However, since the population is small, drift can be a powerful force. Just by chance, the frequencies of the B alleles increase in the population over several generations (and the population moves into a valley in the adaptive landscape). If the B alleles become frequent enough, the population will begin to have BB individuals with high fitness. As this happens, selection begins to increase the frequency of B (the population moves out of the valley and selection pushes it towards the global fitness peak). Eventually, through the action of genetic drift combined with selection, the population moves from one local peak, through a valley of low fitness, to the global fitness peak.
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