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Natural selection can be understood as 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 section, 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:
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large population size
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no new mutations
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no migration
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random mating
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no natural selection
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.
The Hardy-Weinberg Law
The Hardy-Weinberg Law states that under the above conditions, both phenotypic and allelic frequencies remain constant from generation to generation in sexually reproducing populations, a condition known as Hardy-Weinberg equilibrium.
This section will look in detail at the five conditions upon which the Hardy-Weinberg Law is contingent.
A population must be large enough that chance occurrences cannot change allelic frequencies significantly. To better understand this point, consider the random flipping of a coin. The coin is as likely to land on heads as it is on tails. If a coin is flipped 1,000 times, it is likely to land on heads almost exactly 50% of the time. However, as you may know from experience, if the same coin is flipped only ten times, it is much less likely that it will land on heads 5 times. The same holds true for allele distributions in populations. Large populations are unlikely to be affected by chance changes in allele frequencies because those chance changes are very small in relation to the total number of allele copies. However in small populations with fewer copies of alleles, chance can greatly alter allele frequencies.
In order for allelic frequencies to remain constant, there must be no change in the number of copies of an allele due to mutation. This condition can be met in two ways. A population can experience little or no mutation. Alternatively, it can experience balanced mutation. Balanced mutation occurs when the rate at which copies of a given allele are lost to mutation equals the rate at which new copies are created by mutation.
Additionally, individuals must not move into or out of a population to maintain allelic frequencies. Whenever an individual enters or exits a population, it takes copies of alleles with it, changing the overall frequency of those alleles in the population.
In order for all alleles to have an equal chance of being passed down to the next generation, mating within the population must be random. Non-random mating can give an advantage to certain alleles, allowing them to be passed down to more offspring than other alleles, increasing their relative frequency in the population. The processes of natural selection, since they usually select for individuals with the greatest fitness for a given environment, usually work against random mating; the most fit organisms are most likely to mate.
Just as mating must be random, the survival of offspring to reproductive age, or reproductive success, must also be random. Again, natural selection usually works against such randomness.
