If a population is well-adapted to its environment, almost all mutations should lead to deviations from optimal performance in the selective habitat. Therefore, populations at equilibrium are expected to experience selection for genetic robustness. But study of this process in experimental populations is problematic for at least two reasons: equilibrium states are difficult to achieve ͑or definitively prove͒ in lab-evolved populations, and selection for mutational robustness is "second-order" because the benefit is not experienced until offspring carrying mutations arise. 1 However, theory and artificial-life data 9 support the idea that genetic robustness should be strongly favored when populations experience elevated mutation rates, suggesting that RNA viruses would be fruitful systems to explore how genetic robustness evolves. In general, success in these systems has come either from manipulating ecological conditions such that robustness becomes less important and evolves to decrease, or from manipulating competition environments to examine whether elevated mutation rates favor genetically robust populations.