The principal says that allele frequencies in a population are stable and is constant from generation to generation. The gene pool (total genes and their alleles in a population remains a constant, this is called genetic equilibrium. The sum total of all the all the frequencies is 1.
For example-In a diploid p and q denote the frequency of A and a. The frequency of AA is `p^(2)` and aa is `q^(2)` and of Aa is 2pq. This `p^(2) + q^(2)` 2 pq = 1 which means `(p + q)^(2)` = 1. When frequency measured differs from expected frequencies the difference in frequencies indicates the extent of evolutionary change. The equilibrium is disturbed by 1- gene migration 2- mutation 3- genetic drift 4 - natural selection .
Disturbance in genetic equilibrium, or Hardy-Weinberg equilibrium, i.e., change of frequency of alleles in a population would then be interpreted as resulting in evolution.
