Broadly speaking, gene flow maintains populations as cohesive units, whereas genetic drift and natural selection can cause populations to diverge and eventually to split into distinct species. Throughout this chapter we have been concerned primarily with relatively recent adaptive and non-adaptive changes within populations. From time to time we have referred also to the more distant evolutionary histories of populations and species, which can have lasting impacts on genetic structure. This will be explored further in the next chapter, which is concerned primarily with the link between evolutionary histories and the current distributions of species.
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