The spatial scale of adaptation in pond-breeding amphibian larvae

The spatial scale of adaptation is fundamental to our understanding of evolutionary ecology. Traditionally, strong gene flow and weak selection were expected to prevent adaptive evolution at finer spatial scales, but instances of microgeographic adaptation challenge this assumption. We evaluated four alternative predictions about the scale of adaptation to divergent selection from predators for two pond-breeding amphibians. Common garden experiments revealed that wood frogs developed larger tailfins and higher survival along a spatial cline, indicating the importance of selection and gene flow between and within habitats. Spotted salamanders displayed defensive behaviors and higher survival between ponds. Results suggest that adaptation occurs both between and within habitats such as ponds. Evidence for adaptations within traditional habitats contradicts the traditional notion of the habitat patch as a population. Overall, results compel a greater appreciation of fine-scaled adaptation in nature, suggest the need for spatially explicit genetic sampling designs, and reject the assumption that populations are always panmictic within habitat patches.