The Hardy-Weinberg Equilibrium is a biological model which states evolution is not being observed in a population provided that several assumptive criteria are not violated. In simplified terms, the criteria are as follows, in no particular order:
- The population must be large, or even infinitely large;
- Random mating in regards to the observed trait must be occurring;
- Gene flow must not be occurring;
- Mutation must not be occurring;
- Natural selection must not be occurring on the observed trait. When a population adheres to all five criteria, it is not undergoing evolution; the population is in equilibrium. However, if even one criterion is violated, then the population is undergoing evolution.
The Hardy-Weinberg Equilibrium was developed and presented by G.H. Hardy and Wilhelm Weinberg; the model is accordingly named after both. It is a mathematical model that uses the genetic data of a population in order to determine whether it is in equilibrium or undergoing evolution with regard to a particular trait. Thus, in order to test a population against the model, one must have some prior information about the genetic structure of the population, with a minimum knowledge of either the allelic frequencies or the genotypic frequencies.