I have read something interesting about it. Evolution happens when there are crossings between different populations, exposed to different conditions. For instance: one population from North Africa meets a population from Southern Europe, when there was still a connection between the continents. They settle down and reproduce. Some of their children will be more apt than others, because they have gotten the best of the two groups. However, as we approach the concept of a "global population", the differences between the different groups tend to get smaller and smaller, causing these crossings to be less and less effective, evolution-wise, as people get to be more similar over time.