Figure 6 Trajectories for mean fitness relative to the ancestor in 12 replicate populations of E. coli during 10,000 generations. Each curve represents the best fit of a hyperbolic model to data obtained for one population every 500 generations.

directed) opposes passage through a "valley" of maladapted intermediate states, even though a better solution may exist across the way. Theoreticians have identified processes that might facilitate peak shifts, but empiricists know very little about the structure of adaptive landscapes. A key question is, how often are there nearby fitness peaks of unequal height?

To address this question, we examine whether the evolving populations diverged from one another in mean fitness, as they did in morphology. Figure 6 shows the estimated trajectories for mean fitness in the replicate populations during 10,000 generations. All 12 adapted much more rapidly soon after their introduction into the experimental environment than they did subsequently, when their environment had been constant for several thousand generations. We performed analyses of variance to estimate the among-population variance component for mean fitness at each time. If the populations were approaching different fitness peaks from the same initial state, then this variance component should increase from zero to some plateau. Figure 7 shows the fit of the hyperbolic model to the trajectory for the among-population standard deviation for mean fitness. The fit of the model is poor (r = 0.286). However, the twenty separate analyses of variance (after t = 0) yielded estimated variance components > 0 in 18 cases. The associated significance levels were <0.05 in 7 cases and between 0.05 and 0.25 in 8 others.

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