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The Hardy-Weinberg equilibrium principle describes the unchanging frequency of alleles and genotypes in a stable, idealized population. In this population we assume there is random mating and sexual reproduction without normal evolutionary forces such as mutation, natural selection, or genetic drift. In the absence of these evolutionary forces, the population would reach an equilibrium in one generation and maintain that equilibrium over successive generations. The equilibrium for a population with the alleles A and a, for example, would be allele frequencies of .6A and .4a and genotype frequencies of .36AA, .48Aa, and .16aa. By describing specific ideal conditions under which a population would not evolve, the Hardy-Weinberg principle identifies variables that can influence evolution in real-world populations. If a population is not in a state of equilibrium, at least one of the evolutionary forces is at work causing change in the population. Further investigation can determine which variables are influencing the changing population.
For further information, see Palomar College's Hardy-Weinberg Equilibrium Model at their Web site.
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