INDIRECT SELECTION AND ORIGIN OF 

 RESISTANCE 



L. L. Cavalli-Sforza 



Istituto Sieroterapico, Milan 



Adaptation of individuals and of populations 



Adaptation to environmental changes in living organisms 

 can take place both at an individual and at a populational 

 level. Mechanisms of the first type are often efficient enough 

 to cope with the altered situation, but there will be some 

 variation in the individual responses to the changed environ- 

 ment. If the following two conditions are fulfilled: (i) that 

 this variation is at least in part heritable, (ii) that there is 

 differential reproduction of individuals showing different 

 degrees of adaptation, then the population is also bound to 

 change, in the sense that the frequencies with which the 

 variously adaptable types are represented will be modified. 



This mechanism is of course nothing but natural selection 

 and its consequence is evolution. How much evolution takes 

 place in any given population in a given time is dependent 

 on how much heritable variation is available, and this is a 

 property of the population; and on how much variation in 

 the reproduction of the various types is created by the change 

 in living conditions. The use of antibacterial drugs in con- 

 centrations at which they exert their specific effect con- 

 stitutes a fairly drastic change in conditions, which is bound 

 to affect deeply the rates of reproduction of individual cells. 

 One should therefore expect drugs to effect major changes 

 in the composition of a bacterial population whenever this 

 contains — either because of original heterogeneity or because 

 of new hereditary change — types which have different 

 adaptability. 



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