EVOLUTION AND VIRUSES 



However, these differences are clearly not the primary genetic 

 changes; the specificity and genetic continuity lie in the nucleic 

 acid, for infections with the nucleic acid moeity alone give 

 the same results as infections by whole virus. Indeed, when 

 infections are made with nucleoprotein made by combining 

 nucleic acid from one strain of tobacco mosaic virus with 

 protein from another, they produce new virus particles with 

 protein characteristic of particles from which the nucleic acid 

 came. Hence the protein seems not to be concerned with 

 directing the synthesis of the virus particles, and changes in 

 it reflect earlier changes in the nucleic acid. Whether genetic 

 changes are simply rearrangements of the four purines and 

 pyrimidines yet identified in the nucleic acid of tobacco mosaic 

 virus, or whether there are other constitutional changes that 

 affect its genetic and chemical behaviour, only time will tell. 



Effects on the Evolution of Organisms 



It is easy to show in a very short time with bacterial viruses 

 the whole Darwinian philosophy of natural selection operating 

 on variants and perpetuating those forms best suited to survive 

 in a new environment. When a bacterial culture is infected 

 with an appropriate bacteriophage, the culture will become 

 water-clear as the virus multiplies and spreads rapidly through 

 it, apparently killing all the cells. After a while, though, 

 bacterial growth usually starts again, and ultimately there is 

 established a culture of cells immune from that bacteriophage. 

 The new culture derives from variants possessing the property, 

 previously of little value but all important in the new environ- 

 ment, of resisting the bacteriophage. 



A dramatic effect of this type is, of course, demonstrable 

 only with the closed system of an experimental culture in a 

 test-tube, and cannot be taken to represent what happens in 

 less artificial systems, where a bacteriophage will rarely be 

 able to reach the whole of a susceptible population. For 

 example, many samples of soil contain bacteriophages and 

 bacteria susceptible to them, showing that resistance to this 

 bacteriophage is not a prime requirement for the bacterium 

 to survive in that soil. However, there are probably many 

 occasions where such resistance has determined survival at 

 least in restricted localities, and the ability to vary in this 



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