Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli
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Contributed by Richard E. Lenski, April 9, 2008 (received for review March 26, 2008)

Abstract
The role of historical contingency in evolution has been much debated, but rarely tested. Twelve initially identical populations of Escherichia coli were founded in 1988 to investigate this issue. They have since evolved in a glucose-limited medium that also contains citrate, which E. coli cannot use as a carbon source under oxic conditions. No population evolved the capacity to exploit citrate for >30,000 generations, although each population tested billions of mutations. A citrate-using (Cit+) variant finally evolved in one population by 31,500 generations, causing an increase in population size and diversity. The long-delayed and unique evolution of this function might indicate the involvement of some extremely rare mutation. Alternately, it may involve an ordinary mutation, but one whose physical occurrence or phenotypic expression is contingent on prior mutations in that population. We tested these hypotheses in experiments that “replayed” evolution from different points in that population's history. We observed no Cit+ mutants among 8.4 × 1012 ancestral cells, nor among 9 × 1012 cells from 60 clones sampled in the first 15,000 generations. However, we observed a significantly greater tendency for later clones to evolve Cit+, indicating that some potentiating mutation arose by 20,000 generations. This potentiating change increased the mutation rate to Cit+ but did not cause generalized hypermutability. Thus, the evolution of this phenotype was contingent on the particular history of that population. More generally, we suggest that historical contingency is especially important when it facilitates the evolution of key innovations that are not easily evolved by gradual, cumulative selection.
Footnotes
- ↵*To whom correspondence should be addressed. E-mail: lenski{at}msu.edu
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 25, 2006.
Author contributions: Z.D.B., C.Z.B., and R.E.L. designed research; Z.D.B. and C.Z.B. performed research; Z.D.B. and R.E.L. analyzed data; and Z.D.B. and R.E.L. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0803151105/DCSupplemental.
- Received March 26, 2008.
- © 2008 by The National Academy of Sciences of the USA