The structure of clines with fitness-dependent dispersal.

Spatial models commonly assume that dispersal does not depend on environmental conditions or phenotype. For example, these assumptions underpin explanations for clines on the basis of a trade-off between dispersal and local adaptation. We reexamine clines when an individual's decisions over whether and where to disperse depend on its fitness. We compare fitness-dependent dispersal with cases where dispersal responds to juvenile survivorship only. Clines are steeper the more responsive dispersal is to environmental conditions for all dispersal behaviors that we consider. Clines eventually become stepped as the responsiveness of dispersal to environmental conditions is increased for half of the dispersal behaviors we consider, but smooth clines are maintained for the remaining cases. Smooth clines are maintained by the biased movement of individuals out of the hybrid zone when individuals move directionally in response to gradients in juvenile survivorship, which is a different mechanism to that maintaining smooth clines in classic cline theory.