Beneficial mutations.

A reader asked me to post about beneficial mutations as an antidote to the common creationist (mis)conception that all mutations are detrimental. I replied that this isn’t an anti-creationist blog per se (I feel that I have more interesting things to talk about, frankly), and that the issue has been covered by others (e.g., here and here). I will, however, note the following article published recently in Science:

Perfeito, L., L. Fernandes, C. Mota, and I. Gordo. 2007. Adaptive mutations in bacteria: high rate and small effects. Science 317: 813-815.

Abstract: Evolution by natural selection is driven by the continuous generation of adaptive mutations. We measured the genomic mutation rate that generates beneficial mutations and their effects on fitness in Escherichia coli under conditions in which the effect of competition between lineages carrying different beneficial mutations is minimized. We found a rate on the order of 10–5 per genome per generation, which is 1000 times as high as previous estimates, and a mean selective advantage of 1%. Such a high rate of adaptive evolution has implications for the evolution of antibiotic resistance and pathogenicity.

As they note at the end of the paper,

Given the estimates for the overall mutation rate in E. coli and its genomic deleterious mutation rate, our estimate of Ua implies that 1 in 150 newly arising mutations is beneficial and that 1 in 10 fitness-affecting mutations increases the fitness of the individual carrying it. Hence, an enterobacterium has an enormous potential for adaptation and may help explain how antibiotic resistance and virulence evolve so quickly.

It is important to be clear that this is not a matter of mutations occurring in response to need, nor of whole populations of individuals changing to become resistant simultaneously. It is the normal Darwinian process of “random” mutation (with respect to fitness, but not such that all mutations are equally likely to occur) leading to increased or decreased reproductive output of individuals that happen to carry the mutation, with the proportion of genic variants changing over many generations. The main insight is that the pool of mutations on which this blind process of natural selection acts is larger and less biased toward deleterious changes than assumed. In this case, the study not only hits a common creationist misconception head on, it also shows how understanding evolution often has considerable medical importance.



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