I Wish That I Could Turn Back The Clock

A superb paper on the importance of historical contingency in the evolutionary process just came out in the latest issue of PNAS.

This study was done to address an old quibble between Simon Conway Morris and Stephen Jay Gould about what would happen if we could return to the prehistoric past and replay the evolutionary process again.

Will natural selection result in organisms that look fairly similar to what we have on Earth today, or will a series of unpredictable happenstances result in new organisms that look utterly different?

The Lenski group at Michigan State University spent 20 painstaking years to perform this study - I must say that I'm impressed with both the simple elegance of the experimental design and the sheer dogged tenacity of the research team.

I don't have enough time now to write a comprehensive summary (see Pharyngula's excellent post), but just to give you a flavour of what they achieved:

12 separate populations of E. coli bacteria (which normally cannot use citrate as an energy source under oxic conditions) were grown in identical citrate-rich environments for over 30,000 generations.

They froze a sample of each population every 500 generations to act as their "fossil record" so that they can rerun the experiment from that stage, if required.

All 12 populations had been experiencing a declining rate of fitness improvement.

Then, at the 33,127-generation mark, one of the populations was observed to have evolved the ability to use citrate. None of the other 11 populations can do it, not even till today (>44,000 generations).

"Replay" experiments using stored samples earlier than the 20,000th generation were completely unable to yield any citrate-using populations.

In striking contrast, some of later samples easily evolved citrate use within a few thousand generations.

Thus, this evolutionary innovation is produced by mutational change. Natural selection only plays a secondary role in preserving the changes.

Would you like to know more?
- Original research article: Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli (Blount et al. PNAS)
- Historical contingency in the evolution of E. coli (Excellent summary at Pharyngula blog)