EVOLUTION AND VIRUSES 



property has probably often influenced the direction of evolu- 

 tion in many kinds of organisms. 



Whether virus diseases have ever been responsible for 

 destroying whole hnes of susceptible organisms is unknown 

 and is perhaps improbable, because an extremely efficient 

 method of spread would be needed to infect every individual 

 in a population not artificially restricted. An example of what 

 can happen has recently been provided in the United Kingdom 

 by myxomatosis in rabbits. In places where rabbits were 

 plentiful and living in burrows, this newly introduced virus 

 spread rapidly through the populations and eliminated them. 

 In such areas, secondary evolutionary effects are now to be 

 seen in a changing flora, with species previously grazed 

 heavily by rabbits becoming more prominent than previously. 

 In other areas, where rabbits were less abundant and not all 

 in burrows, some escaped, and these are now beginning to 

 re-establish populations in some areas cleared by the disease. 

 It would be idle, and rash, to forecast what will now happen, 

 when we only have to wait to see the event, but there are 

 several possibilities. One is that the virus itself will disappear 

 because the population of susceptible hosts is too small to 

 ensure its survival. Another is that it will produce a variant 

 less virulent than itself, which could lead to an immune 

 population of rabbits by providing a natural vaccine. A third 

 is that the rabbits themselves, although not quite as prolific 

 as bacteria and so less likely to do so, will produce resistant 

 variants. Or, perhaps most likely, there will be a lasting effect 

 on the rabbit population, with myxomatosis virus always 

 about, although never very apparent when rabbit populations 

 are low, but flaring up wherever and whenever large popula- 

 tions occur in burrows. 



These kind of effects are not peculiar to viruses, but general 

 to all kinds of pathogens that cause lethal diseases. There is 

 a more subtle way in which viruses may influence the evolu- 

 tion of organisms. It is by adding to their genetical potentiali- 

 ties. That this happens in bacteria is established. We have 

 already instanced lysogeny, but in addition to this, infections 

 with bacteriophages can also carry other genetic factors from 

 one bacterium to another. There is no evidence for any 

 comparable occurrences in higher organisms, but there is no 



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