TABLE 1.

Number of mutations per genome equals the mutation rate only in the first generation before selection

GenerationAvg no. of mutations/genome in modela:
MultiplicativeThreshold (k = 2)Eigen
1 (before selection)UUU
1 (after selection)UsUdEmbedded ImageEmbedded Image
2 (before selection)2UsUdEmbedded ImageEmbedded Image
Equilibriumb (before selection)Ud/s2 + UdEmbedded Image
  • a Table cells give the average numbers of mutations per genome as functions of overall mutation rate U, deleterious mutation rate Ud, generation number, and model. The models assume that mutations occur as the genome is replicated and packaged, with selection ensuing later, when that genome infects a new cell. Thus, lethal mutations would be observed in the genomes of individual virus particles (hence before selection) but not in a consensus sequence obtained from a plaque (after selection). The process of mutation alone increases average mutation number with each generation, whereas selection reduces it.

  • b Equilibrium number of deleterious mutations only. The number given for the Eigen model assumes that an error catastrophe has not occurred; after the error catastrophe, the equilibrium number of mutations is infinite if unlimited mutations are allowed. The equilibrium for the multiplicative model was given in reference 31 and that for the Eigen model in reference 59.