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 important, they occur in small numbers in some populations and in large numbers in other cultures. With Hershey, Delbruck and Luria are awarded the Nobel Prize in Medicine or Physiology in 1969.
Luria, S. E. and M. Delbruck. 1943. Mutations of bacteria from virus sensitivity to virus resistance. Genetics. 28: 491-511. In Microbiology: A Centenary Perspective, edited by Wolfgang K. Joklik, ASM Press. 1999, p.318 [pdf]