To save: right-click (Windows) or control-click (Mac) on the image and select "Save image."

Pace of evolution hypotheses (2 of 4) Image caption: Hypothesis 1: Phyletic gradualism - slow and steady divergence of lineages. The "burst" of evolution is a geological illusion. It only looks like a burst because a lot of time — say, 5 million years — passed between the times when the two rock layers were laid down. In this period of time, species 3 gradually diverged from ancestor 1 through a series of transitional forms, but these transitional forms were not preserved. << Back to search results

Image credit:
If you use this image in your own non-commercial project please credit it to the University of California Museum of Paleontology's Understanding Evolution (http://evolution.berkeley.edu).

This image is part of a series:

Pace of evolution hypotheses (1 of 4) In many cases, we seem to observe "bursts" of evolution in the fossil record. In this example, in a lower rock layer, you see ancestor 1. In the next rock layer, you see species 2 and 3. Species 2 looks the same as ancestor 1. Species 3 is morphologically distinct, but is clearly also descended from ancestor 1. What happened?

Pace of evolution hypotheses (3 of 4) Hypothesis 2: Punctuated equilibrium — a large amount of change in a short time, tied to a speciation event. Species 2 and 3 are only 100,000 years younger than ancestor 1, and all the evolutionary change connecting them took place in this short time. The "burst" of evolution is really a burst. Transitional forms between ancestor 1 and species 3 did exist, but for such a short amount of time that they were not preserved in the fossil record.

Pace of evolution hypotheses (4 of 4) Hypothesis 3: Macromutation — a big mutation produces sudden evolutionary change skipping over transitional forms. The "burst" of evolution is really a burst — there was a lot of evolutionary change in a very short amount of time. Species 3 was produced by a mutation that radically changed the offspring of ancestor 1 in many ways. Such extreme mutants are sometimes called "hopeful monsters." This hypothesis is consistent with the fossils; however, based on other observations, we do not have clear evidence that such extreme yet adaptive mutations generally occur. Nevertheless, it is possible that mutations affecting development have far-reaching phenotypic effects and have played an important role in the evolution of life.