Lenski and his colleagues have repeated this procedure every day for the past 24 years, rearing over 55,000 generations of bacteria.
I first reported on Lenski’s experiment 12 years ago, and since then I’ve revisited it every few years. The bacteria have been evolving in all sorts of interesting ways, and Lenski has been able to reconstruct the history of that evolution in great detail, thanks to a frozen fossil record. Every 500 generations Lenski and his students sock away some bacteria from each flask in a freezer. They can thaw out these ancestors whenever they wish and compare them to their youngest descendants. Biotechnology has improved drastically since 1988, giving Lenski an increasingly powerful evolutionary microscope. When he started out, it could take months to identify just one of the many mutations that arose in each lineage. These days, he and his colleagues can sequence an entire E. coli genome for a few hundred dollars and find every single new mutation in its DNA.
The scientists determined that the bacteria had made a drastic switch: from feeding on the glucose to another compound, called citrate. Citrate is an ingredient in the broth where Lenski’s E. coli grows. It’s not food; instead, it helps keep the minerals in the broth in the right balance for E. coli to grow.
This was not simply a case of natural selection enabling a species to do something better. This was a case of doing something new.