E coli long term evolution experiment
If anybody’s interested here’s a link to the official webpage for Richard Lenski’s e-coli long term evolution experiment, the one I mentioned a few days ago. The project’s wikipedia page provides a good explanation for laymen.
This ongoing 25 year experiment shows not only that evolution by natural selection occurs, but that it is observable.
Basically, Linski started the experiment in 1988 by taking a specific strain of e coli and then preparing an alternate version of the same strain that was able to grow on Arabinose. Arabinose was not present in the growth medium, the difference between the two strains was simply included as a genetic marker intended to help identify any cross contamination that might occur during the daily procedures required by the experiment. He then took six identical cells from each strain, making twelve different populations, and put one each into a separate test tube filled with a mixed growth medium and placed them into an agitating incubator. In addition to glucose, the DM25 liquid growth medium contained citrate (citric acid.)
Note also that E. coli cannot use citrate to support growth; it serves only as a chelating agent in this medium.
Each day a small sample (about 1% by volume) is taken from each of the twelve populations, alternating between Arabicose and - populations to help identify any potential mistakes in procedure, and placed separately into a new test tube with fresh growth medium. The populations average 6.64 cell divisions per day, and the number of generations at this point now exceeds 56,000. Every 500 generations a long term sample is frozen from each population, providing the ability to go back and test the populations for genetic changes and to repeat any evolutionary steps observed from an earlier, genetically pure sample.
Between generation 30,000 and 30,500 something amazing happened in just one of the twelve populations, some of the e coli cells within it had evolved the ability to feed on the citrate. The inability of wild e coli to feed on citrate is considered a hallmark of the species, it’s one of the standard tests biological laboratories use to distinguish it from salmonella. The citrate eating mutants immediately thrived, and being able to metabolize a food source normal e coli cannot were able to outcompete their parent population, quickly becoming the dominant form within that culture line.
What’s interesting is that the researchers were able to not only prove via genetic testing that the evolution was not the result of any contamination, but were able to revive cells from the previously frozen long term samples from the population in question and watch the final evolutionary step occur again, though it occurred only on the order of one in a trillion cell divisions.
The source material has more detailed information and links to papers. The researchers believe that at least three different mutations were required for the evolution to citrate eating e coli, including at least one potentiating mutation that deteriorated the normal DNA repair mechanisms of the e coli cell, itself not adaptive but increasing the rate of mutation.