Darwin imagined that evolution by a process of natural selection occurs gradually over time. In each succeeding generation there is new variation and shifts and changes in populations, but that the changes are slow and gradual and that the appearance of significantly different types of organisms, i.e. new species, are from a human perspective difficult to observe.
In the early 70's Stephen Jay Gould and Niles Eldredge proposed an alternative mechanism by means of which new species could appear on earth. They called their idea Punctuated Equilibrium because they noted from studying the fossil records for some organisms, that there were longs periods where very little change occurred, and that these periods of "equilibrium" were then "punctuated" by very dramatic changes in a relatively short period of geologic time. In both Gradualism and Punctuated Equilibrium, natural selection is operating to bring about changes in populations over time and the eventual appearance of new species. As scientists gather more and more evidence to prove the FACT of evolution, the evidence suggests that for some species change occurs gradually as Darwin imagined, but that for other species changes can occur much more rapidly, in great leaps if you will, as Gould and Eldridge proposed. In biological science, unlike in religious dogma, there are no absolute truths. The fact that natural selection can operate on some species that are evolving gradually and on other species whose populations exhibit more dramatic changes in much shorter periods of time, only shows that in the natural world there are often many ways in which changes in living systems can occur.

By: Floyd Sandford at: 5th October, 2009