Final answer:
Natural selection acts on the phenotype of an organism. It cannot select for specific genotypes unless they are expressed in the phenotype, thus impacting the genetic variation within a population.
Step-by-step explanation:
Natural selection acts on the phenotype (the observable characteristics) of an organism rather than the genotype (the genetic makeup). This is because it is the phenotype that interacts with the environment. The environment cannot differentiate between organisms with the same phenotype, even if they have different genotypes, such as AA (homozygous) and Aa (heterozygous) for a trait.
Thus, natural selection cannot directly select for or against a specific genotype unless it's expressed in the phenotype. For instance, the 'aa' phenotype might be lethal, but the 'a' allele can be maintained in a population through Aa individuals. Moreover, natural selection can differentiate between dominant and recessive phenotypes, affecting the overall genetic variation within a population.