There is currently an ongoing debate among evolutionary biologists about the rate at which evolution occurs. There are two prevailing theories. Some scientists believe that evolution occurs gradually in very small increments over millions and millions of years. This theory is called gradualism. Others believe that evolution does not occur continuously, but occurs in fits and starts. This theory, called punctuated equilibrium, proposes that there are relatively long periods of time during which no evolutionary changes are occurring; it is during relatively short time spans that there are many evolutionary changes in the species that inhabit the earth.

When Charles Darwin first proposed his ideas on evolution, he expressed the view that evolution and speciation occurred when organisms changed slowly, but at a relatively constant rate through time. He thought that over millions of years, small, incremental variations added up until eventually individuals were distinct enough from their...