A 1943 bacteria experiment proved mutations happen before, not because of, danger
Salvador Luria and Max Delbruck exposed many separate bacterial cultures to a virus and counted survivors in each. If mutations arose only in response to the virus, survivor counts should be steady across cultures; instead they varied wildly, exactly as expected if resistance mutations occurred randomly beforehand. The fluctuation test disproved a Lamarckian view of heredity and helped earn both men, with Alfred Hershey, the 1969 Nobel Prize.