Gene flow is undoubtedly one of the most important processes in population genetics. In the absence of gene flow a combination of mutations and genetic drift will cause populations to diverge genetically from one another, whereas in the presence of gene flow populations can be held together as interconnected units that collectively contribute to a species' evolution. Nevertheless, gene flow does not necessarily make populations genetically homogeneous; populations may regularly exchange immigrants and still maintain a significant level of genetic divergence.
Population differentiation in the face of ongoing gene flow has been one of the most intensely debated subjects in evolutionary biology. How much gene flow is necessary to prevent differentiation by genetic drift? How is local adaptation possible if individuals are dispersing regularly among sites? What are the relative roles of drift and selection in promoting population differentiation? These questions are of fundamental importance to the evolution of populations and the cohesiveness of species. In this section we will look first at how gene flow and genetic drift interact, although this will be a relatively short discussion because genetic drift was covered in some detail in Chapter 3. We will then devote the rest of this section to the seemingly paradoxical co-existence of gene flow and local adaptation.
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