A framework to quantify phenotypic and genotypic parallel evolution

The independent and repeated adaptation of populations to similar environments often results in the evolution of similar forms. This phenomenon creates a strong correlation between phenotype and environment and is referred to as parallel evolution. However, there is ongoing debate as to when we should call a system either phenotypically or genotypically ‘parallel.’ Here, we suggest a novel and simple framework to quantify parallel evolution at the genotypic and phenotypic levels. Our framework combines both traditional and new approaches to measure parallel evolution, and categorizes them into broad- and narrow-sense scales. We then apply this framework to coastal ecotypes of an Australian wildflower, , that have evolved in parallel. Our findings show that populations inhabiting similar environments have evolved strikingly similar phenotypes. These phenotypes have arisen via mutational changes occurring in different genes, although many share the same biological functions. Our work paves the way towards a common framework to study the repeated evolution of forms in nature.