Genetic drift and population size.

This graph shows a hypothetical simulation of random genetic drift and its effects on small and large populations of black (B allele) and white (b allele) mice. In all cases, the starting allele frequencies are B = 0.5 and b = 0.5. The red lines illustrate two populations of mice in which N = 10. The blue line shows a population in which N = 1,000. Genetic drift leads to random changes in allele frequencies, eventually causing either the elimination or the fixation of alleles. This happens much more quickly in small populations than it does in large ones. In this simulation, genetic drift has led to small populations of all-black (BB) or all-white (bb) mice in 50 generations or fewer.