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.