The essence of Darwinism lies in a single phrase: natural selection is the creative force of evolutionary change. No one denies that selection will play a negative role in eliminating the unfit. Darwinian theories require that it create the fit as well.

—Stephen J. Gould

The term "evolution" has multiple meanings; it is most generally used to describe the theory that all organisms are linked via descent to a common ancestor. Evolution also refers to the gradual process during which change occurs. In biology it is the theory that groups of organisms, such as species, change or develop over long periods of time so that their descendants differ from their ancestors morphologically (in form, structure, and physiology) in terms of their life processes, activities, and functions. (Species are the smallest groups into which most living things that share common characteristics are divided. Among the key characteristics that define a species is that its members can breed within the group but not outside it.)

It is important to understand that not all change is considered evolution; evolution encompasses only those changes that are inheritable and may be passed on to the next generation. For example, evolution does not explain why humans are taller and bigger today than they were a century ago. This phenotypic (observable) change is attributable to changes in the environment—that is, improvements in nutrition and medicine—and is not inherited. Similarly, it should also be noted that while evolution leads to increasing complexity, it does not necessarily signify progress because an adaptation, trait, or strategy that is successful at one time may be unsuccessful at another.

In genetic terms, evolution can be defined as any change in the gene pool of a population over time or changes in the frequency of alleles in populations of organisms from generation to generation. Evolution requires genetic variation, and the incremental and often uneven changes described by the process of evolution arise in response to an organism's or species' genetic response to environmental influences.

Evidence of evolution has been derived from fossil records, genetics study, and changes observed among organisms over time. The process produces the transformations that generate new species only able to survive if they can respond quickly and favorably enough to environmental changes. Population genetics is the discipline that considers variation and changing ratios of genetic types within populations to explain how populations evolve. Such changes within a population are termed "microevolution." In contrast, "macroevolution" describes larger-scale changes that produce entirely new species. Although some researchers speculate that the two processes are different, many scientists believe that macroevolutionary change is simply the final outcome of the collected effects of microevolution.

Molecular evolution is the term used to describe the period before cellular life developed on Earth. Scientists speculate that specific chemical reactions occurred that created information-containing molecules that contributed to the origin of life on this planet. Theories about molecular evolution presume that these early information-containing molecules were precursors to genetic structures capable of replication (duplication of deoxyribonucleic acid [DNA] by copying specific nucleic acid sequences) and mutation (change in DNA sequence).