American Scientist

May-June 2017

Volume: 105 Number: 3

Biologists and philosophers of science have long wondered whether evolution’s winding paths that led to today’s living world are predictable and repeatable, or unpredictable because of chance events. In this illustration, two evolutionary paths start out in parallel, and then begin to wend and bifurcate in sometimes similar and other times unique ways. Although evolutionary biologists cannot go back in time to rerun evolution from the beginning and follow its outcomes, they have devised ways to test this idea on smaller scales, most famously in the Long-Term Evolution Experiment with Escherichia coli. In this experiment, 12 initially identical populations of bacteria have been kept under the same unchanging conditions up to the present—through more than 66,500 generations. Previous generations are frozen and can be revived to rerun their evolution when something interesting arises. In "Replaying Evolution," Zachary D. Blount explains how this experiment and other research results are beginning to reveal when, in what ways, and to what extent evolution is contingent. (Illustration by Tom Dunne.)