Without genetic mutations, none of us would be here – we'd be copies of the first single-celled organisms. However, only a few mutations are the beneficial sort that power evolution. In popular culture, mutations are associated with the three-eyed fish from The Simpsons and other undesirable effects of exposure to excessive radiation, but a study of their frequency in simple organisms found neutrality, or minor draw-backs, occur much more frequently than serious damage.
The more complex the organism the harder it is to track mutations, so Dr Lydia Robert from Paris' Sorbonne University started small, using single-celled E. coli bacteria. Tracking the growth and lifespan of vast numbers of these bacteria, using time-lapse imaging and fluorescent tags that cause the bacteria to light up when particular genes are expressed, Robert and colleagues observed 20,000 mutations appear over several hundred generations.
The mutations occurred as a result of errors during replication as cells divided. Although not the only way mutations can happen, it is suspected replication errors are the most common form of mutation.
To observe the mutations individually, the team created what they call a “mother machine”, a chip where bacteria grew in over 1,000 microscopic channels just wide enough to allow cells to grow in a single line. The original cell in each row sat at one end. The design eliminated natural selection since even fairly unfit cells survived.
Part of the purpose, Robert reports in Science, was to explore whether mutations occurred at a random rate, or in bursts caused by fluctuations in the composition of material within the cells. Despite acknowledging that their method would not allow them to detect all types of mutation bursts, the authors report that most types of bursts they could've tracked didn't occur, although they did find that replication error rates were proportional to cell size.
Finally, the authors looked at the frequencies of benefit and harm. The most extreme harm, early death, occurred in around 1 percent of cases. Another 0.3 percent of mutations produced a dramatic slowing in growth rates, defined as a greater than 30 percent reduction. This figure is lower than previous estimates. Nor was there evidence of slightly negative mutations building up to the point where lethal or significantly harmful mutations become more common.
What's more, beneficial mutations were rarer still. Indeed, not a single replication error led to a more than 20 percent increase in the rate of growth.
It's important to note that this study was done on bacteria, not humans, and more complex environments offer greater opportunities for minor mutations to affect survival. Nevertheless, the work provides a first rigorous overview of mutation rates, showing how common it is for them to occur in ways that neither greatly help or harm an organism, making them barely noticeable.