Final answer:
Natural selection acts on individuals within populations by favoring those with heritable traits that increase survival and reproduction, contributing to adaptive evolution.
Step-by-step explanation:
Natural selection is a key mechanism in evolution, acting on the phenotypic variations found within populations. It favors individuals with heritable traits that improve their chances of survival and reproduction, thereby contributing more to the next generation's gene pool. This process, which can lead to evolutionary change, is known as adaptive evolution. Specifically, natural selection acts on individuals within populations, selecting for those who are better adapted to their environment and thus have a higher evolutionary (Darwinian) fitness.