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FIGURE 12.1 Phylogeny and nomenclature of the experimental lineages of E. coli used in this study. The ancestral organism (Anc) was obtained from a lineage that evolved under defined laboratory conditions at 37°C for 2,000 generations. A clone was sampled and cultured in six replicate populations that evolved in each of four thermal environments: 32°C, 37°C, or 42°C, or a daily alteration between 32°C and 42°C. After 2,000 generations, a clone was isolated from each lineage and propagated for another 2,000 generations at 20°C. Note that each of the 20°C selected lineages are equally related to Anc (separated by 4,000 generations) and are equally distant from each other (separated by 8,000 generations).

increment of 42/20 + 3 at 40°C, which went against our a priori expectation, the 40°C measurements were repeated with 12-fold replication for three lines, one that demonstrated a trade-off (42/20 − 1), one that did not significantly change fitness (42/20 + 2), and the single line that incremented fitness at high temperature (42/20 + 3). The previous results were repeated for each line: 42/20 −1 had a negative ΔW (P < 0.001), 42/20 + 2 had no significant change in fitness (P + 0.06), and 42/20 + 3 had a significantly positive ΔW (P = 0.04). Of the six lines selected at 20°C directly from Anc

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