As stated in the introduction to population
genetics, the Hardy-Weinberg Law
states that under the following conditions both phenotypic and allelic
frequencies remain constant from generation to generation in sexually
reproducing populations, a condition known as Hardy-Weinberg equilibrium.
- large population size
- no mutation
- no immigration or emigration
- random mating
- random reproductive success
This section will look in detail at the five conditions upon which the Hardy-
Weinberg Law is contingent.
Large Population
A population must be large enough that chance occurrences cannot significantly
change allelic frequencies significantly. To better understand this point,
consider the random flipping of a fair coin. The coin is as likely to land on
heads as it is on tails. If a coin is flipped 1000 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. But in small
populations with fewer copies of alleles, chance can greatly alter allele
frequencies. In small populations, a change in allelic frequencies and
phenotypes based on random occurrences is called genetic drift.
No Mutation
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.
No Immigration or Emigration
For allelic frequencies to remain constant in a population, individuals must not
move in and out of that population. 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.
Random Mating
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 greatest fitness for a given environment, usually work
against random mating: the most fit organisms are most likely to mate.
Random Reproductive Success
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.
