A tad of gay holds sway

Why are there homosexuals? According to Darwinian thinking, a genetic trait that reduces the reproductive success cannot endure in the long run. The answer sounds crazy: blood relatives of gays and lesbians have more offspring. 

For evolutionary biologists interested in homosexuality there is one big puzzle: That it exists at all, although it should not exist according to the basic assumptions of this discipline. Only such genetic dispositions that increase the reproductive success and the genetic fitness of their carriers can succeed in the evolutionary struggle for existence. But the need to have sex with members of ones own sex is not a good recipe for childbearing. The results of a large-scale study from Australia  offers an explanation for the Darwinian paradox: The gay disposition lasts in the gene pool, because in low doses it increases the sex appeal - and thus the genetic fitness -  in the heterosexual relationship of gays. 

A significant aspect of the homosexual lifestyle is that those affected produce less offspring than most heterosexuals. New surveys indicate that avowed gays have five to ten times fewer children than  the rest of the population. According to a new Australian study, among gays over the age of 50, only about one half had ever raised children. "Homosexuality is practically a form of sterilization", says psychologist Qazi Rahman from Queens College in London. 

These figures represent a major challenge for any genetic theory, because a genetic trait that reduces the reproductive success would be mercilessly eliminated by natural selection, explains psychologist Edward M. Miller of the University of New Orleans. That homosexuality has a genetic basis is evidenced by comparative studies. A study by US-researchers J. Michael Bailey and Richard Pillard reached a clear conclusion: With identical twin brothers of homosexuals, the probability that they are gay too amounts to 52 percent, with fraternal twins it is 22, with adopted siblings 11 percent. 


So can we estimate the proportion of genetic influence on homosexuality? Only to a degree. The estimates range from 31 to 74 percent heritability in men and 27-76 percent heritability in women. The interpretation of these statistics is made  difficult by the fact that no precise figures on the incidence of homosexuality exist, because it is difficult to define who actually is homosexual. Nevertheless, the geneticist Dean Hamer of the National Cancer Institute in 1993 made a splash with the message that he had identified a “gay gene” on the X-chromosome which is inherited from the mother. Of 40 pairs of homosexual brother, 33 had  five common markers in a section of DNA called Xq28. 

The alleged gay gene became so popular, that T-shirts with the words "Xq28 - Thanks for the genes, mom” were worn in the gay scene in San Francisco. Six years later came the disillusionment: Researchers among George Rice at the University of Western Ontario reported that in a study of 52 homosexual pairs of brothers, the alleged connection between homosexuality and Xq28 could not be found. Very likely, the disposition for homosexuality is not mediated by one gene, but by a whole potpourri of genetic factors, each of which makes only a moderate contribution to sexual fate and is therefore very difficult to pinpoint in the "haystack" of the genome. 

The question remains why a fitness-lowering gene combination is not extinguished. One possibility is that homosexuals propagate their own genes "through the back door". Maybe they engage very "selflessly" for their blood relatives. Their increased reproductive rate would have the effect that the "selfless" genes spread in the population and would increase the fitness of homosexuals. But this is just theory, noted anthropologist Volker Sommer of the University of Göttingen. "No one has ever counted how many additional children the parents, brothers or sisters of homosexuals raise, thanks to their help" Moreover, there is a rub in this theory, says psychologist Edward M. Miller: If the genetic merit of being gay would be the fitness increase in relatives, homosexuals would be better off by being completely asexual, and not to assume the risks and costs of the homosexual lifestyle. 

Miller has proposed an alternative explanation quite a few years ago. Genes always exists as doubles  on chromosomes, in the form of two alleles. Genetic factors that promote homosexuality can survive in the gene pool if they mostly occur  in a heterozygous (coupled with other alleles) form and increase the reproductive success of their carriers in this combination. Only in the rare cases where the inheritance is  homozygous –  both alleles are identical - homosexuality emerges and reduces fitness. 

A man who carries a small dose of gay genes in his genome would, according to the theory, improve his success  in the heterosexual mating game. That “certain something” that heightens sex appeal probably consist exactly of those essentials which make homosexuals different from heterosexuals in the first place. According to his theory, the alleged "gay genes" equip men who carry  the heterozygous disposition with an above-average degree of feminine traits such as sensitivity, gentleness and friendliness. Gay genes therefore form a natural antidote against "hypermasculine" genes that turn men into rough machos. They would promote properties that appeal to women and indicate a good suitability as a father and significant other. A lesbian disposition lends women reversed traits that helps their reproductive success. Surveys have already shown that psychologically "masculine" women have more sex contacts. 

Imagine, for example, there were five genes, each of which occurs in duplicate and increases the probability of homosexuality, Miller speculates. Only if a man had all five alleles in duplicate, he would be gay. "That would be an event that occurs with a probability of 1 to 32, meaning in 3 percent of all men." Such a system would already be evolutionary stable if a hint of homosexual disposition would increase the genetic fitness of heterosexuals by only 2 percent. 

What hitherto was pure academic speculation, a team led by epidemiologists Brendan Zietsch from Brisbane in Australia has empirically underpinned with a study of 5000 twin siblings. Metrosexuals,  who in their appearance and lifestyle mix male and female characteristics, are the genetic proxies of homosexuals.
The male and female subjects provided information about their personality traits, their sexual orientation and their total number of sexual partners. 2.2 percent of men and 0.6 percent of women admitted to having a purely gay or lesbian gender identity. There were also 13 percent male and 11 percent female "nonheterosexuals” who reported dating with both sexes. Crucial point: Both the siblings of homosexuals and those of nonheterosexuals possessed remarkably many personality traits of the opposite sex. And they also had a greater number of sexual partners than the siblings of heterosexuals. In the evolutionary past, before the invention of the pill and family planning, they should have had a particularly big reproductive success. 

The androgynous personality traits and above-average rates of sexual contacts which characterized these men and women were, according to the researchers' calculations, primarily due to genetic factors and not to environmental influences. The genetic vacancy which is caused by the reduced reproductive success of homosexuals is probably offset by the increased rate of reproduction achieved by their blood relatives. This, incidentally, also explains a puzzling fact which scientists previously could not figure out: Homosexuals have a larger than average number of relatives. This was first demonstrated for the maternal side, but is also true, according to the latest data, for the paternal side, perhaps even stronger. 

According to the results of psychologist Andrea Camperio Ciani from the University of Padua, not only the siblings, but also the mothers and aunts of homosexuals are offsetting their "reproductive shortcomings". They not only had a larger than average number of births, but had also been affected particularly rarely by miscarriages and infections. Maybe they are blessed with genes that produce a particularly strong "love of men". This would be conducive to their genetic fitness, as it would encourage them to have more children. With their sons, this aptitude could trigger an outright homosexual orientation. But even if those were to become reproductive “underachievers”, that could still be evolutionarily adaptive for mothers: If the same genes would procure them - and their daughters - a larger swarm of children.

The fact that there are gene variants that provide a fitness benefit if they are "heterozygous" and therefore occur only in one edition of the genetic double set has long since been known in biology. Homozygous carriers on the other hand, who inherit the gene from both parents, are exposed to the fitness reducing effect of this dual system. The best-known example of the so-called heterozygous advantage is the gene for sickle cell anemia. People who carry this mutated gene only at one of the two loci are characterized by increased resistance to malaria. This combination can thus be found in clusters in the regions of Africa where malaria is also rampant. The gene for cystic fibrosis, a disease that makes the mucus thick, has been found to possess a hidden heterozygote fitness advantage too: It raises the resistance to diarrheal diseases. In both cases, the genetic factor therefore does not disappear from the gene pool, although it reduces fitness if present in duplicate.

Update: Now this theory is also supported by an experimental animal model. In Drosophila melanogaster, males with a genetic makeup associated with high levels of same-sex sexual behavior produced female offspring with higher rates of reproduction, or fecundity.

http://rspb.royalsocietypublishing.org/content/282/1809/20150429

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