In Science this week, researchers from Michigan State University publish data suggesting that there is no limit to how much evolutionary fitness can increase in a given population. Drawing on 12 E. coli populations that were founded from a common ancestor in 1988, the scientists examined the fitness trajectories for those bacterial populations over the course of 50,000 generations. They found no upper limit on fitness gains, as measured by increases in growth rates. The fitness trajectory followed by the bacteria seems dependent on competition among organisms with different mutations or genetic interactions between individuals carrying multiple mutations, and on the effect of improvements in fitness from a single beneficial mutation decline as overall fitness increases.