Being unversed in biology and evolutionary theory, i am somewhat hesitant in asking this question, but hey, I haven’t posted in a while.
Now, from what I understand, evolution in species is not a slow, trudging, semi constant thing. Rather, organisms stay basically the same for a long time, with relatively sudden evolutionary bursts that enact changes. These bursts are often caused by a changing environment as opposed to random mutations, although mutations that greatly increase a species ability to live in its environment are also common.
Now, generally, species don’t move too much, and so consequently don’t change as much simply because there is no advantage in doing so. However, when species do disperse, large changes can be seen in them, as evident in dogs, bears, birds, and to a lesser extent humans. I was wondering how long it would take for a burst to come, if the change of environment was sudden. So, if we moved a collection of white or asian families to say Ghana, and let them breed only with each other, how long before the descendents would respond to the new climate and begin to darken?
I’ve seen the argument made that skin color varies, with a tendency towards darker amongst those people living in more equatorial locales and lighter the farther north or south one goes, as a result of the difference in the relative amount or intensity of sunlight to which these populations are exposed annually, over the course of many, many breeding cycles.
The OP presumes an evolutionary mechanism that would respond to such an environmental stimulus, given enough time.
Could it not also be the case that mutations delivering lighter skin-toned people inspired those familial groups that produced such to migrate towards less sunny climes?
I’m afraid you have an erroneous concept of the mechanism for evolution. For one thing, you seem to be using a Lamarckian model, in which some advantage of being dark would, of itself, communicate itself to the bodies of each successive generation until some optimum “darkness” had been achieved.
Evolution does not work that way. First there would have to be a mutation in the genes that control melanin production in the skin. If that mutation never occurred, your group would never tend to get darker. Then, the mutated gene(s) would have to confer an advantage on those people who got those changed genes.If the people with the changed gene(s) never produced more surviving offspring than their neighbors, there would never be a “pressure” to keep and use that gene, and, eventually, the mutation would die out without becoming prevalent,
Assuming you are talking in a non-Lamarckian sense, and by “respond” you mean for lighter skin to be selected against via survival of the fittest (ie darker skinned variants being more hardy) then the movement towards a true “dark/darker” could probably be accomplished in around 10,000 years or so, or around 400 generations. This also assumes the possibility of some favorable darker skinned mutations (that would survive better and spread in the population) and potentially speed the darkening process. Without mutations though, the best you could probably hope for is an American Indian type skin hue, and not the near “blue black” of some of the Sub-Saharan population groups.