Let me start this post off with stating that this is completely a factual and empirical post and that it has nothing to do with my personal views on homosexuality. Also, this is dealing with the genetic causes of homosexuality, which is only one of the facets in the ontogeny of sexual orientation in an individual. There are countless other factors and causes outside of genetics, but they are not relevant here.
That being said, scientists have been stumped for years because from an evolutionary perspective, homosexuality shouldn’t exist. Every species is governed by the evolutionary force of natural selection. Selection chooses between variations in a population to select traits that will most help an individual to survive it’s environment and improve its reproductive fitness. (Side note for those unfamiliar with biology, reproductive fitness is how successful an individual is in reproducing and passing their genes on to the next generation.) Keeping all of this in mind and continuing with this logic, homosexuality is an obvious disadvantage to individuals. By being homosexual, individuals prefer their own sex, with who it is impossible to mate with, effectively decreasing their reproductive fitness to a nonexistent level. Normally, when a trait is this harmful to an individual’s reproduction fitness, evolution will select against it and eventually (but still rather quickly), it will disappear from the population entirely.
Despite this logical argument, homosexuality persists in contemporary society, stumping scientists and leading to several explanations as to its continuing existence:
In 1959, ecologist G. Evelyn Hutchinson proposed that gay genes were still persistent in society due to a heterozygous advantage. This hypothesis states that there are different alleles for the gay gene – individuals who are homozygous for this gay gene are more likely to become homosexual (in combination with other factors) and therefore suffer a reduced fitness, while individuals who receive two different alleles for the gay genes and are heterozygous, are heterosexual and experience an average higher fitness than normal. This gives individuals who are heterozygous for the gay gene an advantage over those individuals who don’t possess the gay gene, thus maintaining it in the population.
In 1975, Edward O. Wilson suggested a different hypothesis. This time, he proposed that homosexuality still persisted because it encouraged altruism and kin selection. Both of these mechanisms can be found in other species throughout the animal kingdom. The simplest explanation is that rather than invest in their own reproductive fitness, individuals abandon mating efforts to help and assist in raising the offspring of close kin, like parents or siblings, who share a large portion of the individual’s genetic make-up, allowing them to pass on their genes without actually producing any offspring. However, it appears that homosexuals do not have an overwhelming inclination to contribute to the reproductive success of their relatives compared to others enough to outweigh the costs to their own fitness. So while this hypothesis could help explain the continuity of homosexual orientation in contemporary society, it could not stand alone and would have to be in combination with other factors.
In 1991, it was suggested by two scientists, J. Michael Bailey and Richard C. Pillard, that homosexuality in both men and women was a polygenic trait, meaning that it wasn’t controlled at one, single genetic locus. If this is the case, then it is possible that natural selection wasn’t favoring homosexuality, per se, but rather favored the genes for testosterone production or some other adaptive trait that is also associated with some of the same genetic alleles. If a certain combination of these alleles at multiple loci can help contribute to homosexuality than it would continue to persist, despite its detrimental effects to a individual’s fitness.
Lastly, a hypothesis of antagonistic pleiotropy has been recently proposed and revised in both 1994 and 2004. This refers to that though a gay gene could decrease the fitness of homosexuals, it could increase the fitness of their closest, non-gay, male relatives (brothers, fathers, uncles etc.) An alternative to this, is that it increases the fitness of sisters and other female relatives of homosexuals. This alternative is especially probably as research shows that the gay gene most likely resides on the X chromosome, which females posses two copies of, unlike their male counterparts. In a study at the University of Padova in Italy in 2004, researchers found elevated reproductive success in the maternal relatives of homosexual men (both male and female) — significantly higher reproductive success of the mothers, maternal aunts, and non-gay maternal uncles of homosexual men, supporting the antagonistic pleiotropy hypothesis.
Puts, David A. “The Ontogeny and Evolution of Homosexuality.” The Evolution of Human Sexuality: An Anthropological Perspective. Second ed. Dubuque,Iowa: Kendall/Hunt, 2009. 278-94. Print.