The word “evolution” is often used in a general sense to mean simply “change,” but in a biological context “evolution” refers specifically to change in the genetic makeup of populations over time. Developmental changes that occur in a single individual over the course of the life cycle are not the result of evolutionary change. Evolution is genetic change across generations of a population—a group of individuals of a single species that live and interbreed in a particular geographic area at the same time. It is important to remember that individuals do not evolve; populations do.

The premise of natural selection was one of Darwin’s principal insights and has been demonstrated to be an important process of evolution, but natural selection does not act alone. Four additional processes—mutation, gene flow, genetic drift, and nonrandom mating—affect the genetic makeup of populations over time. Before we consider how the other processes can change the frequencies of gene variants in a population, we need to understand how mutation brings such variants into existence.