Phenotypes frequencies in a population vary over time.  Phenotypes are determined by genotypes which are determined by  combinations of alleles. If a population is in Hardy- … Weinberg  Equilibrium the allele frequencies will remain constant.Even  though the frequency of each allele remains constant, the way that  the alleles are paired in individuals will vary, changing the  phenotype frequency in each generation.f a population is in Hardy- … Weinberg  Equilibrium the allele frequencies will remain constant. Even  though the frequency of each allele remains constant, the way that  the alleles are paired in individuals will vary, changing the  phenotype frequency in each generation.