Luria-Delbruck experiment (1943) demonstrates that in bacteria, beneficial mutations arise in the absence of selection, rather than being a response to selection. This reinforces the Darwinian notion of evolution by natural selection acting on random mutations.

In their experiment Luria and Delbruck plated equal volumes of bacterial cultures onto phage (virus) containing agar. If virus resistance in bacteria was caused by a spontaneous activation in bacteria then each plate should contain the same amount of resistant colonies. This, however was not what Delbruck and Luria found. Instead, the number of resistant colonies on each plated varied drastically. They concluded that this was a result of random mutations which had occurred in some cultures but not in others.

Max Delbruck and Salvador Luria won the 1969 Nobel Prize for this work.

These experiments introduced the microbiological method of replica plating.