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by gwern 3635 days ago
"So they must have some positive effect on genetic fitness to even exist at all."

That's not true. There's a number of ways variants can exist without being fit. They can be regularly created by mutation, or they can increase due to genetic drift, especially in a bottleneck scenario. And there's even more ways that a phenotype can persist while being highly unfit - if it's a side-effect of a co-evolving pathogen, being one of them, as then the human natural selection is constantly fighting it but the pathogen easily evolves even faster.
2 comments
JoeAltmaier 3635 days ago
Genes can also come along with other stuff - like Neanderthal genes that (must have?) provided some benefit for living in Northern Europe, while also adding depression, alcoholism etc.

Sorry, off topic :)

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Snargorf 3635 days ago
You don't even need to look that hard.

Poor eyesight has no adaptive benefit. Yet many, many people need glasses. I guess it's just 'difficult' for our genes to grow a sharp eye focusing system, so they often get it wrong.

The same may be true of homosexuality. So even if there's nothing adaptive about being gay, it keeps happening because sexual tuning in the brain is a hard target to hit.

Homophobia may be an adaptation to this. It makes parents force their children to act straight even if they're gay.

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gwern 3635 days ago
> Poor eyesight has no adaptive benefit. Yet many, many people need glasses.

Many people... in particular industrializing countries. Our genes have a great sharp eye focusing system. You won't find much myopia in a random tribe.

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