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Evolution in Action
Genetic Equilibrium
Genetic Equilibrium
Hardy and Weinberg's allele pool equation demonstrated mathematically that all populations of baby peccaries (and, in theory, all other populations of organisms as well) were in a state of genetic equilibrium.

Year after year, generation after generation, there should be no change in either the gene pool, or the kinds and types of baby peccaries born. This is what the mathematical formula said; no change, therefore no evolution!

Undoubtedly, some populations of some species "obey" the Hardy-Weinberg principle and never change their gene pools or their ratios of genotypes over millions of years, and thus never evolve. However, many other populations of many other species change and evolve very rapidly indeed. These rapidly changing species are NOT "obeying" the Hardy-Weinberg principle.


Genetic equilibrium, as defined by obedience to the Hardy-Weinberg principle, is the basis of measuring all evolutionary change. Study populations (of real organisms) are compared to the ideal of genetic equilibrium, and if they do not measure up to the ideal in any way, this is a force that is bringing about evolutionary change.

Evolution is measured at the population level with genetic equilibrium as the standard.

According to the Hardy-Weinberg principle, both the ratios of genotypes and the frequency of alleles remain constant from one generation to the next in a sexually reproducing population, provided other conditions are stable.

Conditions for Stability So long as any population remains free of outside interference, it will remain in genetic equilibrium. Real populations like our peccaries, however, are rarely free from outside influences, and they never totally meet the following five basic conditions for stability:

1 Reproduction must be totally random. If either the males or the females show strong preferences during the mate-selection and/or the mating process, then only certain genotypes will get to pass on their genes to the next generation. The genotype frequencies will be different in each generation.

Peccaries mate completely randomly and with no preferences; therefore, they meet this condition for stability.

2 There must be no gene flow. There must be no immigration or emigration of individuals to or from the population. Exchanging individuals between populations acts to reduce variety in the population losing the individual and to increase variety in the population receiving the individual.

Peccaries rarely exchange individuals between groups because these animals have very strong social ties to one another.

3 Populations must be large. In large populations, the elements of pure chance are not a significant factor.

Unfortunately, peccaries exist in small populations; therefore genetic drift (loss of an individual by pure chance) can cause rapid and wide deviations from the original gene pool frequencies.

4 There must be no mutations. Mutations change genes into different alleles, either changing the ratio of alleles or introducing new ones.

No populations are ever free from mutational events, including peccaries. Spontaneous mutations arise all the time in any gene and in any individual.

5 There must be no selection. Natural selection differentially removes certain genotypes from the population, and thus increases the frequency of certain alleles in the next generation.

This is almost certainly true of the peccaries.

In an ideal population left undisturbed by any of the five conditions listed above, there would be complete genetic equilibrium.

The population would always contain the same type of genes represented in the same ratios within the same phenotypes. Over long periods of time, this population would never change.

Clearly, in the real world, this does not happen. A definition of evolution, therefore, could be:

Deviations from genetic equilibrium equal evolution!

© 2001, Professor John Blamire