The Hardy-Weinberg principle says that allele frequencies in a population are stable and constant from generation to generation as long as
The population is large enough and changes in allele frequencies due to chance or accident are insignificant.
Mating occurs at random.
Mutation does not occur or if it does occur it must reach a state of equilibrium.
All the members of the population survive and have equal reproductive rates.
A population will not exist in Hardy-Weinberg equilibrium if the individuals mate selectively as the mating has to be random and influences like mutation, natural selection and genetic drift affect the equilibrium.