So far in this chapter we have learned how to reconstruct evolutionary relationships, but we have done little more than allude to the processes that may have influenced the current distributions of genetic variation. We will now redress this imbalance by taking a more detailed look at what sorts of geographical and historical phenomena might have affected population sizes, population differentiation, gene flow and, ultimately, the distribution of species and their genes. We will begin this section by looking at some of the reasons why populations become isolated from one another, and we will then ask how long it takes for populations to become genetically distinct once reproductive isolation is complete. We will end this section with a discussion of the confounding influence that hybridization may have on our interpretation of past events.
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