Evolution is the gradual, cumulative change over time of the characteristics of groups of organisms in a heritable manner. Eventually, these minute changes add up to produce an individual that is markedly different from its distant ancestors, but almost indistinguishable from its most immediate ancestors. These changes are brought about by the organism's genetic response to the environment, and, over the entire course of history, evolution has given rise to all different forms of life on Earth.

Evolution does not occur rapidly on the individual unit of life; changes are too small and slow to be effective at the individual level. In fact, evolution is more efficient at the population

The Bonneville Salt Flats in Utah were at one time covered by a sea of salt water. Evaporation removed the water, leaving the salt behind. © Buddy Mays/Corbis. Reproduced by permission.

level among groups of organisms that are...