UC Berkeley
Image Caption
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 no medium-sized seeds are available. Populations of all big-beaked individuals have a very high average fitness — they can crack open big seeds. Populations of all small-beaked individuals do well (they can manipulate smaller seeds) — but not quite as well as the big-beaked individuals. Medium-beaked individuals have the lowest fitness — they are not particularly good with either big or little seeds (and no medium-sized seeds are available). A graph of these gene frequencies and the population's resulting fitness levels is shown. This sort of graph is called an adaptive landscape.
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This image is part of a series:
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…
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…