Recurrent evolution is the repeated evolution of a particular trait, character, or mutation. Most evolution is the result of drift, often interpreted as the random chance of some alleles being passed down to the next generation and others not. Recurrent evolution is said to occur when patterns emerge from this stochastic process when looking across multiple distinct populations. These patterns are of particular interest to evolutionary biologists, as they can demonstrate the underlying forces governing evolution.

Recurrent evolution is a broad term, but it is usually used to describe recurring regimes of selection within or across lineages.^[1] While most commonly used to describe recurring patterns of selection, it can also be used to describe recurring patterns of mutation; for example, transitions are more common than transversions.^[1] The concept encompasses both convergent evolution and parallel evolution; it can be used to describe the observation of similar repeating changes through directional selection as well as the observation of highly conserved phenotypes or genotypes across lineages through continuous purifying selection over large periods of evolutionary time.^[1]

Recurrent changes may be observed at the phenotype level or the genotype level. At the phenotype level, recurrent evolution can be observed across a continuum of levels, which for simplicity can be broken down into molecular phenotype, cellular phenotype, and organismal phenotype. At the genotype level, recurrent evolution can only be detected using DNA sequencing data. The same or similar sequences appearing in the genomes of different lineages indicates recurrent genomic evolution may have taken place. Recurrent genomic evolution can also occur within a lineage; an example of this would include some types of phase variation that involve highly directed changes at the DNA sequence level. The evolution of different forms of phase variation in separate lineages represents convergent and recurrent evolution toward increased evolvability. In organisms with long generation times, any potential recurrent genomic evolution within a lineage would be difficult to detect. Recurrent evolution has been studied most extensively at the organismal level, but with the advent of cheaper and faster sequencing technologies more attention is being paid to recurrent evolution at the genomic level.

The distinction between convergent and parallel evolution is somewhat unresolved in evolutionary biology. Some authors have claimed it is a false dichotomy, while others have argued that there are important distinctions.^[2][3][4][5] These debates are important when considering recurrent evolution because the basis for the distinction is in the degree of phylogenetic relatedness among the organisms being considered. While convergent and parallel evolution can both be interpreted as forms of recurrent evolution, they involve multiple lineages whereas recurrent evolution can also take place within a single lineage.^[1][6]

As mentioned before, recurrent evolution within a lineage can be difficult to detect in organisms with long generation times; however, paleontological evidence can be used to show recurrent phenotypic evolution within a lineage.^[6] The distinction between recurrent evolution across lineages and recurrent evolution within a lineage can be blurred because lineages do not have a set size and convergent or parallel evolution takes place among lineages that are all part of or within the same greater lineage. When speaking of recurrent evolution within a lineage, the simplest example is that given above, of the "on-off switch" used by bacteria in phase variation, but it can also involve phenotypic swings back and forth over longer periods of evolutionary history.^[6] These may be caused by environmental swings – for example, natural fluctuations in the climate, or a pathogenic bacterium moving between hosts – and represent the other major source of recurrent evolution.^[6] Recurrent evolution caused by convergent and parallel evolution, and recurrent evolution caused by environmental swings, are not necessarily mutually exclusive. If the environmental swings have the same effect on the phenotypes of different species, they could potentially evolve in parallel back and forth together through each swing.

• Convergent evolution
• Parallel evolution