In Chapter 3 we learned that by quantifying the genetic diversity of single populations we can gain considerable insight into processes as varied as bottlenecks, reproduction and natural selection. We must understand these before we can interpret genetic data in an ecological context, but the genetics of populations are influenced by both intrapopulation and interpopulation processes, and therefore the next step is for us to investigate how gene flow (the transfer of genes from one population to another) influences the evolution of populations and species. We will start this chapter by looking at how we can quantify population subdivision and gene flow. However, we cannot predict the effects of gene flow without adding genetic drift and natural selection into the equation, because these are the three processes that jointly determine the extent to which populations will genetically diverge or converge. In the second part of this chapter, therefore, we shall revisit genetic drift and natural selection, but this time the emphasis will be on the ways in which they interact with gene flow to influence the genetic structure of populations across a species' range.
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