E.coli Evolve

A few weeks ago in my interview with Adam Savage I mentioned the work of Dr. Richard Lenski on E.coli bacteria. Specifically the observation of the evolution of completely novel traits in the bacteria. This work is a fascinating look into both the painstaking nature of laboratory research and the evolutionary process in action. In the experiment (which has been going on for 20 years) the researchers grew colonies of E.coli on a medium that was restricted in the bacteria’s preferred food, glucose, and abundant with an alternative food source, citrate, that they can not normally metabolize. 12 separate but initially identical populations of bacteria were grown in this way and every 500 generations or so a sample of each was frozen to record the evolutionary trajectory each took through time. This situation is how things stood for over 30,000 bacteria generations. To put this number in perspective, in human generations this would take approximately 450,000 years, assuming a generational period to be about 15 years, not unreasonable for much of our history I would think. Could you imagine the patience required to continue this experiment for so long? Watching grass grow or paint dry would be interesting distraction by comparison surely. But I digress, at around the 31,500th generation something happened, one of the populations exploded due to having developed the ability to use citrate for food.

This result is not trivial, indeed the inability of E.coli to use citrate for food is one of it’s defining characteristics and can be used as a method for distinguishing it from other bacterial strains. The large amount of time it took the bacteria to evolve this trait also suggests that is not a path that is easy to take in space of all possible mutations. Further work done by Lenski indicates that it was likely the accumulation of at least three separate mutational events that lead to this outcome. This shows that how a species develops in the future is very much dependent on what has happened to it in the past, or in other words, differences in the makeup of a species constrain how it may change in the future. This may seem to be trivially true, after all, fins have to exist before limbs can evolve and limbs would seem to be the must have accessory if your going to develop wings. However to see how this is significant all we need to do is go back to one of those frozen cultures and see if we get the same result when we play it again, Sam. In fact this is exactly what was done and it was discovered that the trait only “re-evolved” after a certain generation in the population that had the fortune to innovate it in the first place.

This indicates that it is only the presence of certain other mutations in this particular bacterial line that allowed the evolution of this ability at all. And so it is by the perversity of historical accident that a population inherits the conditions upon which further moldings of the clay of life are wrought.

Resources

http://scienceblogs.com/loom/2008/06/02/a_new_step_in_evolution.php

http://www.newscientist.com/article/dn14094-bacteria-make-major-evolutionary-shift-in-the-lab.html

http://scienceblogs.com/pharyngula/2008/06/historical_contingency_in_the.php

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