MUTATION 



existence. This is usually a tedious process but the isolation, 

 for instance, of a streptomycin-resistant colony without ever 

 exposing the organisms to the antibiotic is a most convincing 

 demonstration of the reality of spontaneous mutation. 



3. Population genetics of bacteria and viruses 



In discussing the population genetics of bacteria there is an 

 unlimited range of material to be drawn upon, but this is not 

 my primary field and I am only concerned to draw from it 

 examples of phenomena that are relevant to the problems of 

 virus or body cell. 



One of the important features of any evolving population 

 in which the individuals reproduce asexually is the absence 

 of any means of storing mutational experience that is not 

 immediately helpful to survival. In any large population of 

 higher organisms, the individuals of which can mate freely 

 throughout the population, the genetic pool contains a large 

 reservoir of variation which can be drawn on, as it were, to 

 cope with long-range environmental changes, and which 

 acts as a buffer against too rapid genetic change. Recombina- 

 tion in bacteria is now well known to occur but there is much 

 to suggest that it may be a laboratory artefact shown clearly 

 only in rough E. coli mutants of special character. It has never 

 been shown to play a significant part in the evolution and 

 survival of bacteria in natural environments. On the other 

 hand, transduction by temperate bacteriophage could be 

 a process of evolutionary significance, and one can hardly 

 quarrel with Lederberg and Zinder's view that a large pro- 

 portion of the complexities of the Kauffman-White classifi- 

 cation of the Salmonellas are manifestations of transduction 

 within the group. For most bacteria most of the time, how- 

 ever, reproduction is by simple fission, and change by random 

 and rather frequent mutation. The only stabilizing feature 

 is the environment, and if this remains constant the distribu- 

 tion of types within the population is also likely to remain 

 stable. Anyone who has tried to bring reproducible results 



