Figure 4.

Quasispecies diversity and virulence. (a) Diverse RNA virus populations are often referred to as quasispecies, mutant swarms, or mutant clouds. According to viral quasispecies theory, diversity is a determinant of viral phenotype and virulence. Here, high mutation rates lead to a diverse population, and the mutant spectrum is optimized by natural selection.

Maintenance of a diverse population within hosts enables rapid adaptation and ultimately virulence. (b) An alternative model. Recent work from my laboratory suggests that the evolution of RNA dependent RNA polymerases is shaped by a speed-fidelity trade-off. Selection favors faster polymerases and faster polymerases are inherently more error-prone. Faster replication also leads to more rapid within-host spread and increased virulence. Empiric data across viral systems demonstrates that within-host diversity is rapidly lost or partitioned due to purifying selection, bottlenecks, and population splits. Thus, within-host diversity is neither maintained nor optimized by selection.