One of biology's longest-running experiments suggests that adaptation can be endless, even in extremely stable environments.

To test the assumption that evolution is stimulated by environmental change, Richard Lenski at Michigan State University in East Lansing and his colleagues maintained the same populations of Escherichia coli in a stable environment for 27 years, freezing samples every 500 generations. They found that populations consistently outcompeted their ancestors, indicating that they were becoming increasingly fit. This continued right up to the 60,000th generation, although the rate of fitness improvement slowed over time.

The results suggest that there is no upper limit to adaptation, even in simple environments.

Proc. R. Soc. B 282, 20152292 (2015)