Gene flow is a profound evolutionary mechanism
Gene flow, or migration, occurs when individuals move between populations. When this happens, some of the novel mutations in one population will migrate to the new population with the migrating individuals. The original population may become less diverse as a result of losing one or more of its individuals. Similarly, the migrants are likely to make their new population more diverse through the introduction of new gene variants. There are many examples of populations evolving through gene flow. An appreciation of gene flow can help us understand the global distribution of the CCR5 mutation in humans (discussed above, in the introduction to the chapter). The CCR5 mutation confers resistance to some forms of HIV, yet is not most common in areas with a high prevalence of HIV and AIDS. The mutation is relatively new: biochemical and biogeographic evidence suggest an origin in Northern Europe approximately 1200 years ago. However, the mutation was distributed long before HIV and AIDS were relevant to human health. In fact, the mutation’s distribution pattern mirrors the Viking migration of the 9th through 11th centuries (Figure 8). Thus, we can hypothesize that Vikings carried the mutation with them as they conquered new territories, and passed the mutation to their descendants. But why was this genetic feature prevalent in Vikings? We’ll develop that story further, below.