Hardy Weinberg Theory Explained


The Hardy Weinberg theory was named after G.H. Hardy and Wilhelm Weinberg, who were the first to prove that the concepts contained within the theory could be proved mathematically. The initial goal of this effort was to disprove a commonly-held theory at the time that dominant alleles would automatically increase as time passed.

The theory states that genotype or allele frequencies within a specific population will remain constant throughout each generation if there are no other evolutionary influences that are present.

Several evolutionary influences are possible that would change the genotype or allele frequencies, including something as basic as mate choice. Hardy and Weinberg offer selection, mutation, gene flow, genetic drift, and meiotic drive as additional evolutionary influences that could alter the frequencies.

The 7 Assumptions Made by the Hardy Weinberg Theory

For the theory proposed by Hardy and Weinberg to work, there are 7 basic assumptions that form its underlying foundation. They are as follows.

1. Organisms must be diploid.
2. The only form of reproduction that occurs is sexual reproduction.
3. Generations do not overlap one another.
4. Mating is a randomized activity.
5. The population size being considered is infinitely large.
6. Both genders have allele frequencies that are equal.
7. There is not any mutation, selection, or migration involved in the process.

If there is a violation of one of these 7 basic assumptions, then a deviation from the expected outcome will occur. The severity of the deviation and how it affects the general population depends on which assumption was violated. Here are some examples.

Violation of Assumption #4 (Randomized Mating):
The most common outcome of this assumption violation is inbreeding. When people are not allowed to choose a random mate, then they look inward to the family unit. This produces an increase in homozygosity.

Violation of Assumption #5 (Population Size):
When a small population is present instead of an infinite population, then random changes in allele frequencies may occur. This happens because of genetic drift.

Violation of Assumption #7a (Mutation):
A mutation may occur at any time in a population base during the cycle of reproduction. Some mutations may be positive, but many of them create a subtly negative effect on the population. When a recurrent mutation occurs, it will maintain the alleles within the population base, even when strong selective processes against them are implemented.

Violation of Assumption #7c (Migration):
An incidence of migration has a subtle effect on the general population. Because it links 2+ populations together, the allele frequencies become more homogeneous and could eventually lead to non-random mating.

Certain genetic traits are also gender-linked and that creates changes to the outcomes that are predicted by the Hardy Weinberg theory as well. The most-cited example of sex linkage in humans is red/green colorblindness. About 1 in 200 women in Western Europe have this colorblindness, which is closed to what the theory predicts will proportionally happen.

Because human men are the heterogametic sex of the species, they have only one copy of the gene. Since red/green colorblindness is an X-linked trait and men only have 1 copy of it, the trait affects them with greater frequency. About 1 in 12 men from Western Europe are affected by this form of colorblindness.

Why This Theory Is Important Today

The Hardy Weinberg theory is primarily used to test for forms of non-random mating, such as population stratification.

When considering the concept of inbreeding within a species, it looks beyond the direct family unit. A species located on the North American continent, for example, would not mate with the same species located on the African continent if there was no way to travel between the two locations. This would create inbreeding on those two continents because the entire population is not randomized.

Now the two individual continental populations can form randomized sub-groups, but this changes the evolutionary course for the entire population. Each subgroup will focus on the traits that are necessary for continued survival in their region of the planet, eventually forming a subspecies on each continent that is completely different from the first overall population group that started both subgroups.

Discovering these subgroups is necessary, especially within humans, because inbreeding creates a susceptibility to dangerous diseases. The Ashkenazim Jewish population is an excellent example of this. Half of all people with this genetic background are a carrier of up to 38 different genetic diseases.

By understanding who we are and from where we have come, it becomes possible to create consistent genotypes and alleles that are passed to new generations. That is the foundation of the Hardy Weinberg theory.