Additional evidence for functional roles for many of these SNPs came from studies of apparent effects of natural selection. A total of 31 SNP in 26 haplotypes displayed evidence for negative selection. Thirty-four SNPs in 19 haplotypes displayed evidence for positive selection. Signals of recent positive selection provide information about the adaptation of humans to local conditions and have been implicated in phenotypic variations [34]. Thus, the 6 genes located in these regions of positive selection may be of particular interest in studying addiction vulnerabilities.
