"Since skin colors are associated with other distinctive physical features, I doubt that the tropical-sun theory is a complete explanation."
This association is largely coincidental, based more on the founder effect, genetic drift and inbreeding than on natural selection or gene linkage. As for the tropical sun theory, let's take a simplistic case. A species could have adapted to tropical conditions by duplicating the genes of the melanin metabolic pathway so as to produce large amounts of the pigment to cut back on UV radiation damage and the over-production of vitamin D. If a population of that species moved into temperate conditions, the loss of vitamin D could select for individuals who loose those extra genes and so make less melanin.
However, it should be pointed out that skin color is a complex metabolic process, so these kinds of simplistic scenarios are not completely accurate.
Kevin L. O'Brien
"Good God, consider yourselves fortunate that you have John Adams to abuse, for no sane man would tolerate it!" William Daniels, _1776_