According to natural selection mutations happen to populations so that the offsprings from the mutated folk inherits the mutation. If the mutation is an asset to the being's survival, other beings without the mutation will die and the mutated will be the new "stronger" kind.
This is a good thread topic.
So what about the whites in Europe that once had black ancestors? Did the black ones in Europe die because of vitamin D deficit and the ones who luckily were born albinos survived?
This part sucks. But what the hey. In a word, albinism is not the complement to Vitamin D deficiency, since it's a harmful mutation that carries severe disabilities including blindness. I don't think it's necessary to assume that early inhabitants of Europe were black,or that pigmentation change was particularly significant among the variations in traits that emerged among the various gene pools that became regionally isolated. Besides, the large shift in pigmentation may not have been from Africa to Europe per se, but from southern to northern Europe.
From all of the geographic distributions of skin shade still prevalent in the world today it should be evident that this is closer to the variations seen in any other species that exhibits adaptive radiation.
Natural selection is statistical, so it's not likely that death was necessarily assured by darker skin shades at the higher latitudes, but that the odds favored fair skin over a large number trials and over many generations.
Acting in concert with natural selection is genetic drift, which is seen in other species affecting their coloration. Furthermore, the cause for white fur for the many animals living in snowy environments is camouflage. You wouldn't want to rule this out as part of the process.
It's likely that people who settled around the coastal regions of Europe had plenty of Vitamin D since it's available from seafood. For this reason, dark pigmentation would not necessarily be selected out in those regions of Europe on account of Vitamin D deficiency.
