Figure 1.—

Fitness improvement of random clones, isolated from the 10 viable populations after 5 months of laboratory evolution, relative to their common ancestor. Shown are the mean and standard error of five replicate estimates of the selection-rate constant. Open bars, populations adapted to growth conditions; shaded bars, populations adapted to starvation conditions. Asterisks above the bars reflect the P-value of a one-tailed t-test of the five replicate estimates: *P < 0.05, **P < 0.01, and ***P < 0.001.