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.
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