Selection is the engine that makes evolution go, whether in nature or in the laboratory. But scientists don’t fully understand the mechanism by which it works. Using the example of evolving yellow fluorescent proteins in Escherichia coli so that they fluoresce green instead, a team led by Andreas Wagner of the University of Zurich, the Swiss Institute of Bioinformatics, and the Santa Fe Institute probed how selection strength affects evolvability (Science 2020, DOI: 10.1126/science.abb5962). The researchers subjected E. coli populations to multiple rounds of strong or weak selections based on the cells’ fluorescence intensity—or to no selection. For strong selection, they kept the top 20% of fluorescing cells from one round to the next. For weak selection, they kept all cells fluorescing above the background. In these experiments, the population subjected to strong selection had significant increases in fluorescence levels and more quickly evolved green fluorescence, whereas the population subjected to weak selection experienced initial decreases in fluorescence that then stabilized. The success of strong selection comes from the accumulation of mutations that improve foldability and mutational robustness, which is the ability to resist functional changes upon subsequent mutations. Because the experiments were done at high mutation rates, the researchers don’t know whether their findings also apply to low mutation rates.