In a population, natural selection acts on phenotype of individuals.
Explanation:
Natural selection tends to favour those organisms of a population which carry traits that allow them to be better adapted to the environment and these traits are passed on to their offsprings. Natural selection tends to act on the phenotype of the population whereas evolution is the phenomenon which changes the allele frequencies. Natural selection acts on individual organisms of a population whereas the change in populations, on the whole, arises due to evolution.
