Other experimental studies have used RNA viruses or viroids ͑RNA viruslike plant pathogens that lack proteincoding genes͒ as models to study whether elevated mutation rates favor robust genotypes. One study examined two species of viroids that differed in robustness. 13 The robust viroid species had a low reproductive capacity in plants but a large neutral neighborhood of mutations ͑fraction of one-error mutations that do not change minimum free energy RNAsecondary structure͒, whereas the brittle viroid had the opposite combination of traits. It was hypothesized that brittle viroids should outgrow robust viroids when coinfecting plants under ordinary conditions, but the robust viroids should be favored in the presence of a mutagen because they better maintained phenotype ͑proper structure͒ despite mutational input. As predicted, under standard growth conditions in host plants, results showed that the brittle viroid was selectively favored. In contrast, when infected plants were subjected to irradiation by ultraviolet C ͑UVC; short wave UV light of wavelength 280-100 nm͒, this known viroid mutagen caused the robust viroid to be favored in competition, because it better withstood growth despite mutation. 13 Thus, this empirical outcome in biological populations was consistent with the earlier findings seen in artificial-life studies. 9 A second study used an RNA virus of eukaryotes and two different mutagens, but found very similar results to those described above. 27 Vesicular stomatitis virus ͑VSV͒ is an ssRNA virus that is vector-transmitted by some insects and causes disease in certain mammal species. 28, 29 Two diverged populations of VSV were characterized for reproductive growth on tissue culture cells, and for within-population variance among clones for this phenotype; this effort revealed that one population was slow-growing but relatively less variable in clone phenotypes ͑robust͒ and the other had the opposite characteristics ͑brittle͒. 27 Whereas standard growth conditions allowed the faster replicating brittle virus population to outcompete the slower replicating robust population, exposure to either of two mutagens favored the robust variant. These results echoed the earlier studies where competition under elevated mutation rates was observed to favor robust populations, even though robustness was associated with reduced reproduction. 9, 13 While several of these studies have provided evidence for extant, evolvable differences in robustness, the literature is generally lacking in examples of direct selection for robustness in biological systems. In an effort to examine direct selection for RNA virus resistance to UVC damage, here we present new data from a short-term experimental evolution study with phage 6. These results underscore some of the biological, methodological, and conceptual barriers to observing direct selection for robustness in RNA virus systems.