Arnold (Proceedings of the Second International Conference on Quantitative Genetics, ed. B. S. Weir, E. J. Eisen, M. M. Goodman, and G. Namkoong, pp. 619-636; Sinauer Associates.) suggested an extension of this approach to longer evolutionary time scales. Specifically, he studied variation in the number of body vertebrae and the number of tail vertebrae in populations of Thamnophis elegans from two regions of central California. He found relatively little vertebral variation within populations, but there were considerable differences in vertebral number between populations on the coast side of the Coast Ranges and populations on the Central Valley side of the Coast Ranges. The consistent difference suggested that selection might have produced these differences, and Arnold attempted to determine the amount of selection necessary to produce these differences.