| MadSci Network: Evolution |
Before getting into the specific question of fitness, here's a list of some other responses that have covered most of the genetic aspects of homosexuality:
So let's look at the occurance of homosexuality empirically, and see where it fits in genetics and evolution. First, homosexuality exists in most (if not all) human populations, and appears in several early civilizations, especially ancient Greece, suggesting that it is not a "modern" trait. Furthermore, homosexuality is seen in bonobos (Pan paniscus), suggesting that the trait predates civilization, as well as the strictly human ancestry. Continuing, in his book "Biological Exuberance", Bruce Bagemihl gives instances of homosexual behavior throughout the Vertebrata, and work done on the fruitless gene in fruitflies has demonstrated the existence of the trait in Arthropoda (Yamamoto D, Ito H, Fujitani K (1996), "Genetic dissection of sexual orientation: behavioral, cellular, and molecular approaches in Drosophila melanogaster." Neurosci Res 26(2):95-107). More interestingly, there is evidence that some of these broader instances of homosexuality occured independently. The general lesson is that the appearance of homosexuality coincides with the appearance of courtship behavior, raising the question: is homosexuality an independent trait, or is it simply an inseparable aspect of courtship.
Clearly, if homosexuality appears simply as a result of expanding courtship behaviors, then the fitness aspect is moot - the fitness gains of courtship for the population far outweigh the loss of a small percentage of individuals from the breeding pool. So, for the sake of argument, let's assume that homosexuality is an independent trait, and examine it evolutionarily. As demonstrated above, homosexuality exists throughout the animal kingdom, so it must have either a positive or neutral effect on fitness. On the other hand, the breeding pressure against homosexuality would suggest that it only enhances fitness when present in small portions of the population, i.e. if the precentage of strictly homosexual individuals is high enough to lower the birthrate below the deathrate, the population will eventually die off. Furthermore, many individuals that engage in homosexual activities are not exclusive, and still engage in sufficient hetersexuality to produce offspring (as is evident within our own species), such that it cannot be considered a truly sterilizing trait.
One of the major arguments regarding the fitness of homosexuality, at least in vertebrates, seem to revolve around child rearing. Caring for offspring is very expensive per individual caregiver, while yielding tremendous evolutionary benefits to the population, by directly enhancing the fitness of the next generation. Because the cost to the individual is so great, many populations have evolved ways of sharing the costs of raising the offspring, such that the benefits to the population are equalized by general costs to the population. In several social mammals, this is done through wet-nursing by non-pregnant females of related newborn, sometimes even requiring the forced abortion of the litters of subdominant females to produce more wet-nurses (seen in some Wolves). Likewise, removing a portion of the population from breeding relieves the breeders of some of these costs either directly by assisting with child rearing, or indirectly by taking over other costly activities (like food collection) so that t he parents can spend more resources on their progeny.
There is also evidence of homosexual activities playing a role in maintaining dominance heirarchies, reinforcing familiarity, and playing other roles in organizing and maintaining the societies of some of the more social animals. While the absolute evolutionary value of homosexuality varies greatly between species, it is certainly common enough to suggest a contribution to fitness, even if not directly measurable.
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