At the end of the study, replicated ͑n =4͒ assays were performed to gauge whether the survival of treatment populations had changed during the experiment. Evolved populations were measured for frequency of surviving virions, in assays where samples containing ϳ10 6 pfu of virus were exposed to UVC for six different time periods: 1, 2.5, 5, 10, 15, and 30 min. Results showed that after 20 passages the death curves for the populations in the control treatment were highly similar to that presented by the wild type phage 6 ancestor; Fig. 2 shows a representative outcome for one of the control populations. In contrast, we observed that all three lineages in the high dosage ͑15 min UVC exposure͒ treatment went extinct by passage 10 of the experiment ͑data not shown͒. These data were informative because they indicated that the strong selection created through high dosage of UVC exposure was too overwhelming for viruses to be ecologically sustained, and/or to experience spontaneous genetic variation useful for meeting the environmental challenge. The latter outcome clearly demonstrated an evolutionary constraint for the virus. Last, all of the evolved populations in the low dosage ͑2.5 min UVC exposure͒ treatment were extant by end of the study. However, only one of these three populations exhibited a death curve that differed from the ancestor, as shown in Fig. 2 . This result proved that a response to UVC selection was possible in phage 6 under our low dosage conditions. Furthermore, the data interestingly showed that the adaptation caused ϳ17% gain in survival ͑on average͒ relative to the ancestor at dosages away from the selection challenge of 2.5 min ͑i.e., 1, 5, 10, and 15 min͒. However, this response on the part of the one lineage was not overly impressive, because the same 50% mortality as the ancestor was evident under 30 min UVC exposure, demonstrating an additional constraint in the virus even though an adaptive response occurred at low dosage ͑Fig. 2͒.