Occurrence of natural selection and small size of population do not meet for a population to reach Hardy-Weinberg Equilibrium. For Hardy Weinberg equilibrium to be reached, natural selection should not be occurring. If populations are undergoing natural selection at the locus under consideration, allele frequencies will be changing in a specific direction and continuously, Hardy-Weinberg Equilibrium predicts that allele frequencies will stay constant. It assumes that population size is very large.