The genetic causes of convergent evolution

Key Points Convergent phenotypic evolution often results from similar genetic changes in independent species by a process known as parallel evolution. Sometimes, convergent evolution results from the evolution of a genetic change that is inherited from an ancestral population or from hybridization between species, which, in this Review, are collectively called collateral evolution. Whole-genome sequencing of experimental-evolution populations has provided compelling evidence for the importance of parallel evolution, and parallel evolution at specific genes has also been documented between highly divergent taxa. Collateral evolution by ancestry is likely to be common in species in which a single large population is surrounded by multiple geographical isolates. Collateral evolution by hybridization has been documented only recently and is likely to be widespread in nature. Multiple factors contribute to parallel evolution; it seems that genes which control key developmental decisions and those that interact most immediately with the environment are most likely to contribute to this type of evolution. This Review distinguishes between three distinct routes by which similar genetic changes contribute to convergent evolution and discusses examples from diverse taxa. Convergent genetic evolution might result from the fact that some mutations both minimize pleiotropic effects and maximize adaptation. The evolution of phenotypic similarities between species, known as convergence, illustrates that populations can respond predictably to ecological challenges. Convergence often results from similar genetic changes, which can emerge in two ways: the evolution of similar or identical mutations in independent lineages, which is termed parallel evolution; and the evolution in independent lineages of alleles that are shared among populations, which I call collateral genetic evolution. Evidence for parallel and collateral evolution has been found in many taxa, and an emerging hypothesis is that they result from the fact that mutations in some genetic targets minimize pleiotropic effects while simultaneously maximizing adaptation. If this proves correct, then the molecular changes underlying adaptation might be more predictable than has been appreciated previously.