Important Experimental Evolution Result Detailed

In 1988 biologist Richard Lenski started 12 identical cultures of E. coli bacteria and began propagating them every day to study their evolution. Along the way, he and his students froze cultures so that they could always go back and study them more closely. A few years ago his lab reported that after about 31,000 generations the bacteria had gained the ability to live on citrate as a food source. The results received a lot of fanfare and attracted the interest of anti-evolutionists. However, the paper describing the result was not fully satisfying because it didn't look into the underlying genetic changes that gave rise to the citrate-eating ability.

This week Lenski's lab published a follow-up paper and we now know what happened....mostly. E. coli already have a gene (citT) for a protein that imports citrate into the bacterial cell, but it isn't turned on in the presence of oxygen. When they looked at the citrate-eating bacteria, they found that the area around that gene had been duplicated, but in a way that put the duplicated gene under a new promoter, which is a section of DNA that regulates when genes are turned on. This is illustrated in Figure 2 of the paper:

The new arrangement allowed citT to be turned on in the presence of oxygen and, lo and behold, citrate-eating bacteria were born. However, they weren't very efficient at first so further tweaks to the new arrangement refined their ability. Some of these tweaks included further duplications of the new arrangement to increase the amount of the importer protein produced.

This would all be interesting enough, but they also found that if they went back and inserted the new gene arrangement into ancestral bacteria from before generation 20,000, it hardly worked. But it did work if they put it in ancestral bacteria after generation 20,000. This fits with their previous results where they found that the ability to eat citrate could re-evolve in cultures started from stocks after generation 20,000. It thus appears that other unspecified mutations elsewhere in the genome set the table for the gene duplication and rearrangement to be useful. Unfortunately we may never know what those other mutations were because even with the genome sequence, figuring out which mutations were important would be an enormous amount of work. Interestingly, when they looked at the genomes of the bacteria from the re-play experiments, they found that the same kind of gene duplication and rearrangement often occurred but that no two were exactly alike. In a few cases the genetic event was quite different while giving the same basic result.

The authors propose that evolution often proceeds in a manner that can be divided into three parts: potentiation, actualization and refinement. First, mutations accumulate that are of little significance on their own. Second, some kind of genetic event occurs that results in a new function or new regulation of the function. This genetic event is able to be accommodated because of the previously unimportant mutations. Third, the new ability is refined by further mutation.

The whole study is quite elegant and was clearly a lot of work, and it will go down as a landmark in experimental evolution. Yet, its results are not surprising. These kinds of genetic events can be inferred to have occurred many times in the genomes of all manner of organisms, including humans. But now we have a detailed record of one that occurred (and re-occurred) in a laboratory. This study is also a clear refutation of the ridiculous but frequent claim by creationists that mutations can only degrade genetic information.


See also Carl Zimmer's summary: The Birth of the New, The Rewiring of the Old

---