Quotations are taken from a definition of evolution issued in 1997 by the National Association of Biology Teachers. The full statement is available online.
Natural selection is the primary mechanism driving evolutionary change.
It is caused by variations which exist within a species due to differences in the genetic makeup of its individual members. This causes some individuals to be more likely to thrive in a given environment, and to produce offspring like themselves. Punctuated equilibrium is the concept that evolution did not happen at a steady rate. Rather, the historical record shows long periods of relative stability, such as the present time, when few new species evolved. These periods were broken by short periods of rapid change.