CLONAL PHENOMENA 



more descendants than other clones in the environment 

 provided. The mutations h- -> ^+ and A+ -> h- are not 

 associated with any change in growth rate in optimal medium, 

 but it is found in practice that mutation to improved growth 

 rate is also of frequent occurrence. This probably depends on 

 the fact that no two batches of medium are quite identical, so 

 that there is always scope for a genetic change that will have 

 special survival value in a new transfer. If we start with an 

 h- culture, A+ variants will gradually accumulate, but other 

 mutations are also taking place, and we can think of hr^ 

 mutating to h^ which has a growth advantage and under the 

 circumstances will soon overgrow and replace k^ and all the 

 mutants of unchanged growth rate that were derived from it. 

 The culture will come to be dominated by hi and its mutants 

 almost as definitely as if an artificial selection of a single 

 colony of h- had been used for transfer. This process will be 

 repeated giving ' periodic selection ' at a rate that is sufficient 

 to maintain the A" dominance over A+ at about the 1*3-1 -5 to 

 10^ level. 



Similarly, when a culture is initiated with a high proportion 

 of A+ this shows stabiHty for a period and then a rather 

 sudden reversion to the standard ratio between the h~ and 

 A+ types. 



From the general point of view, the two important features 

 emerging from these experiments are : 



(i) the unique importance of capacity to produce more 



descendants than rival clones — everything else is 



secondary ; 

 (ii) that even if the details of the process cannot be 



analysed, if the population is large enough and time 



allowed an approximate equilibrium can be attained. 



{b) Pseudo-mutation [Novick and Weiner's Experiment) 



The second example I want to take from work on the 

 population genetics of bacteria is from a recent study by 

 Novick and Weiner (1957) in the chemostat based on the 



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