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I need it to justify the locals having pale skin while being a diurnal above ground species. You ever see this show "The Orville"? The main antagonist race has similarities (as they are both pale white and have a super cloudy world) to what I'm trying to have for this race.

I know that having near constant overcast weather on a planet like Earth is not realistic, but what if the planet had a denser (or maybe just less translucent?) atmosphere?

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It happened!

Humans are a diurnal above ground species. A subpopulation of humans inhabiting northerly latitudes evolved pale skin, to compensate for long winters and cooler climates necessitating lots of clothes. These are the caucasians, or white people.

Elaborating:

  1. melanin absorbs UV rays and defends DNA.

  2. UV rays not absorbed by melanin can catalyze a reaction in the skin that produces Vitamin D.

  3. Not much UV on skin = not much vitamin D.

  4. If you are deficient in vitamin D in childhood, your bones will develop abnormally.

  5. If you have an abnormal pelvis from low vitamin D in childhood, later when you are pregnant the baby can lodge in the pelvis during deliver and you will die in childbirth. This is a powerful selective pressure to get more vitamin D; options are to show more skin, go someplace sunnier, or get more vitamin D from the skin you can show.

  6. If you have less melanin, less UV rays are intercepted and more are available to catalyze formation of vitamin D.

If your pale people live in conditions similar to those of Northern Europe they might also gain selective advantage from pale skin.

This is assuming they are of human stock. If they are outright alien aliens they might not have vitamin D, or skeletons, and you can make them any color you like.

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In our solar system, Venus is extremely hot due to its atmosphere, even more so than Mercury, which is much closer to the sun.

If this planet was farther from their sun than Earth is from ours, and it had enough greenhouse gasses to contain all of the heat, not only would the sun be much dimmer due to the greater distance, none of it would be direct due to the constant cloud cover; as such, much paler skin would develop.

Since you mentioned that you wanted the 'antagonist race' to be this pale (which implies that the protagonist race may not be), it could certainly be as a result of a mutation, much like the european 'white gene' which developed a couple thousand years ago. Before then, no humans had white skin, and something similar could have happened here where one race became extremely pale, and due to dense cloud cover and a large distance from the sun, this extreme amount of paleness had few evolutionary downsides, and the trait continued.

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    $\begingroup$ In fact, it would have evolutionary advantages, owing to the metabolic cost of producing melanin, and also easing Vitamin D production. $\endgroup$
    – Mary
    Jun 1, 2020 at 0:06
  • $\begingroup$ I actually said that they were similar to the antagonist race from some show, not that they were the antagonists, but that doesn't affect your answer, just letting you know. $\endgroup$ Jun 1, 2020 at 3:27

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