GENETIC MODELS OF ADAPTATION AND GENE FLOW IN PERIPHERAL-POPULATIONS

We investigate the interplay between gene how and adaptation in periph eral populations of a widespread species. Models are developed for the evolution of a quantitative trait under clinally varying selection in a species whose density decreases from the center of the range to its periphery. Two major results emerge. First, gene flow from population s at the range center can be a strong force that inhibits peripheral p opulations from evolving to their local ecological optima. As a result , peripheral populations experience persistent directional selection. Second, response to local selection pressures can cause rapid and subs tantial evolution when a peripheral population is isolated from gene f low. The amount of evolutionary change depends on gene flow, selection , the ecological gradient, and the trait's heritability. Rapid diverge nce can also occur between the two halves of a formerly continuous pop ulation that is divided by a vicariant event. A general conclusion is that disruption of gene how can cause evolutionary divergence, perhaps leading to speciation, in the absence of contributions from random ge netic drift.