The species Escherichia coli, abbreviated E. coli, is a bacterium that is commonly found in the lower intestine of warm-blooded animals.

The researchers, Zachary D. Blount, Christina Z. Borland, and Richard E. Lenski, all of the MSU Department of Microbiology and Molecular Genetics (East Lansing, Michigan, U.S.A.), used twelve initially identical populations of the bacterial species Escherichia coli from 1988.

The bacteria, over a twenty-year period, evolved within a “glucose-limited medium that also contains citrate.”

The abstract to their paper stated, “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.”

Around generation 31,500, the bacteria began to metabolize citrate, something that the original bacteria could not do. Evidence for the evolution of complex traits, therefore, was shown.

They concluded, “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.”

Dr. P.Z. Meyers, a biologist at the University of Minnesota at Morris, makes commented on the paper. Please turn the page for more details.