FIGURE 2 Genetic differentiation, quantified by the absolute allele frequency difference AFD, between the acidic and basic stickleback pool along an exemplary chromosome. The black circles represent individual SNPs, the blue curve shows average differentiation across sliding windows of 10 kb with 5 kb overlap (windows with fewer than six SNPs were discarded), and the gray line gives the genome-wide median differentiation (0.145). The orange triangles denote the adaptive SNPs on this chromosome, that is, the markers exhibiting extremely strong and consistent acidic-basic differentiation used to explore adaptive standing genetic variation in marine stickleback.