Inducible Mutability and Components of Adaptation Figure 9. Relative fitness trajectories of population samples from select lineages of P. cichorii 302959. Relative fitness of population samples containing ,108 cells from UVR lineages (A) 26 and (B) 30 and non-UVR lineages (C) 26 and (D) 39 under non-UVR (open bars) and UVR (shaded bars) conditions was measured by direct competition with the ancestor. Fitness values are means and error bars represent standard error of the mean. Values are significant by two-tailed independent t-test (d.f. = 5, a = 0.05) where indicated (* P,0.05, ** P,0.01, *** P,0.001). P-paired values correspond to two-tailed paired t-tests (d.f. = 5, a = 0.05) between non-UVR and UVR relative fitness values. doi:10.1371/journal.pone.0015975.g009 lineages would gain additional relative fitness if propagated for a longer time; or (3) the ancestral P. cichorii 302959 genotype is more poorly adapted to the UVR conditions and therefore more adaptive opportunities exist than under non-UVR conditions. Although the first two possibilities lend support to the proposed positive influence of inducible mutability on adaptive evolution, we do not have sufficient information to distinguish them from each other or from the third possibility. In summary, we have investigated the adaptive specificity of P. cichorii 302959 during experimental evolution by measuring relative fitness and corresponding changes in physiology. Our results suggest that adaptive specificity in this organism correlates with discrete growth improvements but does not depend on MDR activation. Adaptive improvements in all lineages were concen- trated in the first 250 generations of experimental evolution and specific increases in relative fitness correlated with distinct improvements in doubling and lag times. Furthermore, UVR lineages exhibited additional gains in fitness after generation 250 exclusively under UVR conditions that were reflected in further doubling and lag time improvements but likely not greater UVR PLoS ONE | www.plosone.org 11 January 2011 | Volume 6 | Issue 1 | e15975