English: Photograph of Richard Lenski's long-term evolution experiment with E. coli taken on 25 June 2008. Each flask harbors one of the 12 evolving populations. The photo was taken about 24 h after the cultures were serially transferred, hence they are at stationary phase. The populations grow in DM25 medium, which contains a relatively low glucose concentration so that the culture is not very turbid. However, one population, designated Ara-3, evolved to use the citrate that is also present in the medium. It has a much greater density of cells than the others, and it is therefore much more turbid, owing to the high concentration of citrate in the medium supporting greater cell growth.