Out of Africa’s Mutational Marks

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.

Figure 3
Differences in the proportion of deleterious alleles by frequency class. (A) The proportion of rare versus common deleterious variants per individual. For a given individual, deleterious variants were divided into common (>10%, solid colors) and rare (<10%, white space). The contribution of common deleterious variants to an individual’s burden is much greater than rare variants. (B) For each population, we calculated the proportional site frequency spectrum by plotting the proportion of deleterious large-effect alleles in each frequency class (translucent coloring) along with the proportion of neutral alleles for each frequency class (opaque coloring). African populations have proportionally fewer rare deleterious alleles than expected from neutrality. Populations with OOA ancestry have proportionally more fixed deleterious mutations.

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.

Figure 1C

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.

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