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CellBioGuy comments on When does heritable low fitness need to be explained? - Less Wrong
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This is true (noted also by Douglas_Knight here). It's a little weakened by the fact that homosexuality likely has several different possible causes, which should also be considered separately. The loss of fitness due to homosexuality is also much less than that due to severe birth defects. I think given these initial data, the question still deserves a quantitative analysis. But I can't really judge because I don't know the relevant math - that's what I hoped someone would give in response to my post.
From the gene's eye view, an allele isn't guaranteed to be selected during recombination. It seems to me that after conception, natural selection should operate on the fitness of the actual child as well as the inclusive fitness of the parents affected by raising the child. But I'm just handwaving and guessing here.
It does?
Since when is anything in biology, neurology, or psychology simple?
Lots of things are simple. If the world is not simple, inference is impossible. Many things turn out to be straightforward; as complex and intricate a phenomenon as AIDS is, 'HIV causes AIDS' is much more accurate than 'AIDS is not determined by any one factor but by a combination of genetic, hormonal, and environmental influences; in recent years, biologically-based theories have been favored by experts...' In statistical modeling, it's far from surprising to discover that a few variables have most of the predictive value and that it's only the last few percent which require extreme complexity to predict or model.
You have explained why inference is hard in biology :-)
A technical term for the "problem" is pleiotropy.
Many small scale biological features are re-used over and over, if they break, many things can break a bit. Primary ciliary dyskinesia is an example if this. In the meantime, many complex adaptive structures (like "the ability to hear") are caused by more than one subcomponent, so any of several different subcomponents breaking can produce a symptomatically similar disruption of the complex structure.
Biological causes and biological outcomes are in a many-to-many relationship, with lots of "best effort" failover systems as backups. The amount of effort to put into fixing up a structure is itself something that most of the animal kingdom has optimized a bit, for example via the poorly named "heat shock proteins" that suppress mutational expression in good times but reveal the mutations in bad times.
In the case of male homosexuality, one cause that I recall hearing debate about was that a male child causes a mother's body to change (current best guess is something immunological), such that later male fetuses appear to have their sexual development mildly disrupted. If I recall correctly, the process looks probabilistically cumulative, so that there's something like a 1/3 chance of homosexuality by the time you get to the fifth or sixth male child from the same mother. Again, if I recall correctly, with modern demography this effect might be enough to account for ~20% of gay men? This is somewhat controversial, but le wik has some of the debate.