A single clone of wild type phage 6 was used to found three replicate populations in each of three experimental treatments ͑nine populations total͒, where UVC exposure was manipulated using our published methods. 16 A 200 ml aliquot containing ϳ10 6 plaque-forming units ͑pfu; i.e., viable virus particles͒ of each population was suspended in Luria broth culture medium in the absence of cells. Aliquots were then placed in wells of a flat-bottomed polystyrene 96well plate. A UV illuminator ͑Spectroline Long Life Filter͒ was placed over the 96-well plate to expose the virus samples to UVC emission. "High dosage" treatment exposed FIG. 2. Death curves for virus strains in response to differing dosages of UVC exposure. Wild type RNA phage 6 ͑filled circles͒ better withstands UVC degradation than does DNA phage X174 ͑filled squares͒. A phage 6 population experimentally evolved for 20 generations under 2.5 min UVC exposure ͑open circles͒ survives better than its ancestor, except under prolonged dosage. In contrast, a representative phage 6 control population ͑gray circles͒ evolved in absence of UVC exposure presents a death curve nearly identical to the ancestor. See text for details. populations to UVC for 15 min, "low dosage" treatment exposed them for 2.5 min, and the "control" treatment populations were never exposed to UVC. Following this manipulation, each population was mixed ͑1:1 volumetric͒ with P. phaseolicola bacteria ͑i.e., ϳ4 ϫ 10 9 cells͒ and incubated at 25°C for an additional 120 min, sufficient time for at least one virus generation consisting of cell attachment/entry, intracellular replication of virus progeny, and cell lysis ͑death͒ that bursts the cell to liberate offspring. A typical burst in phage 6 produces ϳ200 viral progeny per infected cell 36 ͑we note that the host bacteria were never exposed to UVC, eliminating the possibility that UVC negatively impacted cell physiology or caused host mutagenesis͒. After 120 min, the mixture was centrifuged to pellet cells, and the supernatant was filtered to harvest a cell-free virus lysate containing up to ϳ2 ϫ 10 8 pfu of virus progeny. The lysate was then stored at 4°C for 22 h. The lysate was diluted 100-fold and used to initiate the next passage where the phage population was again subjected to treatment or control conditions, and allowed to infect naive bacteria freshly cultured from frozen stock ͑i.e., to eliminate possibility of phage-bacteria coevo-lution͒. Thus, experimental populations were challenged to produce sufficient progeny to sustain themselves in the face of the daily 100-fold dilution; UVC was an environmental stressor that could reduce the likelihood of this sustainability unless viruses responded through adaptation. A total of 20 passages ͑i.e., 20 generations͒ occurred in the short-term selection experiment, and samples at each passage were stored at −80°C for future analysis.