freedom), P < 0.001]. These data therefore indicate a rapid bout of morphological evolution after the population was placed in the experimental environment, followed by evolutionary stasis (or near stasis).

From these data, one might speculate that selection had favored larger cell size per se, although one could not exclude the possibilities that size was a correlated response to selection on some other trait or, more remotely, that cell size was subject solely to random genetic drift. If cell size was indeed a target of natural selection, then the eventual stasis might be interpreted either as a genetic/developmental constraint (such that no new mutations appeared that could increase cell size further) or as stabilizing selection (such that both larger and smaller variants continued to appear but were purged by natural selection). And whether or not cell size was a target of selection, the population may have continued to adapt to the environment (after size was static) by changing other traits.

Replicating History. Inferences concerning the tempo of morphological evolution and the adaptive significance of cell size would be greatly strengthened if similar trends were seen in several independent fossil beds. This opportunity is not usually possible: even when several contemporaneous fossil beds exist, one cannot exclude the possibility

Figure 1 Trajectory for average cell volume in one population of E. coli during 10,000 generations of experimental evolution. Each point is the mean of two assays. Curve is the best fit of a hyperbolic model. Averages were calculated after removing particles of 80.25 fl, except for the ancestor (t = 0) where particles of 80.15 fl were removed; these criteria gave a clear separation between cells and background particles. Cell sizes were measured in stationary-phase populations, at the end of the 24-h serial transfer cycle.



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