Since the dawn of genetics in the early 20th century, biologists have debated if evolution is more driven by random mutations or by the original diversity in the gene pool.
Having many genetic alternatives to choose from can make natural selection much faster in the beginning, but do the genetic mutations that occur over time contribute more to the survival of the species in the end?
To try to solve this long-standing argument once and for all, researchers at Michigan State University have tested the adaptability of 72 different populations of Escherichia coli bacteria over 2,000 generations (about 300 days).
Each bacterial population was constructed to have different amounts of genetic diversity at the beginning of the experiment.
At one end of the spectrum, the population was born from a single clone, so each cell was genetically identical to all other cells.
In the middle of the spectrum, populations were grown from an already existing population of bacteria.
At the far end of the spectrum, E coli populations were created by mixing a few pre-existing populations together, creating the maximum amount of genetic diversity possible.
Each population was fed glucose at the beginning of the experiment. To test the adaptability, different sets of these bacterial populations were taken and propagated in a different growth environment, which provided them with the amino acid D-serine instead of glucose for their energy needs.
At 0, 500 and 2,000 generation points, the populations were tested for their ability to compete for nutritional resources against a common competitor (which was another strain of E coli with a medium level of fitness).
The E coli All samples came from the Long-term Experimental Evolution Project, which was started in 1988 by one of the co-authors of the latest journal, the evolutionary biologist Richard Lenski.
When each population of bacteria was measured for its condition in the D-serine environment before any evolution occurred, the more genetically diverse populations performed better than the clones.
In the early stages of the experiment (about 50 generations in), the richness of genetic diversity in the native population was important for adaptation.
But in the 500th generation, diversity at the beginning of the experiment “no longer mattered” because the new mutations were “large enough”, the authors write in their oppressionwhich is available on BioRxiv before peer review.
At the 500th and 2,000th generations, there were “no differences in fitness” among all the different bacterial populations, despite the variation in fitness in the beginning.
“All the benefits of pre-existing variation in asexual populations can often be short-lived, as we saw in our experiment, because that variation will be cleared away when new beneficial mutations sweep to fixation,” the researchers write.
Although not yet reviewed by others in the scientific community and published in a peer-reviewed journal, this result may close the book on longest-running argument in evolutionary biology when it comes to bacteria.
But there is no “right” answer when it comes to the relative importance of standing variation and new mutations for adaptation in nature, the researchers write.
Researchers working with different models tend to “emphasize one or the other source of genetic variation,” they add.
Researchers studying animals and plants tend to emphasize the diversity of the gene pool as the greatest source of evolutionary capacity because it is not practical to wait hundreds of years for mutations to mix things up.
Those who study bacteria and virus tend to view mutations as the greatest source of evolution.
But really, both forces – mutation and existing genetic diversity – can “contribute sequentially, simultaneously and even synergistically to the process of adaptation through natural selection,” the researchers say.
This repression is available at BioRxiv before peer review.
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