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As we saw in the synthetic theory of evolution, natural selection can be understood in terms of changing allelic frequencies in a population. Thus, the ability to study allelic frequencies in a population is vital to the study of evolution. In this SparkNote, we will look at the ways in which evolutionary biologists conduct such studies.

The Hardy-Weinberg Law is essential to the study of population genetics. It states that in a sexually reproducing population, allelic frequencies, and therefore phenotype, should remain constant under the following 5 conditions:

  1. large population size
  2. no mutation
  3. no immigration or emigration
  4. random mating
  5. random reproductive success
A population that meets all of these conditions is said to be in Hardy-Weinberg equilibrium.

Natural populations rarely experience Hardy-Weinberg equilibrium. Natural selection ensures that mating and reproductive success are not random, large populations are rarely found in isolation, and all populations experience some level of mutation. However, the Hardy-Weinberg Law is still very useful. The fact that populations that meet those 5 conditions will have unchanging phenotypes (will not evolve) is proof that variability and inheritance alone are not enough to cause evolution; natural selection must drive evolution. The law also allows us to estimate the effect of selection pressures by measuring the difference between actual and expected allelic frequencies or phenotypes.