When populations become geographically discontinuous, the free movement of alleles is prevented. When this separation lasts a certain time, the two populations are capable of evolving along different trajectories. Thus, their allele frequencies at many genetic loci become progressively more and more different as new alleles arise independently by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors, differ for the two populations, causing natural selection to favor divergent adaptations in each group.

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