Latitudinal variation in vertebral number has a genetic basis in the Atlantic silverside, Menidia menidia

Variation in the number of vertebrae was examined in field and laboratory populations of the Atlantic silverside, Menidia menidia, drawn from across the species' range along the Atlantic coast of North America. Vertebral counts of wild fish increased and were tightly correlated (r = 0.995) with latitude in accordance with Jordan's rule. This strong correlation persisted in laboratory stocks reared for multiple generations in a range of common environments, revealing a genetic basis to Jordan's rule. Selection over the winter in the field increased mean vertebral number in the northern population, intensifying the latitudinal trend in vertebral number that existed in the fall. The combined field and laboratory data support the hypothesis that vertebral number In fishes is adaptive. Moreover, these data implicate natural selection as apposed to random genetic drift as the cause of Jordan's rule.