Ancestral alleles are determined by sequencing a chimpanzee and gorilla. We depict the probability that the minor allele is the ancestral allele. Bin 1 is unchanged from Fig. 1, but bins 2 + 3 and 4 + 5 are merged together to improve statistics. For each bin, we show the mean frequency as a filled circle. Data are divided into (A) nonsynonymous and (B) synonymous SNPs. Neutral theory predicts a straight line with a slope of 1, but this is observed only for synonymous SNPs.