LONG-TERM EXPERIMENTAL EVOLUTION IN ESCHERICHIA-COLI .2. CHANGES IN LIFE-HISTORY TRAITS DURING ADAPTATION TO A SEASONAL ENVIRONMENT

Twelve populations of the bacterium Escherichia coli were propagated for 2,000 generations in a seasonal environment, which consisted of alternating periods of feast and famine. The mean fitness of the derived genotypes increased by approximately 35% relative to their common ancestor, based on competition experiments in the same environment. The bacteria could have adapted, in principle, by decreasing their lag prior to growth upon transfer to fresh medium (L), increasing their maximum growth rate (V(m)), reducing the the concentration of resource required to support growth at half the maximum rate (K(s)), and reducing their death rate after the limiting resource was exhausted (D). We estimated these parameters for the ancestor and then calculated the opportunity for selection on each parameter. The inferred selection gradients for V(m) and L were much steeper than for K(s) and D. The derived genotypes showed significant improvement in V(m) and L but not in K(s) or D. Also, the numerical yield in pure culture of the derived genotypes was significantly lower than the yield of the common ancestor, but the average cell size was much larger. The independently derived genotypes are somewhat more variable in these life-history traits than in their relative fitnesses, which indicates that they acquired different genetic adaptations to the seasonal environment. Nonetheless, the evolutionary changes in life-history traits exhibit substantial parallelism among the replicate populations.