Speciation resulting from divergence within a population (sympatry) generally assumes that gene flow is a constraint; however, evidence for this has come from correlations that sidestep the possibility that multiple causal pathways could be important. In order to determine whether gene flow can indeed hold back divergence, Nosil investigated this effect in polymorphic populations of the stick insect Timema cristinae in two different habitats. Each population was tracked over several years, and about halfway through this period, the gene flow between one' pair was interrupted. In.contrast to the control groups, the perturbed populations showed a divergence in morph frequencies in following generations that were attributed to a lack of dispersal and reduced population size. This study provides experimental evidence showing that gene flow does constrain adaptive divergence in the wild.
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