Lenski’s Long Term Evolutionary Experiment
Richard Lenski began the LTEE with 12 populations (six and six ) of the bacterium Escherichia coli on 24th February, 1988. The experiment is currently housed at Michigan State University and has run continuously apart from a short break while relocating to the present site and another during the height of the Covid-19 outbreak.
The method is very straightforward. Each 24 hours, from flasks of the bacterium in a growth medium known as Davis minimal broth dosed with glucose at 25 mg per litre (DM25), are extracted by pipette a random sample of 0.1 ml which is added to a new volume of 9.9 ml DM25 in new flasks which are then incubated at 37°C for 24 hours and the procedure repeated indefinitely.
What the experiment does is to provide a consistent, stable and simplified niche for the twelve lines allowing them to proceed in parallel isolation (great care is taken to avoid cross-contamination). This allows the researchers to test whether evolutionary change is inevitable, repeatable or unpredictable.
One objection often made by critics is that, being designed, the experiment is not a true test of natural evolution. But Lenski chose the environment, he does not design the bacteria. A random (the flasks are continuously agitated on a mixing plate to ensure uniform distribution of cells) sample makes it through to the next generation (70,000 and counting in the thirty years the experiment has been running) but, over time, the twelve lines have undergone changes that can be observed. Cells have become larger, redundant (in the experimental niche) parts of the genome have become broken.
The most spectacular change so far has been the arrival of the ability of one line to metabolise citrate aerobically. The change has been well-studied because deep-frozen samples are retained every 75 days and DNA sequences (thanks to cheaper and quicker DNA sequencing) can be compared to match genomic changes against phenotypic changes. The ability to digest citrate involved changes at more than one locus, a beautiful illustration of neutral evolution and genetic drift.
The LTEE also is an excellent refutation for Creationist John Sandford and his “Genetic Entropy” idea. I’m sure others can point out errors but this OP is meant only to provoke discussion and not to be authoritative so please jump in with comments.