[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).

[dalke]

I still think that your definition of beneficial and deleterious is defined post-hoc as "survives/does not survive long enough to reach the stable state."

In my example earlier regarding extinct bird species on New Zealand, were there ever any beneficial mutations? After all, the genes no longer reproduce.

My point is that there is no stable state, so it's better to have a shorter-term definition of "beneficial" and "deleterious" based on relative reproduction fitness compared to others in the species population over a short time frame.

Additionally, beneficial and neutral mutations do not always spread to 100% of the species population. A Y-linked trait won't spread without a male lineage.