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Hardy-Weinberg Equilibrium

Departures From Equilibrium Indicate Evolutionary Forces At Work

With these simple tools, we can look at populations to see if they conform to these numerical patterns. If they differ, we seek the reasons for the difference in some violation of the Hardy-Weinberg assumptions. Two processes, natural selection and genetic drift, are the most common and important factors at work in most populations that are not at equilibrium.

For example, suppose we find a population in which the recessive allele frequency is declining over time. We might then investigate whether homozygous recessives are dying earlier. (Many genetic diseases, such as cystic fibrosis, are due to recessive alleles.) This could be due to natural selection, in which those that are better adapted to the environment survive longer and reproduce more frequently.

Or suppose we find a population in which there is a smaller-than-expected number of homozygotes of both types, and a larger number of heterozygotes. This could be due to heterozygote superiority—where the heterozygote is more fit than either homozygote. In humans, this is the case for the allele causing sickle cell disease, a type of hemoglobinopathy.

Nonrandom mating is another potential source of departure from the Hardy-Weinberg equilibrium. Imagine that two alleles give rise to two very different appearances. Individuals may choose to mate with those whose appearance is closest to theirs. This may lead to divergence of the two groups over time into separate populations and perhaps ultimately separation into two species.

In very small populations, allele frequencies may change dramatically from one generation to the next, due to the vagaries of mate choice or other random events. For instance, half a dozen individuals with the dominant allele may, by chance, have fewer offspring than half a dozen with the recessive allele. This would have little effect in a population of one thousand, but it could have a dramatic effect in a population of twenty. Such changes are known as genetic drift.

Richard Robinson


Hartl, D. L., and A. G. Clark. Principles of Population Genetics, 3rd ed. Sunderland, MA: Sinauer, 1997.


B: dominant allele frequency.

b: recessive allele frequency.

b = (observed homozygote recessive frequency)1/2.

B = 1 − b.

B × b = expected frequency of heterozygotes in the population.

B2 = expected frequency of homozygous dominants in the population.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 2Hardy-Weinberg Equilibrium - Basic Concepts, Assumptions Of The Hardy-weinberg Model, Allele Frequencies Remain The Same Between Generations