An evolutionary history in which a species remains essentially unchanged for hundreds of thousands or even millions of years and then experiences short periods of very rapid evolution. The punctuated, or rapid change periods, are the result of major environmental changes in predation pressure, food supply, climate, etc. During these times, natural selection may favor varieties that were previously at a comparative disadvantage. The result is an accelerated rate of change in gene pool frequencies in the direction of the varieties that became the most favored by the new environmental conditions. See phyletic gradualism.