Microbial cultures swiftly adapt to lethal agents such as antibiotics or viruses by acquiring resistance mutations. Does this remarkable adaptability require a Lamarckian explanation, whereby the agent specifically directs resistance mutations? Soon after the question arose, Luria and Delbrück devised a clever experiment, the fluctuation test, that answered this question in the negative: microbial adaptation, they showed, is entirely consistent with a Darwinian explanation. Their 1943 article is a classic of biology literature, with practical and theoretical implications that continue to expand today. Implementing an updated fluctuation test in a college teaching lab provides a simple experimental setting in which beginning students learn to apply basic principles of evolutionary biology and scientific reasoning, while gaining hands-on experience in core technical advances of contemporary life science.

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