And traditional behavior gives us an imperfect window into the economics of the past, which is what's under discussion when we talk about historical selective fitness.

I think we should keep in mind just how far back we're talking. I'm not saying we inherited homosexuality from our common ancestor with the modern fruit fly, but at least our common ancestor with other great apes. Framing the question as why would it be selected for in the context of human societies is probably wrong, when what we want to know is why it wasn't sufficiently selected against given it already existed (I doubt we'll ever figure what advantage it gave the proto-ape whose social structures we'll never know). Once a trait already manifests in 3% of the population, it takes work to get rid of it, and even within that 3%, it was doubtful the case that 0% of them reproduced while 100% of heterosexual men reproduced. I'm sure it wasn't exactly parity, but it's possible there is no explanation in terms of the organization of human societies except for we're really optimized to enjoy sex, sometimes that wire gets flipped, and it doesn't provide an advantage, but it also doesn't give enough of a disadvantage to completely disappear within 300,000 years.

Don't forget also, that if some gene combo is necessary but not sufficient, and requires other developmental factors to manifest that don't manifest in your brothers and cousins (which seems to be the case if it's only 20% between twins), then when they reproduce, even if you don't, the gene still gets passed on. Take me, for example. I'm not gay, but I am sterile and don't want kids anyway. Nonetheless, I have 3 sisters and 13 cousins that have had kids so far. Without doing the exact math, off the top of my head I'm guessing at least 80-90% of whatever I'm carrying made it to the next generation.

Edit: Also, one last thing is we don't know the prevalence in the ancestral population. Given it's roughly 100% bisexual in such a closely related other species, it could have been fairly high in the common ancestor, obviously not 100% obligate, but more than 3%, and it actually has been selected against, a lot, just not enough to get us to zero yet.

When does heritable low fitness need to be explained?

by DanArmak 2 min read10th Jun 2015147 comments


Epistemic status: speculating about things I'm not familiar with; hoping to be educated in the comments. This post is a question, not an answer.

ETA: this comment thread seems to be leading towards the best answer so far.

There's a question I've seen many times, most recently in Scott Alexander's recent links thread. This latest variant goes like this:

Old question “why does evolution allow homosexuality to exist when it decreases reproduction?” seems to have been solved, at least in fruit flies: the female relatives of gayer fruit flies have more children. Same thing appears to be true in humans. Unclear if lesbianism has a similar aetiology.

Obligate male homosexuality greatly harms reproductive fitness. And so, the argument goes, there must be some other selection pressure, one great enough to overcome the drastic effect of not having any children. The comments on that post list several other proposed answers, all of them suggesting a tradeoff vs. a benefit elsewhere: for instance, that it pays to have some proportion of gay men who invest their resources in their nieces and nephews instead of their own children.

But how do we know if this is a valid question - if the situation really needs to be explained at all?

For obvious political and social reasons, it's hard to be sure how many people are homosexual. Note that we are interested only in obligate homosexuality - bisexuals presumably don't have strongly reduced fitness. The Wikipedia article doesn't really distinguish obligate homosexuality from bi-, pan- and even trans-sexuals. The discussion in the SSC comments used an (unsourced?) range of 1%-3%, which seems at least consistent with other sources, so let's run with that.

The rate of major birth defects in the US, as reported by the CDC, is also about 3%. This counts both developmental and genetic problems, and includes everything from anencephaly (invariably fatal) through Down syndrome (severe but survivable) to cleft palates (minor). But most of these, at least 1.5% of births, were always fatal before modern medicine, and many of the others reduced fitness (via mate selection, if nothing else). Various other defects and diseases, which only manifest later in life, are also thought to be influenced or determined during early development. And so is sexual preference.

(Whether homosexuality is a developmental disorder is not the point; I'm comparing the effect of selection pressure on fatal teratology with its effect on reduced-fitness homosexuality.)

Embryological development is a complex and fragile process, and there are many ways for it to go wrong. We don't wonder how it is possible that selection pressure allows anencephaly to occur in 1 in 4859 births. There are certainly direct causes of anencephaly, explanations of why it happens when it does, but (I think) we don't a priori expect them to be due to tradeoffs yielding benefits elsewhere. It's just as plausible that the tradeoffs involved are against even worse (counterfactual) problems elsewhere - or that there are just no available mutations that don't have these or equally severe problems.

Could it be that linking sexual preference to the biological gender is, for some complex developmental reason, fragile enough that it goes wrong despite all selection pressure to the contrary, that it has no redeeming qualities from the viewpoint of evolution, and that is all there is to it?

When faced with any phenotype with reduced fitness, how can we judge if there is something to be explained - a beneficial tradeoff elsewhere to search for - or merely a hard problem evolution couldn't solve completely? And is there a way to quantify this question, relating it to the known mathematical models of genetics?


1. I'm posting this in the spirit of recent suggestions to post more and accept lower quality of (our own) posts to Discussion.

2. I'm going to sleep now and will start replying to comments about 10 hours from now; sorry for the inconvenience.