I've got an idea and would ask for you evolution enthusiasts: what if humanity could be forced to not travel around the earth and be forced to stay by some alien culture in their home habitats (countries or continents) and not hallowed to intermingle with each other. Centuries goes by and after some time the limitations fade due to external cause. Humans are free once more to travel and communcate, but they're very different from each other. What modification would the body of an African human achieve that a Greenlander human would not?
The idea is of a dystopian future where an alien race (or human themselves perhaps) are so interested in preserving ecosystems that they transform each "biome" in a nature reserve and force all creatures inside these boundaries, therefore hallowing evolution to change themselves into different species.