwarfism, is mostly caused by spontaneous mutation. I believe the last number I heard was 80% is new mutation.
[/hijack]

Next, just because something is caused by the environment doesn’t mean that

  1. You aren’t born with it and
  2. You have any choice in the matter.

Say homosexuality may be caused by in utero hormone exposure leaves room for genetic, environmental, and stochastic factors.

There already have been good descriptions of how a 5-10% homosexuality rate could have advantages for a population. This by proxy would increase the survivability for the individuals of a population.

Also, if we are going to grossly oversimplify this to an autosomal recessive condition, then we can hypothesize all types of reasons why it would not breed itself out of a population. These include advantageous carrier states (like sickle cell disease), low penetrance, it not really being a neutral trait (described in the preceding paragraph), and high mutation rate.

9

Really? Then I guess neither have I, and I dare say I know quite a few more gay folk than our gracious OP.

Esprix

10

Let’s take the OP at face value. I see no reason to suspect trollery, and it’s a potentially interesting question, which as noted has been attacked.

The question is, stripped of potential pyrogenic concepts, if a particular behavioral pattern results in failure to reproduce, why is it not effectively bred out of the gene pool?

Well, consider that the sort of religious devotion that causes someone to espouse a role as a Catholic priest, a monk, a nun, a nursing sister, a friar, etc., results in the removal of the person posssing such devotion from the gene pool. Yet, fluctuations to one side, there have historically been regular “vocations” of these sorts on an ongoing basis.

For many years, a specific woman might find it her proper role to fail to marry and instead stay home and take care of her aged parents in their declining years. I had relatives who did precisely that. The social impetus toward this behavior seems to have slackened, but it was clearly a historical feature that did not breed itself out of the human gene pool.

Obviously, something of value to the race as a whole is garnered by the activities these persons engage in instead of reproduction. The care of the elderly, the various good works pursued by celibate clergy and religious, etc., apparently provide adequate benefit to the species to be selected for.

There was at one time the concept prevailing that having persons who were not tied down to the support of a family and free to pursue other activities, including the voluntary assistance they might render to a family which was in some way impaired (man is gored by mammoth tusk and laid up, hence cannot go out and kill Irish elk for supper; woman dies in childbirth, leaving small children who still need care), was sufficiently beneficial to the species to warrant selection for those genes despite their immediate contra-survival tendency in failure to perpetuate themselves in children.

This is totally speculative but does have some evolutionary sense behind it. And people who were gay would obviously be a part of that group of not-tied-down-to-family people.

One might also note that the criterion for “being gay” in our present mind set is a homosexual orientation. This does not, of course, immediately and mandatorily imply having sex with another person of the same sex, merely the tendency to find them sexually attractive. And historically there have been some awfully large closets. Behaviorally, a bi person or a gay person attempting to cover up his or her gayness (conceivably from himself/herself as well) might marry and engender children. Though this would be somewhat rare, it does provide a means of perpetuating a “gay gene.”

Finally, the idea that gayness is genetic is not proven. In the seventies, we “knew” that it was psychologically based, the result of a domineering or smother-loving mother and a weak or absent father. In the nineties, we “knew” that it was genetic. There seems to be some evidence that it may be congenital, the product of a “testosterone flood” in the brain of the child during fetal development. Me, I’m not holding out any bets for discovery of any particular cause. I suspect it has some parallels with why the dinosaurs went extinct: any one answer has some validity but is not the answer.

Discuss.

11

Well, those more learned than both of us combined have concluded differently, and that is that sexual orientation (homosexual, heterosexual, bisexual and asexual) is caused by a complex combination of nature and nurture, so complex that it will probably never be understood.

Good enough for me.

Esprix

12

Even if homosexuality was caused entirely by genetics, and even if homosexuality were a 100% guarantee that the individual would not reproduce, and even if the “contributor to society/species” argument were completely unfounded, homosexuality could still easily be kept in the gene pool. Genes, as I understand it, are not single task critters. They often have multiple functions, and many genes do nothing but control expression of other genes. Some seem to do nothing at all; they act like placeholders. But they do seem to play off one another in odd ways that we cannot, as yet, always figure out.

Posit for a moment a gene that slightly increases peripheral visual acuity. A second gene makes a protein that increases one’s resistance to a common disease. A third slightly increases the sex drive. Now all three of these would be genes that one would think evolution would favor. Posit a control gene for each that allows the gene to be expressed. There are, of course, a gazillion other genes that offer other advantages to an individual as well, but we would expect that these three genes should survive and spread through much of the population. The final posit in this thought experiment would be that if an individual possesses all three of these genes plus their controls, that individual is born homosexual. In such a scenario, the genes that caused homosexuality would never be bred out of the gene pool. Anyone possessing the genes in any combination other than the full six would be actually more likely to survive and pass on their portions of the whole than others lacking them.

Now, I am certainly not saying that genetics is the cause of homosexuality. There seems to be evidence that environment, especially prenatally, is a big factor. However, if genetics were the sole culprit, I suspect we would see a complex scenario like the one above, probably multiple such scenarios in fact, rather than a “gay gene.” If you ever read the blurbs about scientists finding a gene linked to some disorder, they usually say that “such-and-such gene is present in 60% of the people with blank disease.” It’s not usually a dead-on lock, where “every person with gene A develops condition B.” If there were a single gay gene, we’d have probably identified it by now.

13

Polycarp: an excellent restating of what I meant.

everyone makes valid points, and I thank you for your insight.

One point to argue:

I fail to see how this would result in the passing on of the homosexual gene possessed by the care giver in your first example, if anything it would help proliferate the non-homosexual gene of those who had the child that the care giver would be caring for–assuming that the parents did not have the gene (for future reference, I’m talking only about exclusive homosexuality, not bisexuality). I realize that it is possible for a homosexual to have children, however the vast majority of children come from heterosexual parents, so one would expect homosexuality to decrease in frequency.
Your second example would have the same effect, the heterosexuals (and the homosexuals) would have a longer life, thus having more opportunity to have sex. The heterosexual sex can result in children, while the homosexual sex cannot, therefore the less violent lives of the Bonobo (both hetero, and homosexual) only increase the heterosexual gene pool, not the homosexual one.

So far, the explanation that makes the most sense to me is that of the “complimentary genes”. It would be interesting to see if such a pattern exists where the parents of heterosexuals all have a particular set of genes that combine to form the “gay gene”

14

Brought this up in a GQ thread, but I’ll restate it here. Gays do have children–I’m one of those children, and I’m fairly sure that I exist, and am not alone in this. While the idea of complimentary genes, or inactive genes triggered by some stimuli sounds perfectly reasonable (assuming there’s any genetic compontent), there’s no reason to say it couldn’t be a single gene (or a few genes) that get passed on just like the rest.

15

Ah, but that’s just it. You can’t assume that the parents did not have the gene. Who are these children who the altruistic homosexual individuals will be helping to care for? Probably their own younger siblings, or the children of their siblings. Since they are all related, they will have some of the same genes.

If homosexuality is a genetic, inheritable trait (something which is by no means certain), then it is obviously not a dominant trait. Heterosexuals do produce homosexual offspring, after all. So, if it’s an inheritable but recessive genetic trait, there is a good chance that some of the heterosexual siblings or nieces and nephews of our homosexual altruist will be carriers of the trait.

16

For just about everyone I know (straight, gay, bi, asexual, etc.), their lifestyle is definitely a choice (as far as anything can be a choice).

[duck]

By this, I don’t mean they choose to be [gay, straight, etc.]; they do choose to some degree to be [monogamous, promiscuous, out, closeted, etc.]. The first list is things (most probably) out of any kind of control. The second list is things that are choices (whether rational or irrational).

[/duck] (hoping coast is clear)

17

[quote]
Non-reproducing family members could help care for related young as is seen in many species of birds and mammals.
Homosexuality could allow for deeper bonds between males, as appears to be the case in the homosexual relationships of Bonobo. This makes for less tension, less intra-tribe fighting and a more cohesive hunting/fighting force.

eggo
I know Lamia has already addressed the question, but I’ll try to clarify it myself anyway.

You are assuming in your first paragraph that only the caregiver possesses the gene. This is not the case. Firstly the gene would almost certainly be found in at least some of his siblings, and secondly possessing a gene and expressing that gene are two different things.
To elaborate on the animal examples. In many bird species the young of one gender do not move out and establish their own territories, they remain in their parents’ territories and help their parents care for their siblings. Many individuals never reproduce. This behaviour is not learned, it is inherited. The gene for this behaviour is not only found in the non-reproducing individuals, but in the reproducing individuals. Any bird that lacked this gene would be at a distinct disadvantage over others of its species, because although it may reproduce, its young will immediately set up their own territories and compete with it for resources feeding their own young.
Alternatively the gene for homosexuality may be recessive, and simply not expressed in all individuals, but if even one individual in a thousand expressed the gene then this could have major benefits for his/her family in times of crisis. Often it is the population bottlenecks that really select which genes survive. For example if the entire human population were reduced down to 500 individuals then I could argue that there is a fair chance that one of the survivors would be being cared for by two parents and a gay cousin/brother. That means that there is now one individual in 500 carrying the gene if not expressing it. The population will build up to 1000000 individuals over the next x years, and the gene may be swamped, but the next major catastrophe will see the gene brought back to 1 in 500 again. In this way many genes are preserved that have limited value most of the time.
The same logic can easily be applied to Bonobo. You make a big assumption when you say that homosexuals never reproduce. As several people have pointed out here, they do and I the past probably did so more regularly. Assuming that homosexual genetics was as low as it is now, what exactly is the chance that two homosexuals of the same gender would exist in a tribe of ten people? In those circumstances I can see no more reason why a homosexual wouldn’t ‘resort’ to heterosexual sex just as nominally ‘straight’ people ‘resort’ to ‘gay’ sex in certain circumstances.
Of course many people are bisexual. I can’t see any reason why you would make the huge assumption that the genes that code for homosexuality would code for ‘exclusive homosexuality’, and wouldn’t be linked to bisexuality. Genetics only occasionally works that way. Usually a collection of genes is responsible fro producing a spectrum of characteristics. Genes that code for brown eyes never code for ‘exclusive brown eyedness’, and genes that code for height don’t code for ‘exclusive tallness’ or ‘exclusive shortness’. The only times I can think of where this does happen is where there are two alleles for the attribute. This implies that there is only one homosexual/heterosexual gene, rather than a suite of sexuality genes. Given that we have no evidence for the existence of any sexuality gene or genes and that I can’t think of any human behaviour coded by one gene I can’t understand your assumption. You’ll have to explain that one to me in more detail before I’m prepared to accept it for future reference. It’s a seriously flawed assumption and accepting it wouldn’t be conducive to scientific evaluation of your original proposition.

18

Even allowing that (a) homosexuality is genetic in origin and (b) that it has no survival value (both largely hypothetical), the “sickle cell” effect may come into play. As most of our literate colleagues know, sickle cell anemia is a genetic disease resulting from a reinforced recessive gene largely limited to black people of African descent, which was perpetuated because a single allele of this gene gives increased resistance to malaria. Selection for this gene in malaria-ridden areas statistically overrides selection against sickle cell anemia from reinforced appearance of the gene.

The analogy should be obvious: although people with a GG combination (improperly simplifying it to a single gene pair) “are gay and do not have children” (another oversimplification), people with a GS combination have some inherent advantage over those with a SS monotype, and are therefore selected for. Greater creativity (gays supposedly being on average more creative)? Larger penis size in males (supposedly linked to being gay)? (Is that a genetic advantage?) Improved ability at decorating? :wink: I dunno; and anyway the whole schema involves far too many assumptions and too many stereotypes. But the idea that a gene complex that includes pro-survival traits could include a combination that produced gay orientation is not too far-fetched.

19

Think about bees. Here you have thousands of organisms that never reproduce. They live only to help along their few siblings that will one day become queens, plus a few drones. So 99% of bee population never reproduces, never passes on their genes. Would natural selection select against such a situation? Obviously not, since it’s such an effective way to ensure that the young queens (and their genes) make it.

Now take humans. What if we’ve developed a system whereby, rather than 99% of our population being there strictly in a non-breeding support role, only 10% is? That 10% helps feed, protect, and raise the offspring of their siblings. Thus, it’s beneficial for any parent to have a few gay offspring to increase the chances that their own (the parent’s) genes will be passed on. You might sacrifice 1 chance in 10 to do so, but that 1 out of 10 might greatly increase the chances that the other 9 make it.

I’m not saying that this is the way it is, or that homosexuality is genetically based, or that no homosexuals reproduce. But even if homosexuality was entirely genetic, and no homsexuals ever reproduced–hell, let’s say that 10% of the population is born without sex organs at all–it’s still possible that this situation could have evolutionary advantages, and would not be selected out.

20

I don’t know if anyone has some research they can point to, but I do not believe that gay parents are more likely to have gay children.

So, we can therefore definately rule out the simplistic autosomal recessive model for homosexuality. If homosexuality were a simple autosomal recessive, then two gay people who had children together would ALWAYS have gay children. This is not the case, a gay man and a lesbian who have a child together (for whatever reason) will usually have a straight child.

We also know that it is possible for one identical twin to be straight and another to be gay, although identical twins are much more likely to have matching sexual prefrences than fraternal twins.

It seems to me that almost all gay people and straight people were “born that way”. That doesn’t mean that the cause must be genetic, even if we discover some physiological or anatomical differences. We all know that physiological or anatomical differences can be caused by environmental differences such as prenatal nutrition, prenatal exposure to disease, or prenatal exposure to various chemical substances.

All we know for sure is that it isn’t simple. To declare that we “know” that the cause is genetic is simply extremely premature, since people with identical genomes can have differing sexual preferences. It seems to me that a lot of basic biology and neuropsychology has to be done before we can even ask intelligent questions.