A groundbreaking 1943 experiment by physicists Max Delbrück and biologist Salvador Luria decisively demonstrated that mutations occur randomly in bacteria, rather than being induced by environmental factors. This discovery provided critical support for Charles Darwin’s theory of evolution, which posits that natural variation arises spontaneously, with environmental pressures then selecting for advantageous traits.
The Long-Standing Debate
For decades, scientists debated whether variations in organisms emerged randomly or as a direct response to their surroundings. Darwin’s 1859 On the Origin of Species proposed random variation, while Jean-Baptiste Lamarck argued for induced changes. The question remained unresolved, especially in microbiology, where some researchers believed viruses (bacteriophages) could cause bacteria to develop resistance.
The Fluctuation Test: A Novel Approach
Delbrück and Luria designed a clever experiment, known as the “fluctuation test,” to settle the debate. They cultured Escherichia coli bacteria, exposing some to phages and then observing the resulting resistance levels. If resistance was induced by the phages, all cultures would show similar proportions of resistant bacteria. However, if mutations arose randomly, cultures would vary significantly: some would have many resistant bacteria due to early mutations, while others would have few.
The results were clear: resistance varied dramatically between cultures, confirming that mutations happened spontaneously. The team published their findings in 1943, cementing random mutation as a fundamental principle of evolution.
A Unique Collaboration
The experiment stemmed from an unlikely collaboration. Delbrück, a German physicist who fled the Nazi regime, applied his physics background to genetics. He was fascinated by how easily phages could be studied under a microscope, describing the process as “beyond my wildest dreams of doing simple experiments on something like atoms in biology.” Luria, an Italian-Jewish doctor also escaping persecution, shared Delbrück’s interest in applying quantitative methods to genetics.
Their breakthrough came after Luria realized that the principles of statistics, much like slot machines, could differentiate between induced and random mutations. By carefully tracking the cultures, they proved that resistance wasn’t a response to the phage; it was a matter of chance.
Legacy and Recognition
Delbrück, Luria, and Alfred Hershey later collaborated to reveal that phages carry multiple genes and can exchange them through recombination. Hershey and Martha Chase then confirmed that DNA is the carrier of genetic information. The trio’s work earned them the 1969 Nobel Prize in Physiology or Medicine.
This experiment wasn’t just about bacteria; it validated a core principle of evolution and paved the way for modern genetics. The confirmation of random mutation remains essential to understanding how life adapts and changes over time.
