# Sustained fitness gains and variability in fitness trajectories in the long-term evolution experiment with Escherichia coli

## TL;DR

E. coli populations show sustained fitness gains over 60,000 generations in a constant environment, with no clear upper limit.

## Contribution

The study extends a long-term evolution experiment to show that fitness gains follow a power-law model over 60,000 generations.

## Key findings

- Fitness gains in E. coli populations continue for 60,000 generations with no upper limit.
- The power-law model better fits fitness trajectories than the hyperbolic model.
- Subtle but significant fitness variation among populations suggests ongoing divergence.

## Abstract

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.