Simplified representation of several types of genetic modifications that can alter the composition of viral quasispecies. Simplified representation of several types of genetic modifications that can alter the composition of viral quasispecies. (A) Mutation is a universal class of genetic variation and the basis of the original quasispecies formulation, as described in the text. (B) Hypermutation (generally biased toward some mutation types) is a consequence of cellular editing activities acting on viral genomes. (C) Genome segment reassortment occurs in viruses with segmented genomes and is responsible for the antigenic shift associated with new influenza pandemics. (D) Recombination results in formation of mosaic genomes, either by template switching (replicative recombination [left]; the negative, complementary strand is depicted with darker color) or breakage and rejoining of RNA molecules (nonreplicative recombination [right]). (E) A high multiplicity of infection passages of FMDV resulted in genome segmentation, as described in “Fitness Gain and Genetic Change: Genome Segmentation” in the text (schemes are based on references 5, 189, and 306). Esteban Domingo et al. Microbiol. Mol. Biol. Rev. 2012; doi:10.1128/MMBR.05023-11