PT - JOURNAL ARTICLE AU - Olivier Tenaillon AU - Jeffrey E. Barrick AU - Noah Ribeck AU - Daniel E. Deatherage AU - Jeffrey L. Blanchard AU - Aurko Dasgupta AU - Gabriel C. Wu AU - Sébastien Wielgoss AU - Stéphane Cruveiller AU - Claudine Médigue AU - Dominique Schneider AU - Richard E. Lenski TI - Tempo and mode of genome evolution in a 50,000-generation experiment AID - 10.1101/036806 DP - 2016 Jan 01 TA - bioRxiv PG - 036806 4099 - http://biorxiv.org/content/early/2016/01/15/036806.short 4100 - http://biorxiv.org/content/early/2016/01/15/036806.full AB - Adaptation depends on the rates, effects, and interactions of many mutations. We analyzed 264 genomes from 12 Escherichia coli populations to characterize their dynamics over 50,0 generations. The trajectories for genome evolution in populations that retained the ancestral mutation rate fit a model where most fixed mutations are beneficial, the fraction of beneficial mutations declines as fitness rises, and neutral mutations accumulate at a constant rate. We also compared these populations to lines evolved under a mutation-accumulation regime that minimizes selection. Nonsynonymous mutations, intergenic mutations, insertions, and deletions are overrepresented in the long-term populations, supporting the inference that most fixed mutations are favored by selection. These results illuminate the shifting balance of forces that govern genome evolution in populations adapting to a new environment.