Hardy-Weinberg
The Hardy-Weinberg principle of equilibrium, states that a population’s allele and genotype frequencies are inherently stable, and unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies will change.
The Hardy-Weinberg equation is:
p2 + 2pq + q2 = 1 p to the square plus two time p times q equals 1
This mathematical tool allows us to detect deviation from the expected commonness of gene versions (alleles) and as such, discover evolutionary pressures on a population.
The Hardy-Weinberg principle assumes conditions with no mutations, migration, emigration, or selective pressure for or against a genotype, plus an infinite population; while no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes.
Even Hardy and Weinberg recognized that no natural population is immune to evolution. Populations in nature are constantly changing in genetic makeup due to genetic drift, mutation, possibly migration, and selection. As a result, the only way to determine the exact distribution of phenotypes in a population is to go out and count them. But the Hardy-Weinberg principle gives scientists a mathematical baseline for a non-evolving population to which they can compare evolving populations and thereby infer what evolutionary forces might be at play. If the frequencies of alleles or genotypes deviate from the value expected from the Hardy-Weinberg equation, then the population is evolving.