1943 Fluctuation Test: How Delbrück and Luria Proved Mutations Arise Spontaneously

On Nov. 20, 1943, physicist Max Delbrück and physician-biologist Salvador Luria published a concise but decisive experiment — the "fluctuation test" — that provided direct evidence that genetic changes in bacteria arise spontaneously rather than being induced by environmental stressors. Their work addressed a long-standing debate about whether adaptive traits appear in response to challenges or whether variation appears randomly and is then sorted by natural selection.

The question dated back to the 19th century: Charles Darwin argued in On the Origin of Species that variation is essentially random and that natural selection favors beneficial variants, while Jean-Baptiste Lamarck had earlier proposed that the environment could directly induce useful changes. By the early 1940s most biologists accepted Darwin's view for plants and animals, but some researchers wondered whether microbes — especially bacteria challenged by bacteriophages (viruses that infect bacteria) — might somehow develop directed resistance.

Delbrück, a German émigré scientist, became fascinated with biology because viruses and bacteria allowed the kind of simple, quantitative experiments he had admired in physics. While working with Escherichia coli and its phages, he saw how individual virus particles made visible clearings on bacterial lawns, enabling precise counting and experimental control. In December 1940 he met Salvador Luria at Cold Spring Harbor; both men had fled fascist regimes in Europe and were eager to apply rigorous physical and statistical thinking to genetic questions.

The key insight that made the fluctuation test possible came from a conversation about slot machines. Luria realized that statistical patterns of survivors could distinguish two hypotheses. If exposure to phages induced resistance, independent bacterial cultures exposed to phage would yield similar numbers of resistant colonies only after exposure. But if resistance resulted from spontaneous mutations occurring during growth, different cultures would show wide variation: some cultures would be "jackpot" tubes with many resistant colonies because a mutation occurred early and was amplified during growth, whereas others would have few or none.

To test this, Delbrück and Luria grew many separate E. coli cultures, exposed each to phage, and plated them to count surviving, resistant colonies. The observed large variation in resistant counts across independent cultures matched the random-mutation prediction and contradicted the idea that the phage induced resistance. This result established that, at least in their bacterial system, mutations appear spontaneously and are later selected for or against by environmental pressures.

Legacy and later developments

Later in 1943 Delbrück and Luria began collaborating with Alfred Hershey. Together they showed that phages carried multiple genes and that genetic recombination could occur when two phages infected the same bacterium. Subsequent work by Hershey and Martha Chase identified DNA as the genetic material. For their collective contributions to molecular genetics, Hershey, Luria and Delbrück were awarded the Nobel Prize in Physiology or Medicine in 1969.

While the fluctuation test strongly reinforced the Darwinian framework that natural selection acts on random variation, later research has added nuance. Mutation rates are not uniform across genomes: some essential genes appear to experience lower mutation rates in certain organisms, and adaptive systems such as CRISPR can change how bacteria respond to phages. Nonetheless, the fluctuation test remains a foundational experiment demonstrating that random mutation plus selection is a powerful mechanism of evolutionary change.

Why it matters: The fluctuation test moved genetics from descriptive observation toward quantitative experimentation, helping to establish the molecular and statistical foundations of modern evolutionary biology.