The scientific journal NATURE (Jan. 11, 2007, p. 158) has published a model that assumes the existence of a homosexual gene. The observed 1% incidence of homosexuality in the general population (the frequently bandied 10% is what Kinsey observed at the University of Indiana) is more than expected from random mutation, so it could have a genetic component.
Since a homosexuality gene is sterile, in principle it would be quickly eliminated from any population. Since it does persist, it must have some survival value. The authors suggested three possibilities: overdominance, sexually antagonistic selection, and kin altruism.
Overdominance is analogous to sicle cell anemia genetics. Homozygotes for this allele are moribund, but heterozygotes are resistant to malaria, so the gene persists. For example, homosexuality gene could impart superior intelligence when heterozygous. The tenfold increase in the incidence of homosexuality in the academic environment mentioned above lends support to the idea.
In sexually antagonistic selection, a gay allele would would result in a cost for males (feminization and loss of fitness), which would be counterbalanced by fitness advantage when expressed in females. This possibility seems unlikely, as it would lead to a higher incidence of bisexuality than observed.
In kin altruism, homosexuals would help their own family members, increasing the fitness of their relatives and therefore the probability of the gay gene being passed on to the next generation. This scenarion is viable. It is analogous to a beehive, where the sterile workers help with the offspring of fertile queens, to the benefit of the whole hive.
Future observations will determine the costs and benefits of homosexual behavior in natural populations to decide which of the preceding models, if any, is valid. Costs there must be, as in aboriginal societies leading marginal existence homosexuality is absent.
