In this study, the authors investigate how the translational machinery adapts to genetic or environmental changes. They deleted a tRNAArg gene in a Saccharomyces cerevisiae strain and found that when the strain evolved, the gene of a more commonly used tRNA evolved to match the missing anticodon, thus restoring a wild-type rate of cell growth. Further experiments showed that the evolved strains maintained some tRNAs at low levels, so that translation rate is optimal for proteins that are difficult to fold. Furthermore, analyses in various species indicate that the codon switching mechanism is a common form of molecular evolution across the tree of life.