Homosexuality and natural selection

[TithonusSyndrome]

It seems like a fairly obvious question, so I don't know why I haven't heard it asked before - if homosexual couples on the whole do not bear children, regardless of their species, then how does homosexuality persist? Wouldn't natural selection push their genes out of the gene pool after long enough? Other than the odd self-hating homosexual in denial who does the whole white picket fence with 2.4 kids for 18 years before turning into Tobias from Arrested Development, which is an exclusively cultural thing that can't be said to account for any significant level of Darwinian selection pressures, most homosexual people lead entirely childless lives.

[CaptJodan]

I suppose that depends on if the genes themselves play the key role in determining whether one is gay, straight, or a little of both. Womb studies seem to indicate there may be a connection there to be had, and thus wouldn't be weeded out so long as this continued. And of course there's always the environmental factors.

[Imperial Overlord]

Studies suggest that genetics are important, but not the only factor in determining homosexuality. While homosexuals may rarely reproduce (which is not necessarily true), they can also spread their genes through assisting their relatives and their relatives' offspring.

[Alyrium Denryle]

I just went into this in another thread... So I am just going to copy-paste

It is very easy for us for homosexuality to evolve if it is a byproduct of other processes.

I am only going to be presenting simplified versions of the models here. If I were to go into every detail I would be here all fucking week.

The first model is an alliance building and competition reduction model. Competition for mates and resources is very strong, and males benefit from forming alliances. Alliances can be cemented by sex. This is why you see subordinate males exchanging sex for protection in prisons for example. Under this model, homosexual behavior will be engaged in by almost any male under the right circumstances (as an aside: Any male will also rape under the correct set of circumstances) and if strict homosexuality exists on the tail end of genetic variation that controls the relative willingness to engage in homosexual behavior (IE sets a conditional threshold) then this model slots in with our understanding of biology and human history very well. You would also expect some form of density dependant control, something that makes it so males are more willing to engage in homosexuality under highly male biased sex ratios. The fraternal birth order effect is an in-utero effect that accomplishes this.

The second model is that brain feminization is a selective advantage in agrarian societies.

Once agriculture developed, selective pressures may have shifted, males now needed to have better linguistic skills, social skills etc. These are traits associated with female brains. Prior to this, aggression and other classically masculine traits would have been favored in males. The easiest way to deal with the new selective pressure is to mediate the masculinization of the default feminine brain during development. Selection will favor optimization, where males are still (mostly)heterosexual(see the above hypothesis for how this slots in), but also have social skills. If these mechanisms are subject to a heterozygote advantage you will end up with a normalized distribution of masculine and feminine traits, with a probability of being (obligately) bisexual(not just flexible, but actively seeking same-sex relations in addition to opposite sex ones) or gay that increases as brain feminization increases. Homosexuality and bisexuality exist on one end of the distribution with lower probabilities and potentially (see below)lower fitness, while the hypermasculine brutes are on the other, also with lower fitness. The best place to be is in the middle with the best traits of both. This mechanism is supported by the high degree of feminine traits (such as landmark navigation, and superior linguistic ability) in homosexuals, as well as high degrees of gender atypical behavior in homosexuals. The fraternal birth order effect probably contributes to this one as well as its proximate mechanism is brain feminization.

The third model goes as follows.

Another route is similar, but relies on bisexuality being an alternative reproductive strategy. Bisexuals have more sexual partners in their lifetimes, tend to begin sexual activity at an earlier age, and also tend to have fewer long-term partners. For males this is an advantage, to an extent, which I will go into shortly. By starting sexual activity earlier, bisexuals have one up on their fellow males. While other men are fumbling about trying to get females to mate with them, bisexual men are already sexually experienced, they attract more mates this way, and not just because they have two target groups. If they attract more mates, they potentially have more offspring. Additionally, if and when they do have a long term partner, they mitigate the risks associated with infidelity by being more practiced at deceit. They often have to cover up their own homosexual activity (which in many cultures is not accepted, which I will get into momentarily as well) this gives them practice hiding indiscretions, which allows them to more effectively walk the tightrope of infidelity and as a result, increase their reproductive success. So, why is obligate bisexuality not more common? Density dependent selection. The more bisexuals there are, the lower the advantage there is to being bisexual, due to disease risk, the increased risk of having obligately homosexual offspring, and the simple fact that the more bisexuals there are the lower the advantage over other males there is. This will optimize obligate bisexuality at a stable, but low frequency, depending on environmental factors that affect selective pressure.

[God Fearing Atheist]

Other possibilities include:

A balanced polymorphism theory in which single alleles increase sex drive, charm, etc (Mcknight 1997). This is problematic because it relies on a diallelic model of homosexuality, ignores more probable explanations for homosexual male libido, and cannot account for lesbians.

The genetic variance comes via heritable personality traits like low dominance (Wilson 1981). There is no evidence that human male homosexuals are less dominant.

My perfered explanation is steady-state mutation. Alleles contributing to homosexuality mutate into existence at a rate that offsets losses from lack of reproduction. Turner (1995) makes this argument from Xq28, but the results have not been replicated.

Also, as has already been suggested, the behavior genetic studies consistantly show that most variance in sexual perference is non-genetic.

[Kanastrous]

Said it before, I'll say it again: let's hear it for bisexuals!

[Alyrium Denryle]

Other possibilities include:

A balanced polymorphism theory in which single alleles increase sex drive, charm, etc (Mcknight 1997). This is problematic because it relies on a diallelic model of homosexuality, ignores more probable explanations for homosexual male libido, and cannot account for lesbians.

The genetic variance comes via heritable personality traits like low dominance (Wilson 1981). There is no evidence that human male homosexuals are less dominant.

My perfered explanation is steady-state mutation. Alleles contributing to homosexuality mutate into existence at a rate that offsets losses from lack of reproduction. Turner (1995) makes this argument from Xq28, but the results have not been replicated.

Also, as has already been suggested, the behavior genetic studies consistantly show that most variance in sexual perference is non-genetic.

The balanced polymorphism model is implicit in all three options I list above.

The steady-state mutation wont work, because it would be involve genes contributing to homosexuality to be specially prone to mutate and it is unlikely. Plus, it would be more easily detected if it did exist.

A model I forgot to mention is sexually antagonistic selection, in which females benefit from genes that in males contribute to homosexuality, boosting their own fitness at the expense of the fitness of males.

[Omeganian]

I heard a theory homosexuals are more protective of their siblings' children.

[Kanastrous]

I heard a theory homosexuals are more protective of their siblings' children.

Isn't that what's called the 'Uncle Theory...?'

[Alyrium Denryle]

I heard a theory homosexuals are more protective of their siblings' children.

Has not panned out when tested.

[God Fearing Atheist]

The balanced polymorphism model is implicit in all three options I list above.

Thank you. Now please note the other half of my sentence:

A balanced polymorphism theory in which single alleles increase sex drive, charm, etc (Mcknight 1997).

It is a different balanced polymorphism theory.

The steady-state mutation wont work, because it would be involve genes contributing to homosexuality to be specially prone to mutate and it is unlikely. Plus, it would be more easily detected if it did exist.

Like in hypervariable regions like Xq28?

And no, it wouldn't be more easily detected.