[dalke]

"If evolution didn't build in a powerful urge to make our gender behavior match our reproductive sex, then it made a huge error and missed a very easy and effective optimization."

That's not how evolution works! Evolution works at the level of genes, and not individuals. It's easy to construct a model where a 10% gay population ends up being overall better for a population. Consider this made-up hypothesis: gay people are better at caregiving than non-gay people, so a population with gay people ends up with healthier adults who are able to have more, and healthier, children. For this scenario, gayness won't be "optimized" away because that leads to worse reproductive success for the population of genes involved. Nor is the presence of gay individuals "damage", because the result is an evolutionarily better population than one without gay individuals.

As another example, why does Down's syndrome exist? By your logic, shouldn't evolution have optimized that case away? That it hasn't means that changing how the 21st chromosome works is much harder than the impact of having a 1:733 failure rate. Why do you assume that any genetic component to being gay would be easy to change, without having negative consequences elsewhere in the population?

So your error is the belief that evolution emphasizes the reproductive success of individuals, when it deals instead with the reproductive success of genes. Some individuals don't need to reproduce so long as the overall gene population reproduces itself.

BTW, 100 years ago, pink was a boy's color, and young boys wore dresses too. Quoting from http://www.smithsonianmag.com/arts-culture/When-Did-Girls-St... "yet social convention of 1884, when FDR was photographed at age 2 1/2, dictated that boys wore dresses until age 6 or 7, also the time of their first haircut. Franklin’s outfit was considered gender-neutral."

[wanorris]

> "If evolution didn't build in a powerful urge to make our gender behavior match our reproductive sex, then it made a huge error and missed a very easy and effective optimization."

I think this is actually a pretty reasonable statement, as long as it includes the caveat "on average" or "most of the time". Just as Down's syndrome isn't a huge problem for a population -- as long as it stays relatively uncommon.

A population comprised largely of people with Down's syndrome would likely be poorly adapted, and that's probably the case with a population comprised largely of gay people or transgender people as well. (Obviously, this is complete speculation, so I could be utterly mistaken.)

But yes, a population with a certain percentage of gay people could be better adapted just for having them, or alternatively, it could be better adapted because the same genetic diversity that leads a percentage of the population to be gay could be desirable in other ways.

[bermanoid]

I think this is actually a pretty reasonable statement, as long as it includes the caveat "on average" or "most of the time". Just as Down's syndrome isn't a huge problem for a population -- as long as it stays relatively uncommon.

Certainly. In the field of evolution, "on average", "statistically", or "most of the time" should be assumed to attach itself to almost every sentence (including this one).

All of this can be made much more precise, by the way, I just didn't mention it above because I already put up a huge wall of text. When it comes to deleterious mutations, there's a rule of thumb in evolution, which is to some extent mathematically provable: one mutation, one death. Statistically, what that means is that a single bad mutation will kill (where by "kill" I really mean "cause to not pass on one's genes to the next generation"), on average, one creature, no matter how bad the mutation is. If it's critical, then it will kill the first carrier before it's born; if it's not so critical, something like poor eyesight, then it will spread much further throughout the population before it kills (on average) one being.

This applies even in the face of mitigating factors. Taking the eyesight example, the fact that we have eyeglasses, and can correct poor vision, means that because poor eyesight kills less often than it did before eyeglasses the genes that cause it will spread much further throughout the population. The presence of the mitigating factor (eyeglasses) allows a potentially deadly gene to spread much further, so that on average it still kills one person per mutation.

So the fact that homosexuality has spread relatively far throughout the population either indicates that a) it is not a deleterious mutation overall (there's some significant benefit to the gene(s) that outweighs the lack of reproductive drive), b) that the mutation happens fairly often, so there are a lot of deaths due to it (this is the case with Down's syndrome), or c) that some damage-control mechanism exists so that the "death" rate is fairly low compared to the incidence of the gene.

In reality, it's probably some combination of all three possibilities; like I mentioned above, everything in evolution is statistical, so it never helps to look for single right answers.

[dalke]

I'm a bit doubtful about the statistics, and I think I know why. There's a circularity to your use of "deleterious mutations" and "bad mutation."

Is the mutation which causes sickle-cell anaemia a "good mutation" or a "bad mutation"? It increases reproductive fitness in places where malaria is or was common, so it must be good, in an evolutionary sense.

How many deaths has it caused once the population of people carrying the haemoglobin gene mutation migrated to a location without malaria? Is that mutation now "good" or "bad"? How do you incorporate those numbers into your statistics?

Is the loss of eyesight a deleterious mutation? Definitely for a bird of prey, but not so for cave-dwelling creatures living in absolute darkness. For that matter, some people are attracted to people who wear glasses (and wearing zero-prescription glasses is such a turn-off!), so it might increase reproductive fitness.

Evolution doesn't know the future. If a population loses genetic resistance to a disease that's seemingly extinct, is that a "good" or "bad" mutation? How long does it take to judge that? After 1,000 years, should some thawed carcass reintroduce it and the species become extinct, does that count finally as a bad mutation and a single death?

For a real world example, consider the birds of New Zealand. They filled ecological niches which elsewhere were filled by mammals. Were these good mutations or bad ones? And when rats and weasels and cats and more were introduced to New Zealand, helping make many of those species extinct, then did those mutations retrospectively become deleterious?

If a genetic madness affects the leader of the US Strategic Air Command to issue orders which end up nuking a dozen Soviet cities, then what are the other cases which make that average out to one? If the nuking didn't occur, then what would the average have been?

What of a mutation which causes a speciation event? Is that a good mutation or a bad one? It's better for one environment and worse for the other.

There's a 10^-9 chance (1-in-a-million) that a "bad" mutation will mutate again back to the "good" form. With nearly 7 billion people in the world, that almost certainly happens a few thousand times every generation. In a generation we may be able to cure some genetic diseases through genetic engineering, so a "bad" mutation can be fixed.

With all those in mind, I can't figure out a way to get the numbers to come out "1" unless the definition of deleterious is defined to make it come out that way.

[bermanoid]

Yes, the one-mutation-one-death idea is vastly oversimplified when it comes to the real world, so I shouldn't have presented it as being more meaningful than it is. But while it can't be taken as a mathematical truth in the unsimplified real world, the "moral of the story" will holds (that worse mutations can't spread as far as less bad ones).

It's rather simple to prove in the simplified case, it's just a typical steady state assumption. If a population is in an equilibrium state, then the rate at which any mutation is introduced has to be equal to the rate at which it is removed from the population. So if one mutation has a 1% chance to kill its owner each generation, then to maintain equilibrium (in other words, to make sure the prevalence of the mutated gene in the population is stable), every time the mutation shows up anew, it must spread to 100 people, killing one of them. One mutation, one death.

Yes, that's super simplified, it neglects the possibility of multiple mutations, positive or neutral ones, back-mutation, interactions between members of the population, non-equilibrium states, etc. These will change the details of the math, sometimes quite substantially.

But the basic idea, that the worse a mutation is the less prevalent it will be, should hold.

[dalke]

With the steady-state assumption, doesn't every beneficial mutation also lead to a single death?

[bermanoid]

Beneficial and neutral mutations are essentially left out of the equation - they would spread to 100% of the population, so in the steady state, the probability of mutation newly creating a beneficial mutation has to be 0% (since it's already present in every member).

The motivation for ignoring beneficial mutations (and back-mutations to beneficial states) is that they're extremely rare as compared to deleterious ones - most selection pressure in nature is aimed at merely preserving the functionality in the genome, weeding out new deleterious mutations rather than supporting new beneficial ones (though that is a critical role in the very long term, of course).

[bermanoid]

That's not how evolution works! Evolution works at the level of genes, and not individuals.

Yes, of course, though in many cases genes achieve their own survival by boosting the survival and reproduction rates of their hosts.

It's easy to construct a model where a 10% gay population ends up being overall better for a population. Consider this made-up hypothesis: gay people are better at caregiving than non-gay people, so a population with gay people ends up with healthier adults who are able to have more, and healthier, children.

You're invoking group selection here, which is exactly what The Selfish Gene debunked in great detail; given your comment above, I'm surprised that you would make this argument.

From the point of view of the gene, in a society that contained a 10% gay population who were better at caregiving, a gene that selfishly reduced the probability of its host's homosexuality would thrive, because not only would its carriers benefit from the caregiving boost thanks to the other members of society without that gene, they would not suffer from the reduced reproductive potential. Only in the long term, as the gene spread throughout the population, would the caregiving benefits start to fade, and that's not a present-enough change in fitness to apply any evolutionary pressure against the gene (more precisely, it can't apply evolutionary pressure because it depends on the prevalence of the gene in other members of the population; it's a classic prisoner's dilemma situation, and if you're going to take one lesson from Dawkins, it's that evolution always chooses to defect).

As another example, why does Down's syndrome exist? By your logic, shouldn't evolution have optimized that case away? That it hasn't means that changing how the 21st chromosome works is much harder than the impact of having a 1:733 failure rate.

Down's syndrome would be exceedingly difficult to optimize away, because it falls into the category of commonly-reproduced-mutation; it is not the result of code that specifically causes Down's syndrome, it's the result of our genetic material being evolutionarily close to a state that results in Down's syndrome, so whenever something goes wrong, the maladaptive trait is rediscovered over and over. Same thing with most other chromosomal disorders (most of which end up filtered out very quickly, well before birth).

FWIW, that's another common theory about how homosexuality has survived, that normal people are "one mutation away" from being gay (or rather, of having the mutation that makes them potentially gay). Both of these cases still presume, however, that the negative consequences of the trait, when combined with the probability of the trait manifesting, are negligible enough compared to the genetic changes that would be required to move us more than "one mutation away".

Why do you assume that any genetic component to being gay would be easy to change, without having negative consequences elsewhere in the population?

I quite explicitly assumed exactly the opposite. My whole comment on that matter was predicated on the assumption that it is not easy to change susceptibility to homosexuality, and that social mitigation was a workaround.

The main reason I brought up homosexuality at all was that it is often pointed to as a counterexample to the idea that reproductively negative traits are weeded out of the gene pool; I wanted to make the point that evolution doesn't necessarily need to weed out such traits directly as long as it can find some way to control their side effects.

BTW, 100 years ago, pink was a boy's color, and young boys wore dresses too.

Yup, that doesn't surprise me. I absolutely believe that much, if not most, of what signals male/female in today's society is arbitrary. However, I think that the existence of some set of traits that each sex uses to signal reproductive class is very much innate.

[dalke]

bermanoid wrote: "You're invoking group selection here" ... "it's a classic prisoner's dilemma situation, and if you're going to take one lesson from Dawkins, it's that evolution always chooses to defect)"

It's kin selection, not group selection. Consider Dawkins' "Twelve Misunderstandings of Kin Selection" wherein he writes:

"To stick my neck out a little, it seems to me that, far from genes for altruistic behaviour being implausible, it may even be that a majority of behavioural mutations will turn out to be properly describable as either altruistic or selfish." ... "A gene for altruism, then, is any gene that, compared with its alleles, causes individuals to benefit other individuals at a cost to themselves." ... "But the kind of mutation that could lead to such altruistic restraint could be ludicrously simple. A genetic propensity to bad teeth might slow down the rate at which an individual could chew at the meat. The gene for bad teeth would be, in the full sense of the technical term, a gene for altruism, and it might indeed be favoured by kin selection."

The example I gave seems perfectly aligned with this definition of altruism and kin selection. Indeed, it's a weaker but analogous form of what leads to eusociality. You say "it can't apply evolutionary pressure because it depends on the prevalence of the gene in other members of the population", .... and I think I understand why we disagree. I wrote "population" but sometimes meant "species population" and at other times meant "gene population."

In an extreme hypothetical case, suppose that having a gay sibling help to raise a family meant a 5% improved chance that each child would live to adulthood and children in turn. Suppose also that having two gay siblings meant a -1% improved chance (perhaps because the person consumes more food, which could otherwise go to the children). Then there's strong kin selection here to have some, but not all, gay children. The descendants then become a larger part of the species population.

In this case, I don't see how homosexuality would be a "reproductively negative trait" for the gene, only for some of the individuals carrying the gene.

[bermanoid]

Ah, you're absolutely correct, I did misunderstand what you were saying - indeed, your argument is a classic example of kin selection, I'm just so used to hearing group selection arguments that I jumped the gun [1] (when I wrote "given your comment above, I'm surprised that you would make this argument" that should have been my first clue that you did, in fact, know better). You're 100% right that help-out-those-with-the-same-genes altruism is not only possible, but expected, and your argument makes perfect sense in that light.

Personally, my suspicion is that homosexuality is more directly linked to a positive physical trait in the individual, though I don't have much to really back that up other than a vague sense that kin selection effects in evolution are rarely as strong as direct expressed ones. But yes, the "gay uncle" effect could explain it, too, and it's definitely an interesting enough phenomenon to be worth keeping in mind.

[1] In fact, I probably shouldn't react as negatively as I do against most invocations of the group selection argument, because oftentimes the points would be valid if expressed as kin selection arguments instead.

[dalke]

Whoa! There's agreement on the internet! :)

[bermanoid]

Being proven wrong once is worth being right a hundred times; it's only when we realize we're wrong that we learn anything useful. In this case, I was reminded of an evolutionary fact that I hadn't thought about in quite a long time, and that's fully worth being wrong.

In theory, agreements should be more common than they are (sadly) in practice: http://www.scottaaronson.com/papers/agree-econ.pdf

What I've always wondered is which particular assumption of Aumann's agreement theorem is usually lacking on the Internet: honesty, rationality, common priors, or simply the willingness to continue the conversation long enough to resolve the disagreement.

[dalke]

I lean very much against the idea of rationality, in the precise economic sense used by Aumann and others. I find the work of the behavioral economists more believable. I believe there is some truth in the saying that the only people who make economically rational decisions are economists and sociopaths.

BTW, in this thread I learned that I need to be more careful about how I use the term "population." :)