Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

The frequency of the individual genotypes in the equilibrium state can be readily calculated from the frequency of the alleles in a certain locus. This equilibrium state is called the Hardy-Weinberg equilibrium. In a large panmictic population, i.e. in a population whose members reproduce together quite at random, this equilibrium is established during a single generation. It follows from the laws of combinatorics that, at equilibrium, genotypes a₁a₁, a₁a₂ and a₂a₂ will be present in a ratio of ¦²: 2¦_a¦_a:¦², where ¦_aand ¦_adenote the frequency of alleles a₁ and a₂ in the previous generation (Fig. II.14).  Here, the ratio in which the individual genotypes were present in the previous generation is not in any way important. During a single generation, the same representation of the individual genotypes is established, regardless of whether the original population contained, for example, only homozygotes a₁a₁ and a₂a₂, or only heterozygotes a₁a₂.

If we study several genes, amongst which there is no genetic linkage, for example, if each of them is located on a different chromosome, equilibrium is again established within a single generation; the numbers of the individual genotypes can be calculated for this state according to the simple rules of combinatorics.