The theory, illustrated here, states that much evolutionary change takes place in short periods of time tied to speciation events.

A population of mollusks is in equilibrium, living, dying, and getting fossilized every few hundred thousand years. Little observable evolution seems to be occurring based on fossil evidence. Due to climate change, there is a drop in sea level and a small number of mollusks are isolated from the rest of the population. The isolated population experiences strong selection and rapid change because of their new environment and small population size. No fossils representing the transitional mollusks are preserved because of their relatively small population size, the rapid pace of change, and their isolated location. Eventually, sea levels rise, reuniting the isolated mollusks with their original population. The newly evolved mollusks out-compete their former neighbors. Over time there is a shift to the new mollusks and their adapted traits. More recent fossils show only the evolved mollusk form.

d we did this already