There are many different processes that alter allele frequencies. The most commonly understood concept is Natural Selection. But others such as drift and Founder effects also play an evolutionary role. This is seen in the Out of Africa model of human evolution. As modern humans branched out of Africa and into footholds in Asia and Europe, each small group of migrants took a subset of humanity’s genetic diversity with it, creating a series of population bottlenecks.
A large international team of researchers have gone and sequenced genomes from multiple ancient human populations in Africa (the San and Mbuti), along with a North-African population, the Parthians of Central Asia, and natives of Cambodia, eastern Siberia, and Central America (Cambodians, Yakut, and Maya, respectively). African groups have the most genetic variability based on a number of measurements and the resulting Out of Africa populations haven’t been around long enough for many new genetic changes to spread widely within them.
This group of geneticists have identified signatures of these bottlenecks both in terms of genetic variation and potentially harmful mutations. For example, severe mutations were significantly more common in populations that were farther from our African origin, with the Mayan population having the most. This work is clearly consistent with the idea that humanity settled the globe through migrations of small populations that can be traced back to Africa, creating genetic bottlenecks along the way.