Hitchhiking under positive Darwinian selection

Positive selection can be inferred from its effect on linked neutral variat ion. In the restrictive case when there is no recombination, all linked var iation is removed. If recombination is present but rare, both deterministic and stochastic models of positive selection show that linked variation hit chhikes to either low or high frequencies. While the frequency distribution of variation can be influenced by a number of evolutionary processes, an e xcess of derived variants at high frequency is a unique pattern produced by hitchhiking: (derived refers to the nonancestral state as determined from an outgroup). We adopt a statistic, H, to measure an excess of high compare d to intermediate frequency variants. Only a few high-frequency variants ar e needed to detect hitchhiking since not many are expected under neutrality . This is of particular utility in regions of low recombination where there is not much variation and in regions of normal or high recombination, wher e the hitchhiking effect call be limited to a small (<1 kb) region. Applica tion of the H test to published surveys of Drosophila variation reveals an excess of high frequency variants that are likely to have been influenced b y positive selection.