Figure 4. How much of local marine-freshwater adaptation occurs by reuse of global variants?

a. Classic marine and freshwater ecotypes are maintained in downstream and upstream locations of the River Tyne, despite extensive hybridization at intermediate sites¹⁶. b. Pairwise sequence comparisons identify many genomic regions that show high divergence between upstream and downstream fish (X-axis). Many, but not all, of these regions also show high global marine-freshwater divergence (Y-axis; red points indicate significant CSS FDR<0.05), indicating that both global and local variants contribute to formation and reproductive isolation of a marine-freshwater species pair.