FIG. 3.

Model of evolution of a nucleotide along the chain of infected individuals under purifying selection. The numbers at the top denote cycles of transmission. X signs denote mutants. A mutant base appears spontaneously in person n, who infects person A with the mutant and person B with the wild-type variant. Person A passes the mutant down the chain to person C, after which his/her own virus population slowly reverts to the wild type. In person D, who is stably coinfected with a pair of sequences polymorphous at that base, selection clears the mutant virus rapidly.