The segmented RNA phage 6 is typically grown in the laboratory on the plant pathogenic bacteria Pseudomonas syringae pathovar phaseolicola. To examine whether high-MOI populations of phage 6 will evolve decreased genetic robustness against mutations, virus populations were first propagated for 300 generations under high versus low MOI. 25, 14 The expectation was that high-MOI evolved viruses would become more brittle, in comparison to their low-MOI evolved counterparts. To reveal this assumed outcome, clones were randomly chosen from the treatment populations and used to initiate lineages subjected to mutation accumulation via passage through severe bottlenecks ͑population size= 1 individual͒; this method allows genetic drift to overwhelm selection, such that virus lineages amass mutations that are on average deleterious. 26 The mean magnitude and FIG. 1. Genotype and phenotype spaces are represented schematically in two dimensions. A brittle organism produces a phenotype that is a reflection of the underlying genotype, whereas a robust organism produces a constant phenotype regardless of the underlying genotype. ͓Reprinted with permission from M. Félix and A. Wagner, Heredity 100, 132 ͑2008͒. Copyright ©2008 Nature Publishing Group.͔ variance in fitness changes that occurred because of bottlenecking were statistically compared between lineages founded by presumed robust ͑low-MOI͒ and brittle ͑high-MOI͒ viruses. Results from the mutation accumulation analysis supported the hypothesis that viruses historically evolved under frequent coinfection became relatively less genetically robust than those evolved under rare coinfection. 14 One alternative explanation for the observations would be a higher mutation rate in the viruses evolved at high MOI, causing them to experience a greater number of fixed mutations ͑and hence, greater fitness loss͒ during the mutation accumulation; however, no evidence suggests that these viruses mutate at rates higher than their counterparts evolved at low MOI. 14, 15 Thus, the study provided the first evidence that genetic robustness could evolve to change in biological populations.