LONG-TERM EXPERIMENTAL EVOLUTION IN ESCHERICHIA-COLI .7. MECHANISMS MAINTAINING GENETIC-VARIABILITY WITHIN POPULATIONS

Six replicate populations of the bacterium Escherichia coli were propa gated for more than 10,000 generations in a defined environment. We so ught to quantify the variation among clones within these populations w ith respect to their relative fitness, and to evaluate the roles of th ree distinct population genetic processes in maintaining this variatio n. On average, a pair of clones from the same population differed from one another in their relative fitness by approximately 4%. This withi n-population variation was small compared with the average fitness gai n relative to the common ancestor, but it was statistically significan t. According to one hypothesis, the variation in fitness is transient and reflects the ongoing substitution of beneficial alleles. We used F isher's fundamental theorem to compare the observed rate of each popul ation's change in mean fitness with the extent of variation for fitnes s within that population, but we failed to discern any correspondence between these quantities. A second hypothesis supposes that the variat ion in fitness is maintained by recurrent deleterious mutations that g ive rise to a mutation-selection balance. To test this hypothesis, we made use of the fact that two of the sh replicate populations had evol ved mutator phenotypes, which gave them a genomic mutation rate approx imately 100-fold higher than that of the other populations. There was a marginally significant correlation between a population's mutation r ate and the extent of its within-population variance for fitness, but this correlation was driven by only one population (whereas two of the populations had elevated mutation rates). Under a third hypothesis, t his variation is maintained by frequency-dependent selection, whereby genotypes have an advantage when they are rare relative to when they a re common. in all six populations, clones were more fit, on average, w hen they were rare than when they were common, although the magnitude of the advantage when rare was usually small ( similar to 1% in five p opulations and similar to 5% in the other). These three hypotheses are not mutually exclusive, but frequency-dependent selection appears to be the primary force maintaining the fitness variation within these ex perimental populations.