Sunday, September 30, 2012

E Coli Caught Evolving in the Lab

Today I want to write about a really cool experiment done on E Coli bacteria that managed to observe a new trait evolve. My primary reference is the wikipedia page, but I also read an article from new scientist and I looked at the web page from the actual research lab. The wikipedia page and the new scientist article seem to be saying the same thing in an understandable way. The research lab's page has a wealth of information but it was difficult for me to get through it. Also, they had links to several papers about the work, but the ones I checked were behind a pay wall so I am unable to go look at the source material. As I have said before, I'm not a scientist, but I am a science fan. If I make any mistakes or say something that isn't quite true, please feel free to correct me.

Why E Coli?

E Coli has a handful of properties that make it perfect for this type of study. It only reproduces asexually, and it isn't able to conjugate. This means that any changes over the generations must be from mutations. (I guess with a species that reproduces sexually, you would have to worry about recessive genes for example). It is also possible to freeze and preserve a sample, and revive it at any time in the future. This allows them to essentially have a "frozen fossil record" which they will always have access to.

The Experiment 

They have 12 populations of E Coli, which are initially from the same sample. Every 75 days (about 500 generations) they will freeze a sample of each population. They will analyze the samples and do fitness tests to see how the samples do as time goes on. In roughly the first 20,000 generations, all 12 samples progressed at about the same rate. They became specialized for growing on glucose, growing 70% faster in that medium but performing poorly in other environments.

That part so far didn't seem to be written up very excitedly in the wikipedia page or the article, I guess because the other part of it is so much better, but already I think this is amazing. What I see is that E Coli 'in the wild' have certain trade offs that affect it in various ways and a certain collection of traits will be ideal. In the experiment it had a constant supply of food which means different trade offs will be ideal. All 12 populations were able to get to roughly the same point by evolving through some apparently common mutations in about 20,000 generations. (although the language to use naturally implies some intention on the part of the population, we have to remember it is just random mutation and natural selection driving things)

As far as I'm concerned, what has been described is already best described with evolution. This is exactly the kind of thing that we would expect to see with evolution, and it is what is observed. If you want to try to replace evolution with something else, it would need to have at least this much explanatory power.

But then, at around generation 33,127 something really amazing happened. The experimenters noticed that one of the populations had a huge boost in its growth rate. They discovered that it has evolved the ability to use citrate (which is part of the growth medium) as a source of energy. Normally E. Coli is not able to use this material as food. It makes sense that this particular mutation would thrive once present, it adds a new food source, so the mutant with this ability would thrive compared to the others around it who all have to compete for a common energy source.

An interesting thing with the citrate eating population, is that unlike the other adaptations, there is only one population that developed this skill. The researchers interpret this as the citrate eating skill requires more than one mutation. Furthermore, the early mutation isn't beneficial by itself. It seems that it was more of a sidestep that isn't particularly selected for. They went back to their frozen fossil record and tried to see if they ran a new experiment forward from various points along the way, if the citrate eating skill would evolve again. It seems that the first mutation necessary for that skill happened somewhere around generation 20,000, because if they tried from populations before that the skill didn't develop. One thing this research shows, is that somewhat random changes in genotype, which seem to have no real effect at the time, can have profound effects into the future.

Something I will add that isn't mentioned in any of the articles, this seems to me to be a good example of a path toward speciation. We have a population where some of it's members happen to randomly evolve a way to make use of a new food source. If the old food source is heavily competed for and the new source is not, it is conceivable that the population might dine exclusively on the new source. If this happens, it is entirely possible that they will have very different selection pressures on them and they might be pushed in different directions evolutionarily, this might even happen with them staying in the same physical place.

Well, I certainly think this is an excellent example of evolution happening before our very eyes, I hope I haven't messed up very many of the details here. Hope you enjoyed reading about this as much as I did.


  1. Excellent summary. I first heard about this experiment in The Greatest Show On Earth. Such awesome stuff!

    1. Thanks :)

      I'm probably going to do more of the evolution posts in the future just because I really enjoy digging into the articles myself.


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