Another reason why variation persists is that natural selection acts only on phenotypes. If there is a harmful recessive allele, the only phenotype that suffers is the homozygous recessive individual. The allele is protected from selection when it is in a heterozygote. Natural selection can reduce the frequency of a harmful recessive allele to very low levels but cannot eliminate this allele. This is because when the frequency of the harmful allele is low, most of those alleles are hiding in heterozygotes. This is why genetic diseases persist in populations.