This is probably the most subtle life process of all, and the most powerful in ensuring the continuance of life in one form or another. Adaptation to rapid changes in the environment, which are short in comparison with the lifetime of any one individual, falls under the heading of stimulus and response, just discussed. But there is another way of adapting to more extensive changes, which take place over long time periods compared with the lifetime of an individual: evolution. Without this mechanism, the planet still would be populated only by small localized bits of ordered chemical reactions, and higher life (a self-congratulatory terminology) would not exist. Propagation takes place by copying, and growth by using, the information stored in DNA molecules. This copying of information from one generation to the next is not quite perfect, and a few mistakes, or mutations, occur. These mistakes are the raw materials of evolution.

Populations evolve, not individuals. Within a population of individual organisms at any given time, the majority usually are well adapted to existing conditions. A certain minority will vary genetically, and will be somewhat maladapted in one or more tolerable ways. If conditions change, this variability is sufficient to allow some minor and previously maladapted fraction of the original population to become better adapted than the majority. Over several generations the population will change, and the favored few will become the new majority. A small amount of maladaptation and variability is the insurance premium that is paid by the population against the threat of altered conditions. If the entire population were identical, and all were equally well adapted to the original conditions, then they all would be equally badly adapted to any new environmental changes, a possible lethal uniformity.