What is the effect of genetic drift on the Hardy-Weinberg equilibrium?

If genetic drift is not happening the gene frequencies will stay constant from generation to generation and the genotype frequencies will also be constant, in Hardy-Weinberg proportions. But in a smaller population the gene frequencies can drift around. The average gene frequencies in one generation will be the same as in the previous generation, and it might be thought that the long-term average gene and genotype frequencies will simply be those of the Hardy-Weinberg equilibrium, but with a bit of 'noise' around them. That is not so, however. The long-term result of genetic drift is that one of the alleles will be fixed; the polymorphic Hardy-Weinberg equilibrium is unstable in a small population.

Figure: Hardy-Weinberg proportions of genotypes AA, Aa and aa in relation to the frequency of the gene a.