Nonquasispecies evidence for the evolution of RNA viruses

The quasispecies model of RNA virus evolution differs from those formulated in conventional population genetics in that neutral mutations do not lead to genetic drift of the population, and natural selection acts on the mutan t distribution as a whole rather than on individual variants. By computer s imulation, we show that this model could be inappropriate for many RNA viru ses because the neutral sequence space may be too large to allow the format ion of a quasispecies distribution. This view is supported by our analysis of gene sequences from vesicular stomatitis virus, which is considered a pr ototype RNA virus quasispecies. Our results are relevant to the evolution o f RNA systems in general.