populations are maintained in a incubator in Lenski's laboratory at Michigan State University. Each day, 1% of each population is transferred to a flask of fresh DM25 growth medium. The dilution means that each population experiences 6.64 generations, or doublings, each day. Large, representative samples of each population are frozen with glycerol as a cryoprotectant at 500-generation (75-day) intervals. The bacteria in these samples remain viable, and can be revived at any time. This collection of samples is referred to as the "frozen fossil record", and provides a history of the evolution of each population through the entire experiment. The populations are also regularly screened for changes in mean fitness, and supplemental experiments are regularly performed to study interesting developments in the populations. , the E. coli populations have been under study for over 64,500 generations, and are thought to have undergone enough spontaneous mutations that every possible single point mutation in the E. coli genome has occurred multiple times. Founding strain The strain of E. coli Lenski chose to use in the long-term evolution experiment was derived from "strain B", as described in a 1966 paper by Seymour Lederberg (which incorrectly identified the strain as "Bc251", although later genetic analysis found it to be "B" instead), via Bruce Levin, who had used it in a bacterial ecology experiment in 1972. The defining genetics traits of this strain were: T6r, Strr, r−m−, Ara− (unable to grow on arabinose). Lenski designated the original founding strain as REL606. Before the beginning of the experiment, Lenski isolated an Ara+ variant of the strain in which a point mutation in the ara operon had restored growth on arabinose, which he designated as strain REL607. When beginning the long-term evolution experiment, Lenski founded six populations with six individual Ara− colonies of REL606. These populations are referred to as Ara-1 through Ara-6. Lenski also founded six more populations from six individual Ara+ colonies of REL607. These are referred to as populations Ara+1 through Ara+6. The marker differences permit strains to be differentiated on Tetrazolium Arabinose plates, on which Ara− colonies appear red, while Ara+ colonies appear white to pink. Over the course of the experiment, each population has accumulated a large number of distinct mutations, which permit further means of identifying strains by their population of origin. Results Changes in fitness Much analysis of the experiment has dealt with how the fitness of the populations relative