To test this hypothesis, we use two summaries of genetic variation that have different statistical power to make inferences, depending on the model of selection and associated population demography. Two different methods were used for detecting natural selection at two loci relevant to nicotine dependence, specifically the CHRNA5-A3-B4 region on chromosome 15q25 and the CHRNB3-A6 region on chromosome 8p11. We provide strong evidence for selection in the CHRNB3-A6 region and moderate evidence for selection in the CHRNA5-A3-B4 region. However, there is only a modest correlation between nicotine dependence and score on the Wechsler Adult Intelligence Scale (WAIS) Digit Symbol test in our dataset. Overall, we suggest that one possible explanation for these results is that SNPs in these regions associated with risk of nicotine dependence are associated with natural selection acting at these loci to improve human memory and learning.
