The authors explain that the genes in a population can vary quickly if they are successful. For example blue eyes spread rapidly as humans moved north. It takes larger populations for the effect to propagate rapidly and agriculture allowed population to grow.
They also explain how the Ashkenazi Jews got their smarts. Jews in other parts of the world were not known for intelligence. They suggest that genetic variants facilitate the nervous system. With only one copy of the gene the nervous system is improved but theat gene causes disease with two copies, such as Tay-Sachs and others prevalent among Ashkenazi Jews. This is similar to the gene causing sickle-cell anemia among Africans. The 3/4 standard-deviation IQ difference means the Ashkenazi population has six times as may highly intelligent members
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