Black skin, skin with higher melanin content, is more protected from the Sun than white skin. This is why melanin is also an inducible gene, laying in the sun increases the amount of melanin in your skin temporarily, to protect you from further exposure.
Yes, what I am saying is that a dark skinned population that moved to Norway (back in the day) would be under extreme selective pressure (caused by rickets) to accumulate less functional or less induced alleles of all the genes that put melanin into the skin.
How do you suppose a single human population derived such obvious differences, if not for natural selection of genetic variation?
I guess it begs the question of how humanity developed into three distinct races: Caucasoid, Negroid and Mongoloid. Don’t you think there should be some residual traces in each race? Why are they so different/distinct from each other? You said that black skin is more protected from the sun but then why are black skinned people more susceptible to sunburn? Then too, there are several levels of black skinned people in Africa. Shouldn’t they all be the same?