In the most general sense, biological evolution is change in organisms over time.

Evolution includes the relatively small fluctuations that occur continually within populations, as when the genetic makeup of a population changes from one year to the next. But evolution can also refer to the larger changes that occur as species gradually become increasingly different from their ancestors. Let's explore these two ways of looking at evolution in more detail, focusing first on genetic changes (allele frequency change) and then on how organisms accumulate differences from their ancestors (descent with modification).