Quote: "As it turned out, the research showed that the average IQ difference between black and white Americans -- 15 points -- was nothing unusual. Similar IQ differences could be found between various culturally isolated white communities and the general society, both in the United States and in Britain."
The variation of IQ within races is also genetic and is a result of assortative mating. Continued long enough, it might fission a race, so that there would eventually be two races spawned from a common ancestral race. The only cultural effect you see here is the one that reinforces assortative mating with acquired preference and learned avoidance. Poverty didn't "dumb down" the duller whites any more than it did with most of the blacks.