The evolution of non-random movement along clines

Questions: Simple models of dispersal assume that it incurs evolutionary costs by inhibiting local adaptation. Is this still true when recognizing that individuals' decisions over whether and where to disperse are often a response to the environmental conditions they face? What are the implications of such conditional and directional dispersal behaviours for the evolution of dispersal itself? Mathematical methods: Population genetic model of gene frequency change at two loci in a cline (spatial selection gradient) when dispersal rates depend on local fitness. Key assumptions: The landscape is a linear array of equal-sized patches bounded by core populations. One locus is under direct selection and there is a gradient in juvenile survivorship with different genotypes being favoured on either side of an environmental transition zone. I compare models in which evolutionary changes in the cline spread out to affect dynamics in core populations with models where core populations are insulated from changes that occur within the cline. Population regulation applies after dispersal when there are limited opportunities to reproduce. The second locus governs the evolution of dispersiveness. Dispersing individuals bias their movements to climb local gradients in juvenile survivorship or expected fitness (juvenile survivorship and overcrowding). Predictions: (1) When dispersal is directional and conditional, it does not impede local adaptation as much as when it is unresponsive to environmental conditions. It may even advance local adaptation. But more responsive dispersal call lead to overcrowding. Initially, the effect of dispersal on crowding dominates, but eventually the effect of dispersal on local adaptation becomes more important. (2) Dispersers; abandon the cline centre when dispersal responds to juvenile survivorship, but cluster there when it responds to expected fitness. (3) When core Populations are affected by changes in the cline, sigmoidal (as opposed to stepped) clines are maintained more effectively by dispersal that is responsive to the environment than by random dispersal.