The Evolutionary Genetics of Emerging Viruses

RNA viruses are the main agents of emerging disease. To understand how RNA viruses are able to jump species boundaries and spread in new hosts it is essential to determine the basic processes of evolutionary change in these infectious agents. RNA virus evolution is largely shaped by very high rates of mutation. This, coupled with potentially enormous intra- and interhost population sizes and continual replication, allows the rapid production of genetic diversity, including those mutations that facilitate host adaptation. However, high mutation rates also act to constrain aspects of RNA virus evolution, as the majority of mutations, including many at synonymous sites, are deleterious, which in turn places an upper limit on genome size. Ironically, although RNA viruses are characterized by their mutation rates, these rates may not be high enough to allow the onset of quasispecies dynamics, in which natural selection acts on the viral population as a whole.