Since the dawn of genetics in the early twentieth century, biologists have debated whether evolution is driven more by random mutations or by the original diversity of the genetic background.
Having a lot of genetic options to choose from can make natural selection move much faster in the beginning, but do genetic mutations that occur over time contribute more to the survival of the species in the end?
In an attempt to resolve this long-standing argument once and for all, researchers at Michigan State University have tested the adaptive capacity of 72 different populations of Escherichia coli bacteria for 2,000 generations (about 300 days).
Each bacterial population was designed to have different amounts of genetic diversity at the start of the experiment.
At one end of the spectrum, the population was bred from a single clone, so that each cell was genetically identical to all other cells.
In the middle of the spectrum, populations were cultured from a pre-existing bacterial population.
At the far end of the spectrum, the populations of E. coli were created by mixing some pre-existing populations together, creating as much genetic diversity as possible.
Each population was fed glucose at the beginning of the experiment. To test for adaptability, several sets of these bacterial populations were captured and propagated in a different growth environment, providing them with the amino acid D-serine instead of glucose for their energy needs.
At the 0, 500, and 2,000 generation point, 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 level of fitness).
All samples of E. coli were derived from the long-term experimental evolution project, which began in 1988 by one of the co-authors of the recent article, evolutionary biologist Richard Lenski.
When the suitability of each bacterial population in the D-serine environment was measured before any evolution occurred, the most genetically diverse populations performed better than the clones.
In the early stages of the experiment (around 50 generations), the richness of the genetic diversity of the initial population was important for adaptation.
But in the 500th generation, diversity at the start of the experiment “no longer mattered” because the new mutations were “large enough,” the authors write in their prepress, which is available on BioRxiv prior to the review for pairs.
In the 500 and 2000 generations, “there were no differences in physical fitness” between all the different bacterial populations, despite the variation in fitness at the beginning.
“Any benefit of pre-existing variation in asexual populations can often be short-lived, as we saw in our experiment, because this variation will be purged when new beneficial mutations sweep into fixation,” the researchers write.
While others in the scientific community have yet to be reviewed and published in a peer-reviewed journal, this result may close the book on the longest argument in evolutionary biology regarding bacteria.
But there is no “right” answer as to the relative importance of foot variation and new mutations for adaptation to nature, the researchers write.
Scientists working on different models tend to “emphasize one or another source of genetic variation,” they add.
Scientists studying animals and plants often emphasize the diversity of genetic grouping as the main 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 viruses tend to consider mutations as the main source of evolution.
But really, both forces, mutation and existing genetic diversity, “can contribute sequentially, simultaneously and even synergistically to the process of adaptation by natural selection,” say the researchers.
This prepress is available on BioRxiv prior to peer review.