Scientific discovery has a way of shattering our preconceptions whether we are scientists or not. That is certainly true of homosexuality. As Psychology Today
recently reported, biologists, who long ago concluded that homosexuality could not have been favored by natural selection are being forced to revise their views from two perspectives: evidence of gay
genes and gay animals.
Gay genes on the X-chromosome
Exclusive homosexuality is puzzling for evolutionary biologists because homosexuals leave substantially fewer offspring. Some marry due to social pressures and end up having children but many do not reproduce. Any genes predisposing to homosexual behavior would thus get excluded by natural selection. Current research implies male homosexuality is a sex-linked trait, although genes are far from being the whole story. (Female homosexuality is also heritable but less is known of the genetic mechanisms).
Sex-linked traits, such as color blindness highlight a curious chink in the armor of natural selection. In the normal course of events, any genetic trait that impedes survival or reduces reproductive success gets winnowed out by natural selection operating over many generations. Sex-linked traits are different because they are on the X-chromosome. Females are carriers of the affected genes (since fathers always transmit Y-chromosomes to their offspring in the course of normal fertilization). Females rarely manifest the sex-linked trait, however, because their second X-chromosome masks the mutated gene.
This fact opens up a genetic conflict of interest between the sexes. Why? Because a gene that is harmful to male reproduction can be retained by natural selection if it provides advantages to females. Indeed, if the reproductive benefit to females is greater than the cost to males, then lineages having the sex-linked gene will out-reproduce those lacking it based on simple arithmetic.
A gene promoting male homosexuality could therefore slip through the net of natural selection. For this theory of male homosexuality to be credible, at least two criteria would have to be met. First, it would have to be demonstrated that male homosexuality is transmitted through the female line after the manner of all other sex-linked traits. Second, it would have to be demonstrated that the gene increased the reproductive success of the female carriers.
In recent years, evidence has accumulated that a homosexual orientation is inherited. Study of family history reveals that homosexual men have more homosexuals in their family tree than do heterosexuals. But this is true only of ancestors that can be traced through the mother's side of the family and does not apply to paternal ancestors (1,2). This phenomenon is a smoking gun not only for genetic inheritance of sexual orientation but also for considering homosexuality as a sex-linked trait. In other words, it comes from the maternal line because that is how the X-chromosome is transmitted across the generations (male ancestors conveying Y-chromosomes only). Plenty of evidence now implicates the X-chromosome in male homosexuality but the precise genes have not been identified.
So far, the evidence for male homosexuality as a possible female adaptation lines up perfectly with the first prediction. What of the requirement that females who carry a gene for male homosexuality must enjoy some sort of advantage that allows them to out-reproduce females who are not carriers? Recent studies (1,2) have found that female relatives of male homosexuals do indeed produce more children (and the same is true of bisexual men).
Why exactly the female relatives are more fertile is also interesting. It seems that they are more fertile because they have a comparatively high sex drive. It is tempting to imagine that the high sex drive of female carriers of the putative "homosexual" gene is partly due to their greater attraction to male bodies. This idea is corroborated by similarity in the brains of male homosexuals and female heterosexuals in a part of the brain linked to sexual behavior (3).
The theory that male homosexuality evolves because the gene/s causing it increase/s the sexual activity and reproductive success in female carriers is scientifically neat.
Some researchers nevertheless argue that homosexuality can provide a net biological benefit for the homosexuals themselves whether they are males or females. They point to homosexual behavior of other species. I reserve discussion of gay animals to another post.
1. Camperio-Ciani, A., et al. (2004). Evidence for maternally-inherited factors favouring male homosexuality and promoting female fecundity. Proceedoings of the Royal Society of London, B 271, 2217-2224.
2. Camperio Ciani, A. (2009). Genetic factors increase fecundity in female maternal relatives of Bisexual men as in homosexuals. Sexual Medicine, 6, 449-455.
3. LeVay, S. & Hamer, D. (1991). A difference in hypothalamic structure between heterosexual and homosexual men. Science, 253, 1034-1037.
See Also:
Finding the Switch, by Rob Kunzig. Psychology Today. http://www.psychologytoday.com/rss/pto-20080420-000003.html
I can certainly see how a
I can certainly see how a genetic factor that increased the fecundity of female relatives might become common. However I do not understand why the gene's positive effect on female reproductive fitness is thought wedded to homosexual behavior in male 'carriers' to an extent that makes it inaccessable to natural selection.
If such a gene arose it's negative affects on male reproductive fitness would result in strong selective pressure for the female fecundity without the male homosexuality; over time the gene than links female fecundity and male homosexual behavior would be improved to greatly reduce the tendency to homosexual behaviour.
Homosexuality is orders of magnitude too common for Camperio-Ciani's theory to be tenable.
I have my doubts about the influence of female sex drive on relatively modern birthrates being applicable. Resource provisioning and scarcity must have been more of a factor than sex drive in evolutionary time. Birth order effects of sons interacting with family size may explain the results. An explaination that may be new to you - Origins of male homosexuality - Conclusion
Taking the percentages of
Taking the percentages of heredity of homosexuality from your link (30-45%), a woman who has 20 children (10 male, 10 female) who live to reproductive age would only have 3 - 5 exclusively homosexual male children, and would presumably have around 3 - 5 hypersexual heterosexual female children. The question then is whether the remaining 5 - 7 male children are typically "fit".
If an average woman carrier of the male homosexuality gene has 20% more children than the average non-carrying woman:
1.2 X more boys -> 0.84 to 0.66 X the number of heterosexual boys and 1.2 X more girls.
Taking the lower 30%, this leaves 1.02 X heterosexual children compared to non-carrying women (a selective advantage), and 0.93 X heterosexual children for the 45% figure. (Assuming ~ equal numbers of boys and girls, which is pretty close once they're adults)
Figures from the study seem to show the child advantage in women of today's era (a far cry from even 100 years ago) is ~17% to ~33% more children (for the mothers of the homosexual children and the aunts of the homosexual children, respectively).
For the aunts (assuming they too tend to have homosexual children), the higher 45% heredity gives:
1.33 X more boys -> 0.73 X heterosexual boys and 1.33 X heterosexual girls, which equals an advantage of 1.03 X the number of heterosexual children in general.
The lower 30% figure gives:
1.33 X more boys -> 0.93 X heterosexual boys and 1.33 X heterosexual girls, which equals an advantage of 1.13 X the number of heterosexual children in general.
Both of these figures are above 1.00, giving a selective advantage (in Aunts, at least) to be a carrier or sister of a carrier of the "homosexual" gene(s). Given the tendency for more Aunts in a carrying family, it seems pretty obvious the selective advantage is even greater. (Running the calculation using the 30% figure for homosexual heredity and the 17% figure for more children of carriers gives a 1.00 - on par with non-carrying mothers.)
Not all genes can be under selective pressure to change (many genes are lethal with only minor alterations). Even if they were, perhaps the easiest changes to make would encourage even more sex (and children) among women - enough that a few hundred years ago it would have put them into an early grave (too frequent childbirth being dangerous), thus decreasing their evolutionary fitness even more than normal women.
And oh yeah: sickle-cell anemia.
Eh. I'm pretty sure I
Eh. I'm pretty sure I misunderstood what was meant by "percentages of heredity of homosexuality" from the link you posted. Even so, I don't think it's a detriment to the point above.
Heredity and homosexuality
Isn't this question rather like the old discussion of why there are post-menopausal women in the population, as opposed to a situation where women died when their fertility ended? One can certainly speculate on the advantages of having in the population a group of non-reproducing people who can provide support of various kinds for those who commit their resources to reproduction. Such people may have contributed enough to the group's survival to make up for their consumption of food, and thus made it more likely that recessive or sex-linked factors would stay in a population that was already carrying them.
Heredity and homosexuality
Isn't this question rather like the old discussion of why there are post-menopausal women in the population, as opposed to a situation where women died when their fertility ended? One can certainly speculate on the advantages of having in the population a group of non-reproducing people who can provide support of various kinds for those who commit their resources to reproduction. Such people may have contributed enough to the group's survival to make up for their consumption of food, and thus made it more likely that recessive or sex-linked factors would stay in a population that was already carrying them.
Sorry---didn't mean to post
Sorry---didn't mean to post it twice.
hey! so you posted a 3rd
hey! so you posted a 3rd time!
just kidding. that is really interesting. gay people being around because they help take care of kids. i got that right, right?(twice!) that would also make them a repository for unwanted children aka adoption! i wonder if there is anything linking gay people to being more/less willing(or just as willing as everyone else) to take care of offspring.
Useful Speculation? I have my doubts . . .
I'll admit off the bat: I don't know Camperio-Ciani's work. Nevertheless, from this summary it sounds immensely simplistic, conjectural, and reductive. As for Hamer's and LeVay's absurd, outdated claims about the hypothalamus, that's been widely discredited, not least because, of course, they could study only dead subjects.
As for psychological variance, including a constitutional capacity for bisexuality, is that something evolutionary psychology could ever get its head around? I guess you missed Jesse Reynold's excellent post just after yours, a superb rebuttal, pointing out that the relation between genes and behavior is *astonishingly* complex: it's not all about oversimplified claims for biological determinism.
What's most interesting about this post, to me at any rate, is that it painfully reveals the blind-spots of evolutionary psychological thinking. Evolution, for you, *has* to serve some adaptive end (Darwin's "Descent of Man" actually suggests otherwise), so if homosexuality strikes you as nonadaptive (meaning, in this case, nonprocreative), then there *has* to be a biological explanation for that. But why must there be? And why must the answer be genetic?
What if you shifted your thinking and allowed for behavior that wasn't exclusively biologically driven? You guys would have nothing to say, because evolutionary psychology has written itself into such a corner that it has to claim/speculate on biological explanations regardless. And so it'll be doing this ad infinitum--and we'll have to wade through this crap without end: speculation on homosexuality and twins; speculation on homosexuality and wombs; speculation on homosexuality and genes.
Do us all a favor, eh, and if you must speculate on something, start speculating on why people become exclusively heterosexual. Is there a gene for that? Could we have some studies on twins and wombs and the hypothalamus of exclusive heterosexuals? Because a wee rethinking will reveal that procreation for evolutionary ends doesn't necessitate *lifelong* heterosexuality or even heterosexuality as a sexual preference. Certainly, it doesn't begin to explain variances in sexuality. Our sexuality is just a tad more complicated than that.
Questioning what factors make
Questioning what factors make people heterosexual would certainly open up some new ways of looking at this issue. For that matter, we usually neglect to ask what makes human beings want sex partners at all, when our opposable thumbs make masturbation effective.
same reason making a joke is
same reason making a joke is funner and funnier in a room full of people than making the same joke in response to something on tv, all alone by yourself.
since when do you need thumbs for that? just think of all the examples. :o)
Nature vs. Nurture
This article seems to hark back to the old nature vs. nurture debate. Although it is interesting, we should be careful in that we cannot forget that nurture does play a role into this; it isn't all just nature.
homosexual gene
I have met more gay and bisexual men with ancestry in a small group of towns in a remote section of New England. They include a family with 7 gay/bi brothers and a lesbian sister; a set of twins; an uncle and nephew and at least 30 others whose family history I do not know. Do I think there is a genetic factor? Most certainly, but without any scientific evidence to back me up. It is worth studying.
Homosexuality may be important for families (sounds odd I know)
I just read an interesting study today about how same-sex attraction is linked to evolution.
http://www.sciencedaily.com/releases/2010/02/100204144551.htm
It essentially says that gay men, studied in particular, are considered important to the proliferation of a family's genes even though the men can not reproduce being gay.
It found that gay men are more helpful within a family, such as being more available to nieces and nephews for babysitting, tutoring, and other support. This altruism increases the survival of the family's genes.
The study focused on Samoan homosexuals, who are considered important members of the family. It may or may not have a connection to other cultures, nevertheless and interesting study!
lol
sounds like someone just doesn't like evolution. i love how christian apologists scout these sites and try to discredit everything as a desperate attempt to save their failing ideologies. amusing. very amusing.
That was uncalled for..he
That was uncalled for..he made some valid points and he's entitled to his opinion. You on the other hand have not added anything beneficial to the discussion. Please leave the bickering for another site/time/discussion where it is actually called for.
Why don't you actually offer
Why don't you actually offer an intelligent rebuttal to his remarks rather than perpetuate your own religious discrimination here?
This is an interesting
This is an interesting subject and worthy of more study. I just hope that it won't become taken as an apologietic or a reason for "social evolution" until all the facts are in place.
I am interested in knowing what is the role of nature verses nurture, both in making a man a homosexual, and the apparent higher sex drive of his mother?
And what about issues like screening for environmental factors (eg. estrogen like substances) polluting the environment and affecting the foetus? We know this happens. This question applies to the mother when she was in utero herself, as well as her homosexual child.
Also, is it truely a benefit to reproduction (how do those non oversexed women out there ever get around to it after all...^_^) or just something that is't harmful enough for nature to deselect?
Finally, has anyone done any serious research into whether there might be any inherited weaknesses in the human genome that will cause the same mutation to keep happening from generation to generation? After all, the theory is that many of these traits arise by some sort of a mutation (their words, not mine) and the beneficial ones persist, but are mutations truely completely random, or does there exist within the genes a tendency to reproduce certain human conditions, both good and bad, more often than other possible conditions? Just curious.
Frustrating misinformation...
I wish someone would proof read these articles before they get posted.
"Females are carriers of the affected genes (since fathers always transmit Y-chromosomes to their offspring in the course of normal fertilization). Females rarely manifest the sex-linked trait, however, because their second X-chromosome masks the mutated gene."
This may be a fact if the offspring are female, however fathers also transmit Y-chromosomes to their MALE offspring. So it is possible that the affected gene came from the father. Yes he got that gene from his mother but to say that women only carry the gene is fraudulent at best.
There is no real proof for genetics causing homosexuality
Many cases in families are originated because of copying the gay behavior as " good ". Children´s minds can´t make a difference of what´s good what´s bad. Example, if the father is a thief children that behaviour is normal, that´s what they see.In other words, it´s the environment and not really the genetics which can cause homosexuality.
You really have no idea what
You really have no idea what you just said. Homosexuality is not derived from other homosexuals. How do you explain the homosexuals that came from heterosexual, even homophobic parents and family? Or the straight children that grew up in homosexual families? I have two moms, and I'm a straight woman. I'm not attracted to females. I've been told all my life that it doesn't matter whom I'm attracted to as long as I love them and they love and treat me well- that's not going to change the fact that I find males sexually appealing and females simply as friends.
My birth mother, who is a lesbian, has a gay cousin on her maternal side. My other mom has a gay uncle. Neither of these gay family members had much contact with my parents when they were younger, if at all. Nor was this "behavior" you speak of even mentioned. So please, do try to explain yourself with a little more...sense.
Why does the argument of nature vs. nuture still exist?
I just recently wrote a paper where I utilized Camperio - Ciani's study as to whether or not the gay gene existed or if this was still a nature vs. nuture argument.
This paper, coupled with other research, truly supported that being gay is a genetic issue and the decision is made for us before birth.
I don't understand why people can't understand this and begin to treat everyone equally regardless of sexual orientation.
biology mistake in article
"Females are carriers of the affected genes (since fathers always transmit Y-chromosomes to their offspring in the course of normal fertilization)" BIO FAIL: Fathers pass on one of their X and Y sex genes, sometime the X, making a baby XX girl with one of mum's pair of X, sometimes the Y, making another XY boy with one of mum's Xs. This is how it is for at least 90% of humans, with plenty of other variations of course, but the statement that fathers always transmit Y chromosomes is a major fail in this article's credibility, agree though I do with its conclusion that non-reproductive sexualities are actually positively selected for by the processes of evolution for the advancement of the wider tribe (or gene pool).
Both this article and the
Both this article and the discussions posted are really very interesting, I admire everyone here very much. The one thing always keeping me from "understanding" a gene for homosexuality (homosexuality from an evolutionary perspective) is that I just didn't see how it didn't get weeded out at some point since, clearly, it isn't ideal for reproduction. This article provided a beautiful rebuttal to my concern. Of course, in the argument that one is more likely to be homosexual if their relatives are, it is definitely likely that their homosexual relatives provided a model and paved the way (in terms of family acceptance), adding a nurture perspective.
Thank you for the interesting ideas and discussion, everyone. Really great stuff.
I remember reading this
I remember reading this briefly in the article Finding The Switch. Not only have I rehashed that article multiple times at my own blog I have actually read evidence against any biological explanation as to why homosexuality exists. In a nut shell sexuality can be seen as a mixture of both nature and nurture but the evidence sits highly on the side of nurture factors from my most recent read in My Genes Made Me Do It by Neil and Briar Whitehead. However I am very open to new discoveries and articles like this one.
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