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 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.
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 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.
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 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.