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.
Finding the Switch, by Rob Kunzig. Psychology Today. http://www.psychologytoday.com/rss/pto-20080420-000003.html