Patterns of geographic variation in Florida snakes

Abstract

I analyzed geographic variation within Florida for 15 species of snakes. Contour maps were machine-produced for each of over 200 morphologic and 17 environmental variables. Factor analysis of the maps showed that seven major patterns of geographic variation account for over 60% of the total variation. Each of the patterns of geographic variation can be explained in terms of natural selection and gene-flow characteristics in past or present environments. Disjunct populations showing phenetic similarities are the result of an earlier widespread phenotype followed by differentiation in geographically intermediate regions. Recourse to land bridge hypotheses and retrogressive evolution are not necessary to explain polytopic phenotypes. The geographic localities of primitive species or primitive characteristics within a species are not the centers of origin, but are merely regions in which evolution has proceeded relatively more slowly. A north-south cline on the Florida peninsula is the most obvious pattern of geographic variation, common to all species examined for at least some characters. Most species examined showed aspects of a Suwannee Straits pattern with dramatic character-state shifts occurring in the region of the present Suwannee River.