Population genetics is concerned with the genetic basis of evolution. The objects of study in population genetics are typically the frequencies and fitnesses of genotypes in a population. In the population genetics sense, evolution is the change of genotype frequencies over time, which may or may not be due to natural selection. The time scale of evolution is an exceedingly long one, so it is difficult to follow much genetic change in populations. Rather, population genetics is driven by constructing mathematical models of evolution and then checking if their predictions are met by natural populations.

In a 1991 commentary in the journal Genetics, Richard Lewinton wrote:

"When I entered Theodosius Dobzhansky's laboratory as a graduate student in 1951, the problematic of population genetics was the description and explanation of genetic variation within and between populations. That remains the problematic 40 years later in 1991."
It is no less central to the field in 2001.

Genetic variation is of central importance because evolution can only happen when there is variation to act on in a population. This understanding has helped to make population genetics the basis of modern evolutionary theory.