Sure.
Dark skin is ideal for Africa and other equatorial areas. The Mediterranean had a different climate, so people in that area have a lighter skin color with tanning ability to provide a better adaptation between protection from ultraviolet radiation (dark skin--summer) and production of vitamin D (light skin--winter). Farther north skin colors are lighter still, reflecting adaptation to still less intense ultraviolet radiation and the need for more vitamin D production. These topics are well discussed in books on human races and adaptation. You should check out a few. I can provide references if you really are interested.
As for DNA sequences, talk to the genetics folks.
Tierra del fuego is not equatorial.
But that is a rehash of observation not a relationship of "improvement". You imply that, generally, dark skinned people in the north should have health problems due to lack of vitamin D deficiency. I've never heard such a thing. Plus I have never heard of light-skinned persons, generally, having health problems due to excessive vitamin D production at the equator. So I reject your explanation of the "improvement". You don't even try a just-so story for the DNA sequence improved upon.