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I would imagine that the answer to this question would be population control, especially since even if one sibling is homosexual this does not necessarily mean that the other siblings will be too.

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I suggest you change the question to "How can homosexuality evolve?" –  GriffinEvo Mar 27 '13 at 11:25
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I suggest a title change too. Evolution does not assume a purpose. I suggest "How can homosexuality be maintained genetically in a population?" –  kmm Mar 27 '13 at 13:07
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Why does it need an evolutionary explication? Not everything needs to be selected for. This is like asking why myopia (which in certain parts of the world can have a prevalence of >70%) evolved despite natural selection... –  nico Apr 6 '13 at 7:58
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@nico because it seems like homosexuality would be heavily selected against but it appears to be highly prevalent anyway, seems to me the OP is interested in possible explanations as to why it seems natural selection has failed to get rid of it - i.e. how can a seemingly negative trait persist? does it have some kind of benefit which may slow/reverse the effects of selection? –  GriffinEvo Apr 6 '13 at 18:22
    
@rg255 you're right. –  Brenton Horne Apr 6 '13 at 18:42
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3 Answers 3

Several years back, results of a twin study (Zietsch, et al., 2008; popular press in The Economist) suggested that genes associated with homosexuality make heterosexuals who carry the same genes more reproductively successful. This would explain the observation that, for example, sisters of homosexual males have more offspring.

We show that psychologically masculine females and feminine men are (a) more likely to be nonheterosexual but (b), when heterosexual, have more opposite-sex sexual partners.

So if more sexual partners in heterosexuals is correlated with more offspring, then the genes associated with homosexuality may lead to more offspring in heterosexuals that carry them. So these "homosexuality" genes could easily be maintained in the population by conferring a reproductive advantage to siblings.

The authors conclude:

Taken together, these results suggest that genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population.

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do the authors discuss the possible effects of having a homosexual twin sibling? being raised with a homosexual twin might influence their sexual behaviours in a social and psychological context (i.e. the number of partners, rates of courtship, courtship style...). & how to separate these effects of nature vs nurture? I guess one could compare identical twins separated at birth but this is not perfect and would be highly limiting on sample size. Also non-related adopted siblings in which one is homosexual could test the nurture effect. –  GriffinEvo Mar 27 '13 at 14:17
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Obviously selection would appear to not favour being homosexual, in an evolutionary sense it represents somewhat of a decrease in fitness: Homosexuals fail to reproduce successfully due to the requirement of both male and female gametes and reproductive organs, therefore significantly fewer (0) than the average heterosexual couple (>0). Certainly I don't think it would have evolved as some kind of population control method - group selection theory is generally discarded in favour of a gene-centred theory (see Dawkins for popular science literature)

There is debate about whether someone can be biologically "preprogrammed" to be homosexual, this can occur genetically or epigenetically. Genetic models have used kin selection, overdominance, sexual antagonism in the past and are briefly discussed in the following article on epigenetics. Recent work has looked at possible models by which heritable homosexuality could arise by epigenetic markers and this was covered in a lot of mainstream media (1,2.. just google epigenetics homosexuality). This quote is from their abstract and explains one reason it is difficult to pin down heritable causes of homosexuality:

Pedigree and twin studies indicate that homosexuality has substantial heritability in both sexes, yet concordance between identical twins is low and molecular studies have failed to find associated DNA makers. This paradoxical pattern calls for an explanation.

By their readily testable model they have shown possible (& plausible) conditions under which the "epi-marks" which cause homosexuality could spread through populations.

Genetic studies of twins where one is homosexual have also revealed links between genes and a mating advantage. For example if in a pair of male identical twins one is homosexual, the other has a mating success greater than that of the average male. However, it could be that this link is found because of social reasons (perhaps someone who has grown up with a homosexual sibling may have a different mentality or lifestyle socially which could help them increase reproductive success).


Note: I have no problem with a person's choice of sexual orientation & the rights/ethics of homosexuality is not the topic of this question & answer. This is purely an answer to the obvious evolutionary conundrum.

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Yep, neither do I have any problem. The purpose of asking this question I think is to detach any prejudice because evolution implies genetics and thus a lack of control over said orientation. –  Brenton Horne Mar 27 '13 at 13:58
    
The problem with this though is people with more conservative views think that by looking for (and finding?) biological reasons we can "cure" (not my words) homosexuality so it is an incredibly dangerous subject to get involved. –  GriffinEvo Mar 27 '13 at 14:05
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This is not a subject I know well, but I can point to several recent textbooks on homosexuality in animals, all with an evolutionary perspective. I remember that Bagemihl's book got good reviews when it was published. In general, I think it is important to acknowledge that homosexuality is common in many animal species and not just humans:

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For a good review, see Bailey & Zuk (2009) Same-sex sexual behavior and evolution. Trends in Ecology & Evolution, 24(8):439-446. (link to a pdf: thestranger.com/images/blogimages/2009/09/14/…) –  amc Mar 27 at 5:34
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