GENETIC-STRUCTURE OF THE FROGS GEOCRINIA-LUTEA AND GEOCRINIA-ROSEA REFLECTS EXTREME POPULATION DIVERGENCE AND RANGE CHANGES, NOT DISPERSAL BARRIERS

I describe the genetic structure of two frog species, Geocrinia rosea and Geocrinia lutea, using allozyme electrophoresis to understand popu lation structure and thereby possible mechanisms of divergence and spe ciation. The sampling regimes represented the entire range of both spe cies and provided replicated tests of the impact of ridges, rivers, an d dry forest on gene flow. Geocrinia rosea and G. lutea were highly ge netically subdivided (F-ST = 0.69, 0.64, respectively). In the extreme , there were fixed allelic differences between populations that were o nly 4 km (G. rosea) or 1.25 km (G. lutea) apart. In addition to locali zed divergence, two-dimensional scaling of genetic distance allowed th e recognition of broad-scale genetic groups, each consisting of severa l sample sites. Patterns of divergence were unrelated to the presence of ridges, rivers, or dry forest. I argue that range contraction and e xpansion, combined with extreme genetic divergence in single, isolated populations, best accounts for the genetic structure of these species .