Inducible Mutability and Components of Adaptation Figure 8. Average percent reduction in lag time. Lag time of UVR lineage population samples, UVR lineage isolates, and non-UVR lineage isolates after (A) 250 and (B) 500 generations of experimental evolution was measured under non-UVR (open bars) and UVR (shaded bars) conditions. Values represent the average percent improvement compared to the P. cichorii 302959 ancestor doubling times of 10.2760.32 hrs and 10.4760.26 hrs under non-UVR and UVR conditions, respectively. Error bars represent standard error of the mean. n = 8 for UVR lineage population samples and non-UVR lineage isolates but n = 16 for UVR lineage isolates. Values are significant by two-tailed independent t-test (a = 0.05) where indicated (* P,0.05, ** P,0.01, *** P,0.001). P-paired values correspond to two-tailed paired t-tests (a = 0.05) between non-UVR and UVR conditions. doi:10.1371/journal.pone.0015975.g008 lineages. The resulting fitness trajectories are somewhat punctu- ated, suggesting adaptive evolution due to the successive acquisition of beneficial mutations consistent with similar studies conducted previously [28]. Rapid changes in relative fitness interrupted periods of apparent stasis throughout the history of each lineage. However, these populations remained quite dynamic, as the frequency of F morphotypes and rifampicin- resistant colonies continued to fluctuate [12] without affecting the overall fitness of the population. The fitness trajectories in this study confirm that within communities assembled by adaptive radiation, such as those present in our UVR lineages, changes in individual fitness produce fluctuations in genotype frequency, and the interactions of those sympatric genotypes influence the relative fitness of the population [29]. Furthermore, complete selective sweeps are not required for adaptive improvements to influence the fitness of a population. The fitness trajectories in this study also suggest that adaptive improvements differentially contributed to relative fitness gains under UVR and non-UVR conditions. The ordered acquisition of improvements in the two environments favored the respective conditions under which each lineage was propagated such that UVR lineages first exhibited specific fitness gains under UVR conditions and non-UVR lineages under non-UVR conditions. Based on our previous observations of relative fitness at generation 500, we concluded that non-UVR lineages exhibited comparable fitness gains under UVR and non-UVR conditions due to their adaptation to the shared culture medium [12]. We expected that any phenotypic traits contributing to success under non-UVR conditions should likewise be beneficial during growth under UVR conditions. However, it is clear from fitness trajectories of non- UVR lineages that increases in fitness under non-UVR conditions were not mirrored by comparable increases in fitness under UVR conditions. These results suggest that adaptive specificity does not depend on MDR activation but is in fact a common pattern of adaption in P. cichorii 302959 exhibited by both UVR and non- UVR lineages. Such adaptive specialization can result in fitness trade-offs due to either antagonistic pleiotropy or mutation accumulation resulting in lower fitness under different environ- mental conditions [15,16,30]. However, in this study the adaptive specificity of both UVR and non-UVR lineages did not include any observable negative effect on fitness in the respective alternate environments. Interestingly, when fitness did increase in the alternate environment, corresponding improvements in fitness were not observed in the native environment. It is unlikely that fitness improvements under UVR and non-UVR conditions are independent. Rather, mutations that contribute to fitness observ- able only in the alternate environment may represent general improvements of smaller effect in the native environment. After gaining fitness in the alternate environment, UVR lineages continued to gain fitness specifically under their native conditions while non-UVR lineages did not, producing the illusion of adaptive specificity exclusively in UVR lineages. We can propose three possible explanations for this discrepancy: (1) MDR activation provides access to more beneficial mutations by altering the mutation spectrum of UVR lineages; (2) by increasing the overall mutation rate, MDR activation increases the rate at which beneficial mutations appear in the population and non-UVR PLoS ONE | www.plosone.org 10 January 2011 | Volume 6 | Issue 1 | e15975