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This questions came to me after some discussion on my previous one about a civilization living under Antarctica.

I somehow have the feeling this is a dumb question - sorry if it is -, however I can't figure out what the answer would be...

If a human population whose members are black-skinned came to live underground for generations, would they become paler and paler or would they stay black ?

What I know :

  • The reason why cave creatures are mostly white or extremely pale is because skin pigmentation is a process that takes energy to the body to achieve and therefore in the absence of light, it would be a waste of energy.

  • Natural selection keeps the best genetic options to help animals and humans survive in their environment. I know the process and how it works ( the strongest survive longer and then can have more descendants ; dominant / recessive genes ; etc). However in this case, I can't figure out if natural selection would "chose" dark or light skin.

What I wonder about :

  • Wouldn't natural selection in this case prevent those people from whitening ? I mean, in caves with low light, it is easier to hide (whether for escaping or hunting a creature) when you have dark skin. So would the ones with a darker skin survive easier ?

  • Would there be a natural selection process at all considering this is a united and organized population where the weakest are helped by the strongest ? Also would having a black skin (in the case where white skin would be better for survival) be a sufficiently "dangerous" characteristic to provoke enough death to trigger natural selection on the concerned genes ?

  • Speaking of evolution, could it possibly make new generations whiter, despite all members of this population are black-skinned and then have this characteristic in their genes ?

  • If this process of whitening was to happen, how much time would it take ? I assume it would take longer than other human mutations because of the transition from very dark skin to very pale, but I hardly can put a time scale on it.

I know these are a lot of interrogation but I think they are all important points to consider in order to answer the question

Thanks in advance for your answers :)

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    $\begingroup$ Related - worldbuilding.stackexchange.com/questions/6629/… Also related - worldbuilding.stackexchange.com/questions/394/… $\endgroup$ – ChronoD May 27 '16 at 20:30
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    $\begingroup$ Something that may be important is the level of technology they take with them. If this population has advanced technology, then they don't need to worry too much about Natural Selection. They would be able to hunt and defend themselves easily and survive regardless of their skin color, which negates the color advantages of camouflaged darkness. $\endgroup$ – ChronoD May 27 '16 at 20:34
  • $\begingroup$ @ChronoD Thank you for these links, a quick read brought me some useful information. About the technology, the people I have in mind were advanced as a late middle-age population when they settled but later invented more and more ways to make their survival easier. $\endgroup$ – Ctouw May 27 '16 at 20:43
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Permanent changes in the skin pigmentation of a human population are only likely to result from evolutionary selection processes. Dark skinned humans have evolved into lighter-skinned humans before, when early humans migrated from Africa and the reduction in the amount of sunlight meant the being dark-skinned was no longer the advantage it had previously been (protection from strong sunlight), and lighter skin provided an advantage (better vitamin D production).

In the "united and organized population where the weakest are helped by the strongest" you describe, the natural selection process is likely to be very slight or non-existant as the survival rate for all humans will be relatively high, so the evolutionary pressure is not there and changes in base skin colour would be unlikely.

If for some other reason than natural selection there ended up being a bias toward lighter skin (e.g. social or religious practices) then some change would take place over a long period of time (in the order of 10s of millenia).

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  • $\begingroup$ Even with the weak being helped by the strong (that's not as rare as you might think among social animals), that doesn't change the fact they will have some minor disadvantage, if a trait provides even a miniscule advantage it will eventually dominate the gene pool. $\endgroup$ – Vakus Drake May 30 '16 at 3:23
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    $\begingroup$ As I said, if there was some social or other advantage to a particular skin colour then yes, it would eventually lead to a change in the dominant genes. In the scenario described in the question there is no obvious disadvantage to having a particular skin colour, we would need to know more about this environment, level of technology, possible predators, social/moral behaviours etc. before being able to say whether any skin colour was likely to have an advantage. $\endgroup$ – Nathan Griffiths May 30 '16 at 5:00
  • $\begingroup$ Any trait that isn't actively serving a purpose will eventually be lost, especially one that can change as quickly as melanin level. Some amount of energy has to be spent producing melanin so there will be some slight advantage to producing less of it if it isn't serving any function. $\endgroup$ – Vakus Drake May 30 '16 at 14:51
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    $\begingroup$ "Any trait that isn't actively serving a purpose will eventually be lost" - sorry but that is not how evolution works. Traits that provide an advantage will win out over ones that don't, but if there is no disadvantage in having a particular trait there is no reason it would just disappear. $\endgroup$ – Nathan Griffiths May 31 '16 at 7:36
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    $\begingroup$ If there was absolutely ZERO selection pressure on melanin production then I think the population would eventually come to have a wide range of colors, as the genes for MAXIMUM melanin production would suffer mutations that cause less melanin production in offspring (since the normal function is max melanin, the only possibility is less production), and with zero selection pressure (social, environmental, etc) those genes would be distributed around and you would see some dark individuals and some lighter ones. $\endgroup$ – Jason K Jun 1 '16 at 13:40
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There are a couple of factors to consider.

  1. Founder's effect: I.e., what is the genetic make-up of the individuals who head underground? If it is a small population with absolutely no genes encoding for less melanin production then it would take spontaneous mutations to slowly drive down the amount of melanin (and given the generation gap for humans, this would take a LONG time). But if the population is mixed then lighter color genes would already be present so it could happen fairly quickly if there was selection pressure for lighter skin.

  2. Selection pressure may not act on skin color, but could act on a expression of a gene NEAR the genes for skin color. I'm not aware of any good candidates but since the human genome is mapped you could find one or just make it up. Anyway, individuals with chromosomes with genes for lighter skin color also have a beneficial gene for something else (maybe they resist some toxin in the cave environment better) so there is indirect selection pressure for lighter skinned individuals.

  3. Without selection pressure a gene does tend to favor the least energy utilizing variant, but I don't think dark skin versus light skin has enough of a energy difference for it to matter for humans (who are already pretty energy inefficient if they have a working society and aren't subsistence level scavengers). So social pressure may come into play. Perhaps an albino takes over in a coup and deliberately seeds the colony with his offspring (like Genghis Khan). Or lighter skinned folks are viewed superstitiously and are killed as infants. Those types of social pressures may do more to alter skin color in humans than environmental selection.

  4. Epigenetics may be able to give you a rapid shift with an immediate downregulation of melanin production despite the continued presence of the genes. Of course how epigenetics works is still in its infancy but the gist is that there are mechanisms to up or down regulate gene expression outside of DNA so it could be possible that the cave group could lose their melanin in a generation or two even though they still have the genes for black skin. Highly speculative of course since epigenetics itself is controversial and whether or not there are any mechanisms for skin color is unknown!!

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While natural selection shouldn't be having much effect if the civilisation is organised and has decent technology, there will be a general whitening if they remain underground. The human body is very adaptable, you don't even need natural selection to see the effects. The first generation of children will all be far paler than their parents, as their skin pigmentation will alter dramatically. The parents themselves will be paler than they were when they entered the caves.

Vitamin D intake will be an issue for everybody though, if there isn't enough sunlight or alternative sources of the vitamin to keep people healthy, there will be problems. Conversely, if there is enough sunlight to provide the intake, there is possibly enough sunlight that there won't be a pronounced change in the population. They will get paler, but possibly not enough to be noticable.

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No... well it depends

Everything you know is correct, but the answer depends if the civilization is there before or after sapience evolves.

  • If they were there before sapience and the cave have little to no light, they will pale out. There isn't enough light to see even a light skinned person, so there is no benefit to being dark skinned.
  • If they were there before sapience and the cave does have some natural light, they will not. There is enough light that being dark skinned helps hide you in the rocks.
  • If they were there after sapience, it does not matter the light level, they will choose mates because of "love" instead of best features. So specific appearance traits will be mostly equal. This is why humanity has so many different body plans, fat, slim, short, tall, etc. It doesn't matter, someone will always have opinions on these traits.
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    $\begingroup$ In most people case, "love" doesn't override all the instinctive trait selections. What people tend to find attractive has it's roots in signs of a healthy mate. Furthermore, people choose personality traits almost entirely on animal instinct on the macro scale. $\endgroup$ – Space Ostrich May 30 '16 at 2:29
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No, as far as we know.

The lighening of skin is due to selection pressure regarding sunlight making vitamins, and lack of countering with the need to prevent sunlight from breaking down folic acid.

Some nothern people have not become pale because they get vitamins from their diet and don't get much sun on their skin.

So I would suppose that cave people would be the same way.

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One thing to note in evolution is the principle of "use it or lose it" which, in essence means if a trait (in this context a protein) not beneficial to an organism, it can be mutated over without consequence. Dark skin is a consequence of high concentrations of eumelanin, which are produced by melanosomal proteins.

From Wikipedia on the origin of dark skin:

In the heat of the savannas, better cooling mechanisms were required, which were achieved by the loss of body hair and development of more efficient perspiration. The loss of body hair led to the development of dark skin pigmentation, which acted as a mechanism of natural selection against folate depletion, and to a lesser extent, DNA damage. The primary factor contributing to the evolution of dark skin pigmentation was the breakdown of folate in reaction to ultraviolet radiation; the relationship between folate breakdown induced by ultraviolet radiation and reduced fitness as a failure of normal embryogenesis and spermatogenesis led to the selection of dark skin pigmentation.

So humans developed darker skin in response to ultraviolet radiation from the sun. Without the selection factor of the sun being involved in underground dwelling species, the melanosomal proteins are more likely to be mutated over and become "junk" proteins. This would lead to a paler and paler looking human.

Also, in underground caves and caverns, there will be little light, so having any kind of pigmentation at all would simply be a waste of resources.

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Yes, because ultimately variation of darkness would occur, which in terms of survivability would be of no difference in this environment. As long as society wouldn't be selective itself (they would not prefer to mate with white mates due to racism, they might actually prefer white people initially if they are rare), populations will light up. They won't turn completely white however. Keep in mind though, this is only based on the change not affecting survivability at all. We do not have enough information to ultimately decide on this, to be honest.

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