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ChristianKl comments on A Medical Mystery: Thyroid Hormones, Chronic Fatigue and Fibromyalgia - Less Wrong
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(1) Sure, but that sort of thing will just random-walk, it would take ages to go from one mutation to 50% of the population. It has almost no fitness effect. It will probably get gambler's-ruined out.
(2) Absolutely, and we see those things in animals. You can evolve to extinction. In the particular case of a male-causing gene, I think it would have to stabilize very low (because the more successful it is the more harmful it is to the carrier) , but you can certainly imagine (and find) driving genes that just become rapidly prevalent and wipe out the species.
(3) Yes, but that's just the random walk walking. It has to get very lucky to become prevalent, and if it's actively harmful, it won't get that lucky, and that will kill it off eventually.
A mutation needs an edge to spread fast.
The official name of a mutation winning despite having no selection benefit is genetic drift. When I had genetics lessons in university the concept that was taught was that a significant amount of our genetic changes are due to gene drift but there's no exact way to quantify how many.
Furthermore some genes aren't stable and can easily mutate. Evolution doesn't succeed in bringing color blindness to zero despite it being no useful mutation.
Yes, an obvious one is the inability to manufacture Vitamin C. Universal in great apes, including us, but every other animal and plant can do it, except guinea pigs.
I imagine that at some point our ancestors lived in a vitamin C rich environment, so losing this was no immediate handicap. But even then, the random drift should have taken ages. Is there some reason why losing this pathway would be a benefit?
Same for colour-blindness. Is it drifting, or is it actually good for something in an environment where it does no harm? (These poor children, none of them will ever be commercial pilots or qualified electricians....)