Bacteria sometimes evolve in a remarkably repeatable manner: When challenged by the same environment, independent populations of bacteria can utilise identical mutations to recover. However, the ability to evolve predictably is not often shared by a bacterial strain’s close relatives. The bacterial relatives, known as homologs, often harbour many synonymous differences in their DNA. This means that the DNA sequence itself is different, but the encoded protein product remains the same. As such these changes are referred to as “silent”. My work reveals that far from being silent bystanders, these synonymous differences in DNA can determine the repeatability of evolution.