Final answer:
Evolution occurs when the trait averages within a population change over generations and species adapt to environments, driven by natural selection rather than individual adaptation.
Step-by-step explanation:
Evolution occurs when the average value of a trait in a population changes from one generation to the next, and when species gradually become better adapted to their local environment over evolutionary time. This process occurs due to differential reproduction of individuals with certain alleles within a population. Therefore, individuals do not evolve; rather, change is seen in the genetic composition of the population over time as a result of natural selection.
For example, in medium ground finches in the Galápagos, natural selection may lead to a change in average bill size over generations. Such changes in traits are due to the differential survival and reproduction of individuals that are better adapted to environmental pressures. These changes accrue over time, leading to the evolutionary adaptation of the species.