Flexi Says: Natural selection acts on the phenotype (the traits or characteristics) of an individual. On the other hand, while natural selection changes the percentages of genes within a whole population, it does not act directly on the underlying genotype (the genetic makeup) of an individual. For example, in many traits the homozygous genotype (AA) has the same phenotype as the heterozygous genotype (Aa). If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot select for a homozygous individual over a heterozygous individual. Even if the "aa" phenotype is lethal, the recessive allele may be maintained in the population through heterozygous Aa individuals. The mating of two heterozygous individuals can produce homozygous recessive (aa) individuals. However, natural selection can and does differentiate between dominant and recessive phenotypes.