RT Journal Article SR Electronic T1 Sustained fitness gains and variability in fitness trajectories in the long-term evolution experiment with Escherichia coli JF bioRxiv FD Cold Spring Harbor Laboratory SP 027391 DO 10.1101/027391 A1 Richard E. Lenski A1 Michael J. Wiser A1 Noah Ribeck A1 Zachary D. Blount A1 Joshua R. Nahum A1 J. Jeffrey Morris A1 Luis Zaman A1 Caroline B. Turner A1 Brian D. Wade A1 Rohan Maddamsetti A1 Alita R. Burmeister A1 Elizabeth J. Baird A1 Jay Bundy A1 Nkrumah A. Grant A1 Kyle J. Card A1 Maia Rowles A1 Kiyana Weatherspoon A1 Spiridon E. Papoulis A1 Rachel Sullivan A1 Colleen Clark A1 Joseph S. Mulka A1 Neerja Hajela YR 2015 UL http://biorxiv.org/content/early/2015/09/22/027391.abstract AB Many populations live in environments subject to frequent biotic and abiotic changes. Nonetheless, it is interesting to ask whether an evolving population’s mean fitness can increase indefinitely, and potentially without any limit, even in a constant environment. A recent study showed that fitness trajectories of Escherichia coli populations over 50,000 generations were better described by a power-law model than by a hyperbolic model. According to the power-law model, the rate of fitness gain declines over time but fitness has no upper limit, whereas the hyperbolic model implies a hard limit. Here, we examine whether the previously estimated power-law model predicts the fitness trajectory for an additional 10,000 generations. To that end, we conducted more than 1100 new competitive fitness assays. Consistent with the previous study, the power-law model fits the new data better than the hyperbolic model. We also analysed the variability in fitness among populations, finding subtle, but significant, heterogeneity in mean fitness. Some, but not all, of this variation reflects differences in mutation rate that evolved over time. Taken together, our results imply that both adaptation and divergence can continue indefinitely— or at least for a long time—even in a constant environment.