POPULATION GENETICS 



(ii) In thinking about mutational and physiological 

 changes in micro-organisms or body cells, it must be re- 

 membered that replication gives two individuals in which 

 only slight change from the phenotypic character of the 

 parent can occur. Irrespective of how it comes into being, 

 a new character will only emerge when the old character is, 

 as it were, diluted out over several generations. 



(iii) In bacterial cultures only mutants with an immediate 

 selective advantage will have a chance to establish them- 

 selves in the population provided the environment remains 

 constant. In discussing the general ecological situation of 

 bacteria in nature, however, Stanier (1953) pointed out that 

 the discontinuous nature of appropriate micro-environments 

 in both space and time put a premium for survival on three 

 salient characteristics of bacteria : {a) large populations and 

 short generation time; (b) relatively high mutability; and 

 (c) great powers of dormancy. These will ensure that when the 

 appropriate micro-environment appears it will be speedily 

 colonized by a well-adapted micro-flora. 



Perhaps it will be of interest to restate those three salient 

 characteristics of Stanier's, but no longer with reference to 

 bacteria. The salient characteristics of the mesenchymal cell 

 system in man are large populations and short generation 

 time, relatively high mutability and/or adaptability, and 

 great powers of dormancy. These are all matters which are 

 closely relevant to any discussion of mesenchymal cell 

 function in relation to immunity. 



(c) O-D change in influenza virus A 



There was a curiously long delay before the clonal or, to 

 speak more conventionally, the pure-culture approach was 

 applied to studies of viruses. This was to a large extent a by- 

 product of the general experience that material from a given 

 case of virus disease in man or a domestic animal was most 

 unlikely to contain more than one type of virus which was 

 recognizably pathogenic for the experimental animal to which 



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