FIGURE 4 Frequency of the acidic allele at adaptive and control SNPs. (a) The blue lines give the frequency of the acidic allele at each of the 50 adaptive SNPs in each sample pool, and the orange line indicates the median frequency. The gray lines show the acidic allele frequency at 500 baseline SNPs exhibiting a magnitude of acidic-basic differentiation near the genome-wide median (their median frequency is indicated by the black line). The first two sites from the left are the freshwater pools from North Uist used to identify the adaptive SNPs. The other locations represent marine stickleback (NU combines individuals from the marine North Uist samples ARDH and OBSM). The marine locations are ordered by increasing approximate swimming distance from North Uist. Note that the subtle allele frequency differentiation between the acidic and basic pool at the baseline SNPs is expected technically because at these markers too, the acidic allele was defined as the one relatively more frequent in the acidic than the basic pool. Panel (b) follows the same format as (a) but shows data only for the subset of adaptive SNPs at which the acidic allele is the minor allele within all marine sample pools. Both graphs convey that the frequency of alleles important to acidic adaptation is not elevated in marine stickleback close to North Uist than further away.