Lenski replays evolution in long term E. coli populations

Richard Lenski's longterm evolution experiments on E. coli are a hallmark example of evolutionary biology. Lenski and colleagues have maintained 12 parallel lines of E. coli for 50,000 generations now. Initially, these E. coli populations were founded by clones, and over decades, researchers have watched evolutionary dynamics on a scale observable in real time.

This particular paper, published in 2008, describes the acquisition of a novel phenotype - the ability to metabolize citrate in addition to glucose as an energy source.

Lenski's experiments on the evolution of citrate use are particularly elegant for the following reason: The lab maintains frozen samples of the E. coli populations at time points throughout the history of the populations. These samples are not growing (and therefore not mutating) while frozen, but can be pulled from the freezer and reconstituted. This allowed researchers to go back to previous timepoints in the evolution of this phenotype and "replay evolution" to see if the same phenotypes arise again ...

• This is the paper :)