Researchers involved in the E. coli long-term evolution experiment (LTEE) have debunked an established evolutionary theory with a study that provides a ‘high-resolution’ view of the molecular details of adaptation.
This illustration depicts a 3D computer-generated image of a group of Escherichia coli. Image credit: James Archer, CDC.
“Evolutionary adaptation is driven by the accumulation of mutations, but the temporal dynamics of this process are difficult to observe directly. Our study provides a high-resolution view of the molecular details of adaptation over substantial evolutionary timescales,” said study co-author Dr. Michael McDonald, from the School of Biological Sciences at Monash University, Australia.
“The insights we provide into the rate, repeatability, and molecular basis of adaptation will contribute to a better understanding of these evolutionary processes and challenges.”
Dr. McDonald and his colleagues are involved in the LTEE experiment, an ongoing study led by Michigan State University evolutionary biologist Richard Lenski that has been tracking genetic changes in twelve initially identical populations of E. coli bacteria since 1988.
“LTEE is the longest running microbial evolution experiment with more than 67,000 generations of E. coli, which is equivalent to over one million years of human evolution,” Dr. McDonald explained.
In the current study, the scientists analyzed the dynamics of molecular evolution in the twelve LTEE populations of E. coli.
To do this, they sequenced mixed-population samples taken at 500-generation intervals across 60,000 generations of the bacteria.
“We found that even though the E. coli populations have been evolving in a very simple environment for a long time, they are still adapting to their environment. In other words the fit get fitter,” Dr. McDonald said.
“But the established theory tells us that adaptation should have stopped by now since there should be a ‘fitness peak’ that the E. coli should have reached by now — and our work shows that this is not the case.”
According to the team, one explanation is that as E. coli evolve, they change the environment that they are growing in.
This change to the environment then drives further evolution, so that the populations may never stop adapting.
The findings were published online this month in the journal Nature.
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Benjamin H. Good et al. The dynamics of molecular evolution over 60,000 generations. Nature, published online October 18, 2017; doi: 10.1038/nature24287
