The abstract for that paper clearly identifies the primary correlation with skin pigmentation as UV levels; furthermore the conclusion continues the same premise. Pages 72 and 73 identify the alternative reason for how both native Greenlanders and the Inuit maintain a darker skin tone in such northern climates (maintain, not develop) in the high degree of Vitamin D available in their diet. Therefore, if a fair-skinned population were to move north, there would be no overriding pressure to increase pigmentation, and thus they would [i]retain[/i] a pale skin-tone. i.e. a darker skintone moving north is the baseline taken when assessing changes in relative melanin content in humans; increased Vitamin D in the diet would not have the same effect.

Otherwise, the mass domestication of chickens would have had a really strange effect by now.