Final answer:
The Hardy-Weinberg equilibrium occurs when a population is not evolving and allele frequencies remain constant, assuming no mutations, no migration, large population size, random mating, and no natural selection.
Step-by-step explanation:
When a population is in Hardy-Weinberg equilibrium, it means that the population is not evolving. According to the null hypothesis, if a population is in Hardy-Weinberg equilibrium, then allele and genotype frequencies in the population will remain constant over time, provided that five specific conditions are met: no mutations, no migration, large population size, random mating, and no natural selection. Essentially, the Hardy-Weinberg principle serves as a mathematical model to help scientists identify when a population may be evolving by comparing actual genetic data to the expected frequencies if these conditions are met.