As Darwin frequently emphasizes in his writings, although selection may be the
mechanism through which evolution occurs, it depends on there being variation in individual
characteristics. Without variation there is nothing to select. Most applications of Darwin’s
theory thus build on a cyclical model that allows for the variation, selection, retention, and
modification of species characteristics. Variations in a species typically arise as a result of
cross-reproduction and random variation of characteristics. Some of these variations may
confer a competitive advantage in the survival process, leading to a better chance of
selection, or of evolving along with changes in the environment. Because the surviving
members of a species, or emerging new species, provide a foundation for the next stage of
reproduction, there is a strong chance that the new characteristics will be retained. hi turn,
these characteristics will be subject to random modification, creating the variety that allows
the process to continue. In this way, new species and ecological patterns evolve from
variations in the old.