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 response to the environment, and, over the entire course of the history of life on earth, evolution has given rise to all the different forms of life that have ever existed.

Evolution does not work on the individual unit of life; changes are too small and too slow to be effective at that level. In fact, evolution works at the population level--in other words, among groups of organisms that are capable of successfully breeding with each other. With organisms that do not breed with other individuals, the rate of evolutionary change is slower than it...