Final answer:
Evolution occurs at the population level as genetic composition changes over generations, while individuals do not evolve but develop. Population genetics studies these evolutionary processes, with natural selection being a key mechanism driving adaptive evolution.
Step-by-step explanation:
Evolution acts on the population of a species rather than individual organisms. Population genetics is the science that combines evolutionary theory with Mendelian genetics to study this process within populations. It focuses on changes in the genetic composition over generations due to differential reproduction of individuals with certain alleles. Individuals, while they develop and change during their lifetime, do not evolve as their genetic composition does not change.
Natural selection, a mechanism of evolution, acts on the population's heritable traits by selecting for beneficial alleles that increase in frequency within the population, which is known as adaptive evolution. This process can lead to changes in characteristics such as bill size in finches or other adaptations that enhance survival and reproduction. These changes contribute to the shift in average characteristics within the population over large spans of time.