Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
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Dynamics of Adaptation and Diversification: A 10,000-Generation Experiment with Bacterial Populations
Dynamics of Adaptation and Diversification: A 10,000-Generation Experiment with Bacterial Populations
Pages 253-274
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From page 253... ...
Lenski is the John Hannah Professor of Microbial Ecology at Michigan State University, East Lansing, Michigan. Michael Travisano is a postdoctoral researcher at the RIKEN Institute, Wako, Japan.
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You could assess which phenotypes promoted ecological success, and you could evaluate the similarity of the adaptive solutions achieved by the replicate populations, thereby disentangling the roles of "chance and necessity" (Monod, 1971) in evolutionary dynamics.
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Fitness is the most important property of any organism according to evolutionary theory. The mean fitness of a population was obtained by allowing it to compete against the common ancestor.
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(1991) for details concerning the ancestral strain, culture conditions, methods for exclusion of contamination from external sources and cross-contamination between replicate populations, and procedures for estimating mean fitness of derived populations relative to their ancestor.
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From page 257... ...
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 o.s Lo ~ 0.6 o > J 0.5 ~ 0.4 cam 0.3 D .
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Figure 2 shows the estimated trajectories for average cell sire in the replicate populations during 10,000 generations. All 12 independently evolved larger cells and, moreover, all 12 underwent much more rapid change soon after their introduction into the experimental environment than when their environment had been constant for several thousand generations.
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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)
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Mean fitness evolved rapidly for ~2000 generations in the experimental environment but was nearly constant for the final several thousand generations. Although the fitness data are subject to more "noise" than the data on cell size, the hyperbolic model explains ~70% of the variation in mean fitness (r = 0.843, n = 21, P < 0.001; relative to linear model, partial F = 17.9, 1 and 18 df, P < 0.001~.
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FIGURE 5 Finer scale analysis of the trajectory for mean fitness in one population of E cold during its first 2000 generations of experimental evolution.
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Figure 6 shows the estimated trajectories for mean fitness in the replicate populations during 10,000 generations. A1112 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.
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Each point represents the among-population SD for mean fitness (i.e., the square root of the among-population variance component) ; negative values indicate that the estimated variance component was negative.
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, whereas both average cell size and mean fitness of a population are measured with error. Although there is no general solution to this problem of "model II" regression, a precise solution exists when the error variances associated with measurement of each variable are known (Mandel, 1964~.
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From page 265... ...
All regressions were performed according to model II procedures that are applicable when both variables are measured with error, but the corresponding error variances are known (Mandel, 1964~. From analyses of variance performed on repeated measures, the ratio of the error variances for average cell size and mean fitness was 0.138 (adjusted for sample sizes of two and three, respectively, and averaged over all populations and generations)
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Therefore, these analyses do not support the hypothesis that the functional relationship between size and fitness is causal and rigidly fixed (Figure 8B) but suggest instead that the replicate populations have diverged in this relationship (Figure 8C)
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From page 267... ...
Sustained divergence in mean fitness supports a Wrightian model of evolution (Wright, 1932, 1982, 1988; Barton and Hewitt, 1989; Mani and Clarke, 1990; Wade and Goodnight, 1991) , in which replicate populations found their way onto different fitness peaks.
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From page 268... ...
For about 2000 generations after their introduction into the experimental environment, all 12 populations underwent rapid changes in both morphology and fitness, whereas these properties were nearly static between generations 5000 and 10,000. The initially rapid evolution was presumably due to intense selection triggered by the sudden environmental changes imposed at the start of our experiment.
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From page 269... ...
This perturbation precipitated rapid adaptive evolution, while diversification resulted from the stochastic effects of mutation and drift that pushed replicate populations into the domains of attraction of different adaptive slopes and fitness peaks. [Diversification would presumably be even more pronounced if the populations were not only isolated but in different environments (Bennett et al., 1992~.]
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From page 270... ...
In fact, we observed several hallmarks of macroevolutionary dynamics, including periods of rapid evolution and stasis, altered functional relationships between traits, and concordance of anagenetic and cladogenetic trends. For now, the generality of our results remains an open question: one might well wonder what outcomes would be observed with a sexual organism, with larger or smaller population sizes, with different population structures, or with a more complex environmental regime.
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Although evolving in identical environments, the replicate populations diverged significantly from one another in both morphology and mean fitness. The divergence in mean fitness was sustained and implies that the populations have approached different fitness peaks of unequal height in the adaptive landscape.
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(1993) Adaptation and divergence in experimental populations of the
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(1994) Long-term experimental evolution in Escherichia coli.
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