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The genetic causes of convergent evolution
David L.Stern
Abstract | 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.
Different species often evolve similar solutions to environmental challenges. Insects, birds and bats evolved wings, and octopi, vertebrates and spiders evolved focusing eyes. Phenotypic convergence provides compelling evidence that ecological circumstances can select for similar evolutionary solutions1,2. Historically, convergent evolution was thought to occur primarily by divergent evolution of genetic mechanisms. For example, multiple instances of wing evolution almost certainly reflect evolution mainly through different genetic mechanisms in different taxa. Also, if convergence is considered at a sufficiently general level, such as convergence of organismal fitness to similar environmental challenges, then multiple divergent genetic mechanisms might often contribute to increasing fitness. However, at a more finegrained level, recent studies have revealed that morphology and physiology often converge owing to the evolution of similar molecular mechanisms in independent lineages. In microorganisms, even fitness convergence often evolves through similar genetic changes. These new data reveal that genetic evolution may be more predictable than was appreciated before the application of molecular biology to evolutionary questions.
Convergent evolution at the genetic level can result from one of three processes: first, evolution by mutations that occurred independently in different populations or species; second, evolution of an allele that was polymorphic in a shared ancestral population; and third, evolution of an allele that was introduced from one population into another by hybridization, a process that is known as
introgression (FIG.1). It is worth distinguishing between these scenarios because each provides evidence for a different evolutionary path3. The first...