c2 (if t > T2) + … + cn (if t > Tn)] with three steps provides an excellent fit to the trajectory (r = 0.978; 3 and 17 df; P < 0.001). What accounts for the initial delay and the seemingly discontinuous jumps? Both features are expected from population genetic theory, given the uniformity of the founding population, its asexuality, and the resulting dependence of the selection response on new mutations (Muller, 1932; Lenski et al., 1991). Any favorable allele must first appear and then increase from a very low frequency. Assuming constant selection, it takes as long for a favored new allele to increase from a frequency of 10-7 to 10-6 as it takes it to increase from 10% to 90%; and yet only after the allele has reached high frequency does it appreciably affect the mean properties of a population. Thus, the smoothness of the trajectories shown in Figures 1 and 4 is, to some extent, a product of relatively infrequent sampling; discontinuities revealed by more frequent sampling (Figure 5) indicate nothing more than the dynamics of selection when a population depends on new mutations (rather than abundant standing variation) for its response.

The Adaptive Landscape. Wright's (1932, 1982, 1988) concept of the adaptive landscape (or fitness surface) provides one of the most vivid images in all of evolutionary theory, but it is also one of the most difficult to firmly grasp and study. The essential idea is that natural selection tends to drive a population to a local optimum, which is not necessarily a global optimum. Thus, a population may be stuck with a suboptimal solution to its environment because natural selection (which is not goal

Figure 5 Finer scale analysis of the trajectory for mean fitness in one population of E. coli during its first 2000 generations of experimental evolution. Each point is the mean of 10 assays. Solid lines indicate the fit of a step model.



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