The Hardy-Weinberg equilibrium helps scientists determine when natural selection or genetic drift is at work. If the results deviate from the prediction, you know that one of the following situations has occurred, because any of them would cause a deviation:
i Gene flow: Gene flow is one of the factors that can lead to a deviation from the Hardy-Weinberg equilibrium. If one genotype rather than another moves into or out of a population, the genotype frequencies can't be predicted from the allele frequencies.
Suppose that all the individuals with the aa genotype fly away. You're left with a population that consists only of AA and Aa individuals, and you can't predict the frequency of genotypes accurately from the frequency of the alleles. What's important is not just that movement of individuals into or out of the population is occurring, but that this movement is related to the genotypes of the individuals.
i Selection: Imagine that individuals with the aa allele die without reproducing (which would be the case with lethal recessive alleles). In such a case, only AA and Aa individuals would be left in the population. Again, the frequency of genotypes can't be accurately predicted by the frequency of the alleles. A deviation from expectations under Hardy-Weinberg indicates that some evolutionary force is at play.
Inbreeding: An example of nonrandom mating
Inbreeding, which is a special case of nonrandom mating, occurs when individuals mate with relatives. Because related individuals are more likely to have similar genes, they're also more likely to have similar deleterious recessive genes (which is why the offspring of close relatives tend to have reduced fitness). If these individuals mate, chances increase that two individuals with the recessive trait will get together and produce offspring that end up with two copies of the recessive genes and the condition that the recessive genes cause.
It's been estimated that the average human has perhaps a few lethal recessive alleles. But these alleles are rare in the population and express themselves only if a person happens to have children with someone who has the same lethal recessive gene.
1 Nonrandom mating: The Hardy-Weinberg equilibrium assumes that individuals mate randomly. If that's not the case — if AA individuals prefer other AA individuals, for example — after one generation of such mating, the proportion of individuals that is heterozygous will decrease. (Only matings between heterozygous individuals can produce more heterozygous individuals, but not all their offspring will be heterozygotes; some will be AA and aa.) Matings between homozygous individuals always produce homozygous offspring.
1 Random events (that is, genetic drift): When population sizes are small, random events can cause a deviation from Hardy-Weinberg. For example, while each allele in an individual has an equal chance of getting into a gamete, when only a small number of offspring are produced, just by chance one or another allele might be over (or under) represented, leading to yet another deviation from the expected genotype frequencies. Head to Chapter 6 for more information on genetic drift.
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