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 laborat ory populations of the Atlantic silverside, Menidia menidia, drawn fro m 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 genera tions in a range of common environments, revealing a genetic basis to Jordan's rule. Selection over the winter in the field increased mean v ertebral number in the northern population, intensifying the latitudin al trend in vertebral number that existed in the fall. The combined fi eld and laboratory data support the hypothesis that vertebral number I n fishes is adaptive. Moreover, these data implicate natural selection as apposed to random genetic drift as the cause of Jordan's rule.