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Chapter 17 : How Bacteria Revealed Darwin's Mistake (and Got Me To Read On the Origin of Species)
Charles Darwin really could have used bacteria, organisms for which you can really measure mutation rates. Astronomical populations of bacteria can be grown with and without stress and their mutation types can be characterized. In fact bacteria helped Lederberg, Luria, and Delbrück show that new mutations can form without any required "stress." Mutants arise continuously in perfectly happy bacterial populations. With great excitement Darwin showed the data (obtained using bacteria) suggesting that Lederberg, Luria, and Delbrück had missed something-Darwin might have been closer to the mark than people realized. The key to evaluating Cairns claims lies in the details of his selection conditions. Small-effect mutations are extremely common, and they can contribute serially to very fast strain improvement. It seems that the mechanisms for DNA replication and repair have evolved to be most effective at preventing mutation types with large phenotypic effects. All of laboratory bacterial genetics use the same principle strong selection. The trick is to block all parent cell growth (no new mutations) and prevent growth of those annoying frequent small-effect mutations. Darwin’s idea of stress-induced mutation may be wrong, but natural selection can take on breathtaking power when common small-effect mutations are allowed to contribute. These effects are revealed by bacterial populations. Under selection, the high speed of genetic adaptation is easily mistaken for an increase in mutation rate—maybe even Darwin underestimated the power of selection.