Inherited, Not Acquired

Early in the twentieth century it was noticed that tiny percentages of certain strains of bacteria would continue to survive when an entire colony was placed in a toxic setting. Although it was generally understood that random mutations provide the raw material for natural selection among higher organisms it was still controversial whether the ability of some bacteria to survive a hostile environment was due to the ability of a few to adapt to the killer virus or whether favorable random mutations that occurred all along prior to the toxic exposure is what allowed a few to survive. In a landmark paper published in 1943 biologists Luria and Delbrück [24] successfully carried out some experiments to resolve this question in favor of spontaneous mutations. In effect they vindicated the idea that even with micro-organisms evolution works “not as a designer but as a gardener”, an apt quote attributed to Jeff Bezos by Thomas Friedman (New York Times, May 19, 2012). The details of their investigation are given below. The key insight for interpreting their results is a very simple property of the Poisson distribution, namely that the mean and variance are the same, and this is the reason why this is of interest to us.