contingencia histórica waaaa

Historical contingency and the evolution of a key
innovation in an experimental population of
Escherichia coli

Zachary D. Blount, Christina Z. Borland, and Richard E. Lenski*
Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI 48824
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 25, 2006.
Contributed by Richard E. Lenski, April 9, 2008 (sent for review March 26, 2008)

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.


PNAS
June 10, 2008
vol. 105
no. 23
7899–7906


www.pnas.org/cgi/doi/10.1073/pnas.0803151105

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