Since the dawn of genetics in the early 20th century, biologists have debated whether evolution is driven more by chance mutations or by the original diversity of the gene pool.
Having lots of genetic options to choose from might speed up natural selection in the beginning, but do genetic mutations that occur over time contribute more to species survival in the end?
In an attempt to resolve this long-standing argument once and for all, researchers at Michigan State University tested the adaptive capacity of 72 different populations of Escherichia coli bacteria over 2,000 generations (about 300 days).
Each population of bacteria was designed to have different amounts of genetic diversity at the start of the experiment.
At one end of the spectrum, the population arose from a single clone, so each cell was genetically identical to all other cells.
In the middle of the spectrum, the populations were cultured from a pre-existing population of bacteria.
At the end of the spectrum, E.coli the populations were created by mixing a few pre-existing populations together, thus creating the maximum possible genetic diversity.
Each population was fed glucose at the start of the experiment. To test adaptability, various sets of these bacteria populations were taken and propagated in a different growth environment, providing them with the amino acid D-serine instead of glucose for their energy needs.
At generation points 0, 500, and 2000, the populations were tested for their ability to compete for nutritional resources against a common competitor (which was another strain of E.coli with an intermediate fitness level).
The E.coli the samples all came from the Long-Term Experimental Evolution Project, which was started in 1988 by one of the co-authors of the recent paper, evolutionary biologist Richard Lenski.
When each population of bacteria was measured for its fitness in the D-serine environment before any evolution, the most genetically diverse populations fared better than the clones.
In the early stages of the experiment (about 50 generations), the rich genetic diversity of the initial population was important for adaptation.
But, by the 500th generation, the diversity at the start of the experiment “no longer mattered” because the new mutations were “large enough”, write the authors in their preprintwhich is available on BioRxiv prior to peer review.
At the 500th and 2000th generation, there was “no difference in fitness” between all the different populations of bacteria, despite the variation in fitness at the start.
“Any benefit from pre-existing variation in asexual populations can often be short-lived, as we have seen in our experience, as this variation will be purged when new beneficial mutations move into fixation,” the researchers write.
Although it has not yet been approved by other members of the scientific community and published in a peer-reviewed journal, this result could close the book on the oldest argument in evolutionary biology with respect to bacteria.
But there is no “right” answer in terms of the relative importance of permanent variation and new mutations for adaptation in nature, the researchers write.
Scientists working on different models tend to “emphasize one or another source of genetic variation”, they add.
Scientists who study animals and plants tend to emphasize the diversity of the gene pool as the main source of evolutionary capacity, because it is impractical to wait hundreds of years for mutations to mix things.
Those who study bacteria and viruses tend to regard mutations as the main source of evolution.
But in reality, the two forces – mutation and existing genetic diversity – “may contribute sequentially, simultaneously and even synergistically to the process of adaptation by natural selection”, say the researchers.
This preprint is available at BioRxiv before peer review.