A paradox that isn’t

This question was posted on a forum, and I’d originally planned to just ignore it. It bothered me later.

Why are there homosexuals? Natural life has no purpose for homosexuals and thus evolution would have wiped them out.

Let’s establish the basic assumption that would allow us to go on and examine the question. That being, there exists a genetic component to homosexuality. Let’s also be clear that this assumption, supported in the literature, doesn’t necessitate or imply a “gay gene”. More likely, a behavioral phenomenon on this order of complexity is tied to a whole host of genetic and regulatory factors that in a combinatorial fashion, increase or decrease an individual’s propensity towards the given behavior. It’s a statistical exercise, riddled with anomaly. Certainly not as cut and dry as some would like to believe, in terms of simple experimental pathways molded by rows of binary switches. In certain lower organisms, simpler explanations could be seen to hold weight. For example, in Drosophila, point mutations in well-characterized genes are firmly and directly linked to homosexual behavior.

Back to the original question. How might a phenotype significantly decreasing the probability of eventual offspring be explained by means of natural selection? How would the genotypic basis of this phenotype survive and avoid dilution? It’s a fair question, but it also doesn’t require very much creativity to propose a number of satisfactory models and explanations.

As a starting point, if the etiology of homosexuality arises from a collection of traits, one could imagine how the optimal regulation of various loci in concert could- as a relatively insignificant consequence- result in a subset engaging in a specific behavior not aligned with any given definition of evolutionary fitness. The trade-off would be beneficial to the population as a whole, as the robust fitness advantages conferred to a majority would outweigh the statistical risk of sexually non-productive members weighted within the gene pool.

Some specific hypotheses, differentially supported in the research literature, propose explanations such as overdominance, epigenetic maternal effects, kin selection (refuted), and sexually antagonistic selection. Conveniently, instead of describing and speculating on the strengths of each argument, we can reference a populations genetics paper that goes about testing each model in a very rigorous manner (Camperio et al. PLoS ONE. 2008 Jun 18;3(6):e2282).

Antagonistic selection and Darwin for the win.