How long does evolutionary adaptation take?
Why are Northern Europeans so much lighter than Inuits?
A while back, John Hawks wrote about Native Americans and skin pigmentation.
A really interesting question is why Inuits are swarthy. One hypothesis is that Eurasians acquired their skin pigmentation from Neandertals. Another is that Inuits simply didn't have enough time to evolve light skin. As Hawks quotes William Boyd:
Unless the selection of light skin as opposed to dark were fairly intense, the time elapsed has simply not been enough to allow for much adaptation to occur (12). As a matter of fact, the populations which might have been expected to become lighter, namely the Fuegans and the Eskimo, have probably had a shorter time in which to achieve this end than other American aborigines, for it is reasonable to suppose that the Fuegians did not reach their present home until long after their northern neighbors were well installed. And all students of the Eskimo agree in recognizing them as probably the most recent (aside of course from the whites) arrivals in America. It could well be that there has just not been enough time for selection to bleach the skins of the American aborigines. [emphasis mine]I have a much simpler hypothesis. The Inuit diet consisted of a lot of fatty fish, which is a wonderful source of vit D. So they would've been under zero selection pressure to have blindingly white skin like my Irish grandma.
While the Celts are now famous for occupying the edges of Europe (Ireland, Scotland, Wales and South Boston) they originally occupied much of western and central Europe, including here in the Czech Republic. The Austrian town of Salzburg derives its name from ancient Celtic salt mines where mummified Celtic miners are still occasionally discovered.
It seems to me that the landlocked Celts of central and western Europe (in those days anyone not living on the coast was landlocked although there is evidence of abundant trade), and would have been under strong evolutionary pressure to generate vit D with the lightest skin possible.
So my gut feeling on all of this, I think things like melatonin adaption can happen rather quickly if there's enough evolutionary pressure but the Inuits skin pigmentation doesn't represent this sort of pressure. They were probably more at risk for (and selected for resistance against) hypervitamosis D.
Oh yeah, and you can't go wrong eating plenty of fish.