LONG-TERM EXPERIMENTAL EVOLUTION IN ESCHERICHIA-COLI .4. TARGETS OF SELECTION AND THE SPECIFICITY OF ADAPTATION

This study investigates the physiological manifestation of adaptive ev olutionary change in 12 replicate populations of Escherichia coli that were propagated for 2000 generations in a glucose-limited environment . Representative genotypes from each population were assayed for fitne ss relative to their common ancestor in the experimental glucose envir onment and in 11 novel single-nutrient environments. After 2000 genera tions, the 12 derived genotypes had diverged into at least six distinc t phenotypic classes. The nutrients were classified into four groups b ased upon their uptake physiology. All 12 derived genotypes improved i n fitness by similar amounts in the glucose environment, and this patt ern of parallel fitness gains was also seen in those novel environment s where the limiting nutrient shared uptake mechanisms with glucose. F itness showed little or no consistent improvement, but much greater ge netic variation, in novel environments where the limiting nutrient dif fered from glucose in its uptake mechanisms. This pattern of fitness V ariation in the novel nutrient environments suggests that the independ ently derived genotypes adapted to the glucose environment by similar, but not identical, changes in the physiological mechanisms for moving glucose across both the inner and outer membranes.