Figure 3 Trajectory for diversification of the 12 populations in their average cell volume. Analyses of variance were performed to partition the observed variation in cell size into its components. Each point represents the among-population SD for cell volume (i.e., the square root of the among-population variance component). Curve shows the best fit of a hyperbolic model.

plateau. Figure 3 shows the trajectory for the among-population standard deviation (i.e., square root of the variance component). The fit of the hyperbolic model to these data is very good (r = 0.987, n = 9, P < 0.001; relative to linear model, partial F = 15.6, 1 and 6 df, P = 0.008). Thus, we conclude that the populations have diverged, not only from their common ancestor but from one another, in cell size. After 10,000 generations, the among-population standard deviation was 0.14 fl, as compared with the average change in cell size from the ancestral state of 0.44 fl.

The observation that all 12 independently evolving populations responded similarly, even if not identically, seems to rule out random genetic drift, unless we invoke some profound asymmetry in mutational effects on cell size. However, we still cannot determine whether cell size was the actual target of selection (or merely a correlated response to selection on other traits), nor can we discern whether adaptation to the environment continued apace for the entire 10,000 generations (but without producing further changes in cell size).

The Quick and the Dead. A remarkable feature of our experimental system is that we can measure the mean fitness of a derived population relative to its actual ancestor. That is, populations of cells can be ''resurrected from the dead" (i.e., removed from the freezer) at any time and placed in direct competition. This ability to measure fitness permits investigation of a host of intriguing questions. Has mean fitness im-



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