UC Berkeley
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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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This image is part of a series:
Genetic drift example (1 of 4)
BB individuals have big beaks, Bb individuals have medium-sized beaks, and bb individuals have small beaks. These birds live in a place where large and small seeds are abundant, but…
Genetic drift example (2 of 4)
Now imagine a small population of all small-beaked individuals (all bb genotypes). They have a high fitness (they are at a local peak), but not as high as a population…