UC Berkeley
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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 of big-beaked individuals. Through gene flow some B alleles are introduced to the population. If selection alone were acting, it would weed these alleles out of the population since they would show up in Bb individuals with lower fitness. Under selection alone, the population could never reach the higher BB 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 (3 of 4)
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…