Here we test the hypothesis that natural selection has acted on these genes. But if the null hypothesis of neutral evolution or demographic processes is rejected, why would nature seemingly select for this trait, especially given the fact that it is believed that nicotine has not been a part of our evolutionary history long enough, and in large enough quantities, for its effects to be visible in our genomes? One hypothesis is that selection acted on another phenotype and the effect on nicotine addiction was secondary and incidental, a genetic phenomenon termed hitchhiking.
