A population will not exist in Hardy-Weinberg equilibrium if:

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.