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Population genetics is the study of genetic variation within populations. Genetic variation provides the basis for evolutionary processes, leading to changes in allele frequencies over time.

The genetic makeup of a population is defined by its gene pool, which includes all alleles for every gene in the population.

At a given locus, individuals carry two alleles, one from each parent. If both alleles are the same, the individual is homozygous. If the alleles are different, the individual is heterozygous.

Consider an example of a population of squirrels in which a single gene controls coat color. This gene has two alleles: R and B.

Squirrels that are homozygous for the R allele have red coats. Those that are homozygous for the B allele have dull brown coats, and heterozygous squirrels have red-brown coats.

Now, imagine that in a population of 100 squirrels, there are 40 red, 30 brown, and 30 red-brown squirrels.

Each squirrel carries two alleles for this gene, so there are 200 total alleles in the population.

Allele frequency is the proportion of a specific allele in a population. To calculate the frequency of the R allele, we count how many copies of the R allele are in the population.

Each of the 40 red squirrels carries two R alleles, giving a total of 80 R alleles. The 30 red-brown squirrels each carry one R allele, adding 30 more. So the population has 110 R alleles.

When we divide 110 by the 200 total alleles, we get 0.55, or 55 percent.

Using the same method, we count 90 B alleles out of the 200 total alleles. That equals 0.45, or 45 percent.

By measuring allele frequencies across generations, we can tell whether the population is evolving or remaining genetically stable.