Repeatability and coevolution between E.coli and Phage T3

Similar environmental pressures can cause independent lineages to evolve the same characteristics due to natural selection. Many examples of such parallel phenotypic evolution have been documented in nature. Molecular studies have revealed that parallel phenotypic evolution can result from similar changes occurring at different scales of genetic organization – from pathways 1, to genes 2,3, codons 4, and nucleotides 5. However, the factors that determine the scale at which evolution is repeatable remain unclear. Here, we use whole-genome sequencing and an experimental system of bacteria and bacteriophage to understand the genetic mechanisms underlying repeatable phenotypic evolution. Although past studies have proposed that genome size may influence patterns of repeatability, our observations do not support this hypothesis. We find that the scale at which evolution becomes repeatable depends upon the underlying genetic architecture of traits that are beneficial in the selective environment.