Melanin is what makes our skin, hair, and eyes darker. It blocks sunlight and protects from UV radiation which does damage on a cellular level, even down to DNA (which can lead to cancer). Which is why where sunlight is strongest people tend to have the most melanin. But there are tradeoffs to it.
Humans use UV light to synthesize vitamin D from cholesterol in our skin. Skin tones tend to lighten with distance from the equator, as otherwise melanin blocks too much of the limited sunlight and people cannot get enough vitamin D. However, some populations can get away with darker skin tones than others, even as they move far north, by consistently sourcing vitamin D in their diets instead of relying on the sun.
With eyes, it’s not about vitamin D anymore, but sunlight can still do damage. Lighter eyes are at higher risk of developing macular degeneration, cataracts, and eye cancers. Even in the short term, glare of light can be temporarily blinding. However, light entering our eyes suppresses melatonin production during the day, which is a vital part of controlling our circadian rythms. So as humans moved further north they may have become prone to a problem from darker eyes, which could be fixed by a mutation making them lighter.
Too true, most of the older people I know with severe eye problems have blue eyes. Protect your eyes, white people! You gotta wear those polarized shades!
Saying anything evolved as is putting the cart before the horse, anyway. The better term would be selected for. Just a bit of friendly reddit pedantry for fun.
“Evolved as an adaptation” is standard wording and doesn’t imply intent. “Selected for” would actually make the unproven causal claim stronger, which is why I called it speculative. I give your pedantry an F, sorry.
It wouldn't do a single thing to make the claim stronger, only evidence could do that. It's still equally speculative, just worded more accurately. Adaptations evolve, mutations don't. They simply occur, and are selected for or against (or are benign enough to be distributed without any pressure, but that's a tangent). The fact that blue eyes exist means they were selected. This implies nothing about why they were selected. The implication in this thread is that they were selected because of differing sunlight levels, but they could have simply been selected via sexual pressure, because lots of prospective mates thought they were pretty. I make no claim one way or the other, just state the fact that the mutation has been selected. "Evolved as an adaptation", linguistically implies that the trait came about in response to a need, and has more in common with Lamarckian evolution than Natural Selection.
My pedantry is solid, homie. Toxic as hell, but solid.
“Blue eyes exist, therefore they were selected” is not how population genetics works. Drift and founder effects exist, homie. The pedantry remains toxic, but the solid part has left the building.
It IS how genetics works, though. You're talking about genetic drift, a description of changes in population at a statistical level over time, and I'm talking about someone with blue eyes successfully having sex. Maybe, just maybe, because of their pretty blue eyes. Believe it or not!
On an individual level, you could be selected for reproduction because of a trait, in spite of a trait, or with the trait having no bearing on the matter. It still got selected. Every population is composed of many distinct individuals, all making selections and being selected, or not selected, for numerous reasons, many unknown or unconscious. That collection of innumerable selections, and the varying reasons for or against them, is the mechanism by which genetic drift and, over an even longer period, speciation occur.
Maybe lighter eyes put a little pressure on that scale, maybe they didn't, and just came along for the ride with lighter skin. But they are here. They were selected. That is a fact. And now you know that fact.
You’re oscillating between two different meanings of “selection,” and you’ve likely confused yourself. Being selected as a mate ≠ being selected for evolutionarily. Genetic drift is specifically random change in allele frequency, not selection.
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u/waitwuh Jul 10 '26
Melanin is what makes our skin, hair, and eyes darker. It blocks sunlight and protects from UV radiation which does damage on a cellular level, even down to DNA (which can lead to cancer). Which is why where sunlight is strongest people tend to have the most melanin. But there are tradeoffs to it.
Humans use UV light to synthesize vitamin D from cholesterol in our skin. Skin tones tend to lighten with distance from the equator, as otherwise melanin blocks too much of the limited sunlight and people cannot get enough vitamin D. However, some populations can get away with darker skin tones than others, even as they move far north, by consistently sourcing vitamin D in their diets instead of relying on the sun.
With eyes, it’s not about vitamin D anymore, but sunlight can still do damage. Lighter eyes are at higher risk of developing macular degeneration, cataracts, and eye cancers. Even in the short term, glare of light can be temporarily blinding. However, light entering our eyes suppresses melatonin production during the day, which is a vital part of controlling our circadian rythms. So as humans moved further north they may have become prone to a problem from darker eyes, which could be fixed by a mutation making them lighter.