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Hardy-Weinberg Equilibrium

The relationships described above, and the equations that describe them, are known as the Hardy-Weinberg equilibrium. But this brings us to an important point: if a population is in Hardy-Weinberg equilibrium, the allelic frequencies are not changing and therefore evolution could not occur. Hardy and Weinberg identified five tenants that, when violated, would result in the equilibrium being disturbed and, thus, a shift in the allelic frequencies, which may result in evolution. The five principles necessary to maintain Hardy Weinberg equilibrium in a population are:
  • No mutation: Mutation would cause the production of new alleles in a population and thus change allelic frequencies.
  • No migration: Migration, either immigration or emigration, would result in the exchange of alleles between populations, thus changing allelic frequencies.
  • No selection: If particular alleles were selected for, then the frequency of these alleles would increase over time. Similarly, if alleles were selected against, their frequency would be reduced over time.
  • Large population: With small populations, the allelic frequencies can change by chance due to random fluctuations in the gene pool.
  • Random mating: If mating is not random, then some alleles would increase in frequency while others would decrease. Usually random mating is violated when sexual selection is in play, which we will discuss shortly.
As you may have guessed, most populations violate at least one of these tenets. However, the principles and equations of Hardy-Weinberg equilibrium are still useful in describing and studying a population.
One notable violation of Hardy-Weinberg equilibrium is population size. Several processes have been helpful in understanding genetics of small populations. Some of these include: 
  • Genetic drift: Random chance will cause fluctuations in allelic frequencies. Smaller populations are more susceptible to these chance events, and frequencies can change significantly.
    For example, natural disasters can decimate small populations. If, by random chance, all the individuals with a certain variation were killed during a hurricane, then the allelic frequency of that trait would be dramatically reduced. Larger populations would not face such drastic changes.
  • Bottlenecks: When the size of a population is dramatically reduced, usually due to disease or natural disaster, alleles may be lost. The population may be able to recover and increase in number. However, the genetic variation has been reduced and cannot be restored. This is the problem with endangered species today. Although we have protected many species that were on the verge of extinction, diversity has been reduced. If the environment changes, there may not be enough variation within the population to allow the species to survive. Bottlenecks (and genetic drift) can be helped by gene flow. With endangered species, zoos will often exchange animals for mating purposes to increase genetic diversity.
  • Founder effect: When a few individuals from a population form their own new population, the only genetic variation that exists is what these individuals bring. In essence, they do not bring the whole gene pool to the new population, but rather only a fraction of it. Again, this will reduce diversity and variation.

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