Final answer:
Evolution occurs when the genetic composition of a population changes over time, often observed as a change in the average value of a trait, due to natural selection.
Step-by-step explanation:
Evolution occurs when the average value of a trait in a population changes from one generation to the next. This change is the result of the differential reproduction of individuals with certain alleles, which is a cornerstone of evolution by natural selection. While individual organisms can and do adapt to their environment during their lifetimes, these changes are not considered evolution as they are not heritable changes to the genetic composition of the population. Instead, when we talk about evolution, we are referring to genetic changes that influence an entire population over generations. To illustrate, the bill size of medium ground finches in the Galápagos may change over time, not because individual finches' bills are changing, but because the average bill size in the population reflects the reproductive success of finches with certain bill sizes under specific environmental conditions.