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So, pale skin is, fundamentally, an adaptation to living in low light conditions. Pros: reduce energy spent on producing melanin, increase vitamin D production. Cons: more vulnerable to sunburns and skin cancer.

Do populations with this sort of adaptation have an increased rate of other low light adaptations? Specifically, I'm thinking of vision in low light situations. Examples of possible adaptations: increased rod count in the retina (pro: better low light vision, con: worse bright light vision), or faster production/resetting of rhodopsin (pro: better low light vision, con: greater energy expenditure).

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    $\begingroup$ Light intensities that are "low" for vitamin D production are still far more intense than needed for vision. Indeed, one might reasonably regard the evolution of dark skin as an adaption to living in light that's too intense. $\endgroup$ – jamesqf Oct 3 '19 at 18:44
  • $\begingroup$ @jamesqf That seems to be a semantic difference, to me. Though it suggests another possible adaptation: greater light transmission through lens/fluid in eye. Pros: more sensitivity, cons: more vulnerable to retina burn/snow blindness, possibly also cataracts. $\endgroup$ – Sean Lake Oct 3 '19 at 18:47
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    $\begingroup$ Not really semantic, just looking at the question from a different angle. Why do dark-skinned people native to tropical countries wear sunglasses? $\endgroup$ – jamesqf Oct 4 '19 at 1:50

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