  We scanned through the genomes of 29,141 African Americans, searching for
loci where the average proportion of African ancestry deviates significantly
from the genome-wide average. We failed to find any genome-wide significant
deviations, and conclude that any selection in African Americans since
admixture is sufficiently weak that it falls below the threshold of our power
to detect it using a large sample size. These results stand in contrast to the
findings of a recent study of selection in African Americans. That study, which
had 15 times fewer samples, reported six loci with significant deviations. We
show that the discrepancy is likely due to insufficient correction for multiple
hypothesis testing in the previous study. The same study reported 14 loci that
showed greater population differentiation between African Americans and
Nigerian Yoruba than would be expected in the absence of natural selection.
Four such loci were previously shown to be genome-wide significant and likely
to be affected by selection, but we show that most of the 10 additional loci
are likely to be false positives. Additionally, the most parsimonious
explanation for the loci that have significant evidence of unusual
differentiation in frequency between Nigerians and Africans Americans is
selection in Africa prior to their forced migration to the Americas.
