These continental groups reflect the leading roles of geography and endogamy in shaping human races. As long as everyone intermarries, as would doubtless have been the case in the ancestral human population, there is a single genetic pool. New diversity—that is, new alternative versions of genes— accumulates through mutation, and old diversity is eliminated by drift, but these changes occur within a common pool. Any substantial bar to intermarriage, however, whether a mountain range or a religious ban on marrying outsiders, will set up two genetic pools. Since mutation and drift are both random processes, the changes in the two pools will now take place independently. From that point on, the two populations may follow different evolutionary paths. Migration between the two will sharply reduce genetic difference; time and distance will increase it.
The starting point for the emergence of human races would have been the dispersal, within Africa, from the ancestral homeland some 50,000 years ago. Before people left for the world beyond, the human population in Africa had apparently fragmented, doubtless by geographical distance, into several different populations. As already noted, those who left Africa belonged to just one of these populations, those descended from the L3 branch of the mitochondrial DNA tree. They carried away in their genes only a subset of the African genetic diversity, meaning only some of the alleles of each gene. That fact alone set them on a potentially different evolutionary path. The emigrants eventually spread out over the rest of the globe and themselves fragmented into many even smaller populations. The smaller a population, the greater is the force of genetic drift, which reduces the number of available alleles. Without interbreeding to keep the human gene pool mixed, the populations of each continent or region would over time have become more distinct and less like the others. The importance of drift in differentiating a static population has
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