This is known as the Hardy-Weinberg Theorem and it can be demonstrated mathematically to be true. The population must be isolated . There must be no migration of individuals into or out of the population. There are no net changes to alleles due to mutations . Mating is random .
All genotypes are equal in reproductive success .
Whilst these conditions can be approached in nature, they are never achieved for very long . Population Genetics Population genetics is the scientific study of gene pools and the genetic variation within populations. Because the five conditions for Hardy-Weinberg equilibrium are seldom maintained for many generations, gene frequencies in gene pools do change . If a population is small, genetic drift may occur. Chance events in matings may cause the allele frequencies to drift randomly from generation to generation .