Salvador Luria and Max Delbruck provide a statistical demonstration that inheritance in bacteria follows Darwinian principles. Particular mutants, such as viral resistance, occur randomly in bacterial populations, even in the absence of the virus. More importantly, they occur in small numbers in some populations and in large numbers in other cultures. The results, known as fluctuation analysis, show that resistance occurs before exposure to the phage and argues against the adaptation hypothesis of mutations. With Delbruck and Hershey, Luria is awarded the Nobel Prize in Medicine or Physiology in 1969.