The proportions of alternate forms of a gene (alleles) in a large population will not change from generation to generation, unless they are influenced by mutation, selection, emigration, or immigration of individuals from other populations. If these conditions have no effect and if mating is random, the proportions of genotypes in the population will also remain the same after one generation.

It is important to recognize that this principle states the conditions under which evolution will not occur. Deviations from the above situation indicate that evolution is occuring as a result of Natural Selection or other factors.

Theoretically, any population can be described by the equation p2 + 2pq + q2 = 1 where p is the frequency of one allele and q is the frequency of the other. To get the gene frequencies, take the individual # of each respective allele and divide it by the total # of alleles (both types) in the population. If these frequencies are calculated for two consecutive generations and they appear to be changing significantly, evolution is occuring (a change of up to 0.05 is considered equilibrium).