Gradualism and Punctuated Equilibrium

0:00Speciation is the formation of new species. But how fast does a new species form?

0:07Time runs upward. Sideways shows body form: the further apart two species are, the more different they look.

0:16The first model is gradualism: species change slowly and steadily, in small steps.

0:23Each line is a lineage: a chain of ancestors and descendants traced through time.

0:30Where a line forks, one species splits into two. This is a speciation event.

0:36Each branch keeps changing at a steady pace, so the lines slant and slowly spread apart.

0:43Gradualism is the classic view of neo-Darwinism, which joins Darwin's natural selection with genetics.

0:51If change is gradual, rocks should hold many fossils of in-between forms.

0:57The second model, proposed in 1972, is punctuated equilibrium.

1:03In this model, a species barely changes for millions of years at a time. This is called stasis.

1:11When a new species branches off, most of its change happens in a short burst.

1:18Equilibrium means the long calm; punctuated means the calm is interrupted by bursts.

1:26Short means short in geological time: a burst can still last tens of thousands of years.

1:33Bursts leave few fossils of in-between forms, so new species seem to appear suddenly in the rocks.

1:41Which model is right? Both happen, and the fossil record has examples of each.

1:47Gradualism is slow, steady change; punctuated equilibrium is long stasis broken by short bursts.