FIGURE 1 (a) Typical stickleback ecomorphs from marine, acidic freshwater, and standard freshwater (here called ‘basic‘) habitats, highlighting the particularly strong reduction in bony armor and body size in acidic stickleback. Key external skeletal elements (dorsal spines, lateral plates, pelvic complex) are shaded in gray. (b) Image of North Uist (left), indicating the acidic (red) and basic (blue) lakes from which freshwater stickleback were sampled. The sites ARDH and OBSM represent locations at which marine stickleback were collected. The other five Atlantic marine sample sites are located in the map (right; North Uist is indicated by the small rectangle). (c) Unrooted maximum likelihood phylograms showing the genetic similarity among 44 total marine, acidic and basic stickleback individuals. The left tree is based on 200,000 SNPs selected at random across the genome, whereas the right tree uses 120,448 SNPs filtered to be little influenced by selection (low allele frequency differentiation in both marine-freshwater and acidic-basic genome scans, and location in chromosome regions exhibiting high recombination rates).