system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If a dark-skinned population moved to an area currently populated by fair-skinned people (eg Scandinavia) and didn't interbreed, would they eventually become white-skinned? If so, how long would it take? And would the same be true for a fair-skinned people moving to, say, central Africa?
gpt
TL;DR - Probably not, because modern food, clothing, and shelter cancel out most of the survival benefits for light/dark skin in different regions. 

The first part (black people becoming white) would probably not happen under modern conditions.  The major evolutionary advantage of being white is that it allows you to more easily use UV light to metabolize vitamin D, which is important if you live in a region with limited sun exposure.  This would have been an important issue for early humans, but doesn't really matter now that we can get plenty of vitamin D from food and supplements.  Ultimately, there would be no survival or reproductive advantage for people with lighter skin.

For white people moving to tropical regions, there might be slightly stronger selection towards darker skin.  Dark skin helps to protect against melanoma in areas with lots of sun, which is obviously a good thing.  However, evolution only happens if a new trait provides a reproductive advantage.  Humans can begin reproducing at age 14-16 (biologically speaking), and there is a very low incidence of melanoma among people that young.  What this means is that basically everybody will be able to reproduce successfully before developing melanoma, so there is only a marginal benefit for darker skin.  If you take into account the fact that modern people live mostly indoors, wear clothing that covers much of their skin, and can use sunblock, the evolutionary advantage for dark skin could very well disappear.  

It's hard to come up with numbers for a hypothetical question like this, but the kind of evolutionary changes you're thinking about would take hundreds of generations, if they happened at all.