Final answer:
Natural selection acts on phenotypes—the observable characteristics influenced by alleles. Beneficial phenotypes lead to increased allele frequency in a population's gene pool, while disadvantageous phenotypes result in decreased allele frequency.
Step-by-step explanation:
Natural selection selects for or against certain phenotypes. Phenotypes are the observable characteristics that are determined by the genotype and can be influenced by the environment. Natural selection directly interacts with these traits to determine whether they give the individual an advantage or disadvantage in its environment. If a phenotype resulting from certain alleles provides a reproductive advantage, those alleles become more common within the gene pool of the population over generations. Conversely, alleles that result in disadvantageous phenotypes tend to decrease in frequency within the gene pool.
Furthermore, natural selection acts on populations by selecting individuals with greater evolutionary fitness; those who are more likely to survive and reproduce contribute more to the gene pool of the next generation. Therefore, while natural selection does affect the gene pool and can influence which alleles are present in a population, it is specifically the phenotypic expression of those alleles that natural selection acts upon.