Breeding 2,000 Generations of Bacteria May Have Solved This Major Debate in Biology

Biologists have disputed whether evolution is driven more by accidental mutations or by the initial variety in the gene pool since the beginning of genetics in the early twentieth century.

With so many genetic alternatives to pick from, natural selection may proceed more quickly at first, but can genetic mutations that occur throughout time contribute more to species survival in the long run?

Researchers at Michigan State University investigated the adaptive capabilities of 72 distinct populations of Escherichia coli bacteria across 2,000 generations in an attempt to settle this long-running debate once and for all (around 300 days).

At the outset of the experiment, each colony of bacteria was created to have a distinct degree of genetic variety.

On one end of the scale, the population was produced from a single clone, resulting in genetically identical cells.

Populations were produced from a single pre-existing population of bacteria in the middle of the spectrum.

On the other end of the scale, E. coli populations were generated by combining a few pre-existing populations to achieve the greatest genetic variety.

At the outset of the trial, each population was given glucose. To demonstrate adaptation, separate populations of these bacteria were obtained and cultured in a different growth environment, with their energy demands met by the amino acid D-serine rather than glucose.

The populations were assessed for their capacity to fight for dietary resources against a common rival at the 0, 500, and 2,000 generation points (which was another strain of E. coli with an intermediate fitness level).

The E. coli samples were all obtained from the Long-term Experimental Evolution Project, which was founded in 1988 by evolutionary scientist Richard Lenski, one of the paper's co-authors.

When the fitness of each community of bacteria in the D-serine environment was assessed before any evolution, the genetically varied populations performed better than the clones.

The founding population's genetic variety was critical for adaptation in the early phases of the experiment (about 50 generations).

The scientists write in their preprint, which is published on BioRxiv ahead of peer review, that by the 500th generation, the diversity at the start of the experiment "no longer mattered" since the new mutations were "sufficiently large."

Despite the disparity in fitness at the outset, between the 500th and 2,000th generations, there were "no differences in fitness" across all the diverse populations of bacteria.

"Any benefit of pre-existing variation in asexual populations may often be short-lived, as we saw in our experiment, because that variation will be purged when new beneficial mutations sweep to fixation," the researchers write. 

While it still has to be validated by others in the scientific community and published in a peer-reviewed publication, this finding might put an end to one of evolutionary biology's longest-running debates over bacteria.

The researchers conclude that there is no 'correct' answer in terms of the relative relevance of existing variation and new mutations for adaptation in nature.

They go on to say that scientists working on various models tend to "emphasize one or the other source of genetic variation".

Because it isn't practicable to wait hundreds of years for mutations to change things up, scientists studying animals and plants prefer to stress the variety of the gene pool as a significant source of evolutionary capability.

Mutations are often cited as the primary cause of evolution in bacteria and viruses.

But, as the researchers point out, both mutation and genetic diversity "can contribute sequentially, simultaneously, and even synergistically to the process of adaptation by natural selection".