Evidence for punctuated equilibrium

Cheetham has studied in detail the evolution of a group of the sessile aquatic invertebrates called Bryozoa (also called Polyzoa). He concentrated on a genus called Metrarabdotus; some members of the genus are alive in the seas today and they also have an extensive fossil record.

The stratigraphic record is reasonably complete: the average time interval between sample populations was 160 000 years, and the completeness was 0.63 for time units of 160 000 years. He measured up to 46 morphological characters per specimen in a total of about 1000 specimens, from about 100 populations. He concluded that these bryozoans mainly evolved in the punctuated equilibrial mode.

Most of the species did not change in form over long periods of several million years, and most of the new species appeared suddenly without intermediate transitional populations. If there were intermediate forms they lasted (on average) less than 160 000 years.