The Hardy-Weinberg theorem states that the frequency of alleles and genotypes in a population's gene pool remain constant over the generations unless acted upon by agents other than sexual recombination. For example, take a population of mice that consists of 1,000 members. A specific allele, albino allele, is recessive within this species. 80% of the population expresses the normal phenotype- brown coloring, while the remaining 20% are albino. 640 members of the population have the genotype AA, 320 have Aa, and 40 have aa. If completely random mating were to occur, there would be an 80% chance that a gamete...
more common. For example, if resistance to a particular disease allows certain mice to live in an environment that has been introduced to that disease, the allele for resistance may become more frequent in the population.

In order to create a population in Hardy-Weinberg equilibrium, five conditions need to be met- a very large population, isolation from other populations, no net mutations, random mating, and no natural selection. However, because of certain factors, such as genetic drift, gene flow, unavoidable mutations, nonrandom mating, and natural selection, having a population in Hardy-Weinberg equilibrium is impossible. Populations will evolve over time.