Vertebral number is used frequently as a diagnostic species character in fishes, but it varies not only among but also within species. Much of the variation in vertebral number is associated with geographic gradients such as latitude, and it likely reflects both environmental and genetic influences on phenotype. Hence, comparison of intra- and interspecific geographic variation and its genetic basis in closely related species may contribute not only to taxonomy of the group concerned but also to understanding how such differences evolve. In this study, we examine intra- and interspecific latitudinal variation in vertebral number of two geographically contiguous, closely-related species: the Atlantic Silverside Menidia menidia (a northern species) and the Tidewater Silverside M. peninsulae (a southern species). In accordance with Jordan's rule, vertebral numbers of wild individuals increased with latitude within and among species. Mean vertebral number varied along a common linear trajectory both within and among species and could be predicted with very high accuracy from latitude alone (r = 0.98), indicating that vertebral number is not a diagnostic species character in Menidia. Common-garden experiments revealed that this latitudinal cline persists across all temperatures, demonstrating a strong genetic basis within and among species. The continuous variation throughout the entire geographic range across the species suggests that natural selection and adaptation is the most likely explanation, and that populations and species are free to evolve any vertebral number required in each environment.

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