Dark skin is ideal for Africa and other equatorial areas.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.
No, fair skinned people in equatorial regions don’t suffer from excessive vitamin D production. They suffer from chronic sunburns and the various ailments resulting from that...eventually cancer.
There are exceptions to the fair skinned people dominating the northern latitudes rule of thumb. I’d have to check to make sure, but I think people that hunt whale get plenty of vitD from their food. So lighter skin wouldn’t give them any benefit.
You have to remember that in colder climates, people wear lots of clothes. They must absorb all their sun light through their faces. So there are two things happening. Less skin exposed to sunlight, and less sunlight. Also, forests and climate probably play a role. People that live in heavily forested regions are going to get less sunlight. People that live in cold forests where fog and mist is common and the nights are long in the winter are really going to get limited sunlight.
Tierra del fuego is not equatorial. Those folks haven't been there very long. They still retain the general skin color of their Siberian ancestors. The skin color changes throughout the Americas are (or were) a "work in progress" as these things generally take time.
You imply that, generally, dark skinned people in the north should have health problems due to lack of vitamin D deficiency.
Modern groups have vitamin D fortified milk (that's the reason its there!). Groups in the far north supplement their diets with fish oils. They live too far north to get any appreciable vitamin D from the sun, and they can't expose their skin to the sun for very long anyway, lest it freeze.
I've never heard such a thing.
That means nothing. Try reading some books on human races.
Plus I have never heard of light-skinned persons, generally, having health problems due to excessive vitamin D production at the equator.
The problem for light-skinned people at the equator is not excessive vitamin D production, it is cancer caused by excessive ultraviolet radiation reaching the lower skin layers. That is why the skin needs to be dark. Of course, modern light-skinned folks can avoid this problem (and sunburn) by wearing clothes, hats and using sun screens.
So I reject your explanation of the "improvement". You don't even try a just-so story for the DNA sequence improved upon.
Reject away. But until you learn something about the subject, your rejection means nothing.
Really, I have 20-30 books on human races and closely related topics at the office. I would be happy to provide some titles.
There is no such thing as excess vit D, but dark skinned people do have a much higher incidence of many kinds of cancer that appears to be due to their minimal production of Vit. D.
Nothing in this has any connection to evolution, since for both groups, the effect is the same regardless of where they live.