Heritable increases in branchial bone lengths in freshwater sticklebacks. (a) Wild Fishtrap Creek (FTC) fish have significantly longer dorsal (EB1) and ventral (CB1–CB5) branchial bones relative to size-matched wild marine Little Campbell (LITC) fish. All six bones are significantly longer in FTC fish than LITC fish (p < 10−10 for all bones, nLITC = 27, nFTC = 40, Welch's T-test). (b) Increased bone lengths are heritable in adult laboratory-reared fish, and longer bones are also found in a second laboratory-reared freshwater population, Paxton Benthic (PAXB). All bones are significantly longer in each freshwater population relative to LITC marine (Tukey HSD: p < 10−5, nLITC = 32, nFTC = 25, nPAXB = 36). PAXB and FTC bone lengths do not significantly differ (p > 0.05). Error bars = standard deviation of the mean. Red, LITC; light blue, FTC; dark blue, PAXB.