Recurrent evolution is the repeated

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

of a particular trait, character, or

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...

. Most evolution is the result of

drift Drift or Drifts may refer to: Geography * Drift or ford (crossing) of a river * Drift, Kentucky, unincorporated community in the United States * In Cornwall, England: ** Drift, Cornwall, village ** Drift Reservoir, associated with the village ...

, often interpreted as the random chance of some

allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...

s 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 Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity of life on Earth. It is also defined as the study of ...

, as they can demonstrate the underlying forces governing evolution. Recurrent evolution is a broad term, but is usually used to describe recurring regimes of

selection Selection may refer to: Science * Selection (biology), also called natural selection, selection in evolution ** Sex selection, in genetics ** Mate selection, in mating ** Sexual selection in humans, in human sexuality ** Human mating strategie ...

within or across lineages. While most commonly used to describe recurring patterns of selection, it can also be used to describe recurring patterns of

; for example, transitions are more common than

transversions Transversion, in molecular biology, refers to a point mutation in DNA in which a single (two ring) purine ( A or G) is changed for a (one ring) pyrimidine ( T or C), or vice versa. A transversion can be spontaneous, or it can be caused by ioni ...

. The concept encompasses both

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

and

parallel evolution Parallel evolution is the similar development of a trait in distinct species that are not closely related, but share a similar original trait in response to similar evolutionary pressure.Zhang, J. and Kumar, S. 1997Detection of convergent and paral ...

, and can be used to describe the observation of similar repeating changes through

directional selection In population genetics, directional selection, is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under dir ...

as well as the observation of highly conserved phenotypes or genotypes across lineages through continuous

purifying selection In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through rando ...

over large periods of evolutionary time.

Phenotypic vs. genotypic levels

Recurrent changes may be observed at the

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...

level or the

genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

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 DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. Th ...

data. The same or similar sequences appearing in the

genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...

s 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 In biology, phase variation is a method for dealing with rapidly varying environments without requiring random mutation. It involves the variation of protein expression, frequently in an on-off fashion, within different parts of a bacterial populati ...

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 Evolvability is defined as the capacity of a system for adaptive evolution. Evolvability is the ability of a population of organisms to not merely generate genetic diversity, but to generate ''adaptive'' genetic diversity, and thereby evolve throu ...

. 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.

Convergent, parallel, and recurrent evolution

The distinction between convergent and parallel evolution is somewhat unresolved in evolutionary biology. Some authors have claimed it is a

false dichotomy A false dilemma, also referred to as false dichotomy or false binary, is an informal fallacy based on a premise that erroneously limits what options are available. The source of the fallacy lies not in an invalid form of inference but in a false ...

, while others have argued that there are important distinctions. These debates are important when considering recurrent evolution because the basis for the distinction is in the degree of

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

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. 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. 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. These may be caused by environmental swings – for example, natural fluctuations in the climate, or a

pathogenic bacterium Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and are often beneficial but others can cause infectious diseases. The number of t ...

moving between hosts – and represent the other major source of recurrent evolution. 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.

Examples

At the phenotypic level

On the island of

Bermuda ) , anthem = "God Save the King" , song_type = National song , song = " Hail to Bermuda" , image_map = , map_caption = , image_map2 = , mapsize2 = , map_caption2 = , subdivision_type = Sovereign state , subdivision_name = , e ...

, the shell size of the land snail Poecilozonites has increased during glacial periods and shrunk again during warmer periods. It has been proposed that this is due to the increased size of the island during glacial periods (as a consequence of lower sea levels), which results in more large vertebrate predators and creates a

selection pressure Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...

for larger shell size in the snails. In

eusocial Eusociality (from Greek εὖ ''eu'' "good" and social), the highest level of organization of sociality, is defined by the following characteristics: cooperative brood care (including care of offspring from other individuals), overlapping generat ...

insects, new colonies are usually formed by a solitary queen, though this is not always the case. Dependent colony formation, when new colonies are formed by more than one individual, has evolved recurrently multiple times in ants, bees, and wasps. Recurrent evolution of polymorphisms in colonial invertebrate bryozoans of the order

Cheilostomatida Cheilostomatida, also called Cheilostomata, is an order of Bryozoa in the class Gymnolaemata. They are exclusively marine, colonial invertebrate animals. Cheilostome colonies are composed of calcium carbonate and grow on a variety of surfaces, ...

has given rise to zooid polymorphs and certain skeletal structures several times in evolutionary history. Neotropical tanagers of the genera '' Diglossa'' and ''

Diglossopis ''Diglossopis'' was a genus of blue flowerpiercers in the family Thraupidae. They are now usually placed in the genus '' Diglossa''. They were formerly classified in the bunting and American sparrow family Emberizidae, more recent studies have s ...

'', known as flowerpiercers, have undergone recurrent evolution of divergent bill types. There is evidence for at least 133 transitions between

dioecy Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

and

hermaphroditism In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have separ ...

in the sexual systems of

bryophytes The Bryophyta s.l. are a proposed taxonomic division containing three groups of non-vascular land plants (embryophytes): the liverworts, hornworts and mosses. Bryophyta s.s. consists of the mosses only. They are characteristically limited i ...

. Additionally, the transition rate from hermaphroditism to dioecy was approximately twice the rate in the reverse direction, suggesting greater diversification among hermaphrodites and demonstrating the recurrent evolution of dioecy in mosses.

C4 photosynthesis carbon fixation or the Hatch–Slack pathway is one of three known photosynthetic processes of carbon fixation in plants. It owes the names to the 1960's discovery by Marshall Davidson Hatch and Charles Roger Slack that some plants, when sup ...

has evolved over 60 times in different plant lineages. This has occurred through the repurposing of genes present in a C3 photosynthetic common ancestor, altering levels and patterns of

gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

, and adaptive changes in the protein-coding region. Recurrent

lateral gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring ( reproduction). ...

has also played a role in optimizing the C4 pathway by providing better adapted C4 genes to the plants.

At the genotypic level

Certain genetic mutations occur with measurable and consistent frequency. Deleterious and neutral alleles can increase in

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

if the mutation rate to this phenotype is sufficiently higher than the reverse mutation rate; however, this appears to be rare. Beyond creating new

genetic variation Genetic variation is the difference in DNA among individuals or the differences between populations. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources of genetic variation, ...

for selection to act upon, mutations plays a primary role in evolution when mutations in one direction are "weeded out by natural selection" and mutations in the other direction are neutral. This is known as

when it acts to maintain functionally important characters but also results in the loss or diminished size of useless organs as the functional constraint is lifted. An example of this is the diminished size of the

Y chromosome The Y chromosome is one of two sex chromosomes (allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species, since it is the presence or abse ...

in mammals, which can be attributed to recurrent mutations and recurrent evolution. The existence of mutational "hotspots" within the genome often gives rise to recurrent evolution. Hotspots can arise at certain nucleotide sequences because of interactions between the DNA and

DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA dam ...

, replication, and modification enzymes. These sequences can act like fingerprints to help researchers locate mutational hotspots. Cis-regulatory elements are frequent targets of evolution resulting in varied morphology. When looking at long-term evolution, mutations in cis-regulatory regions appear to be even more common. In other words, more interspecific morphological differences are caused by mutations in cis-regulatory regions than intraspecific differences. Across ''

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species ...

'' species, highly conserved blocks not only in the

histone In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosomes in turn are wr ...

fold domain but also in the N-terminal tail of centromeric

histone H3 Histone H3 is one of the five main histones involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and a long N-terminal tail, H3 is involved with the structure of the nucleosomes of the 'beads on a stri ...

(CenH3) demonstrate recurrent evolution by purifying selection. In fact very similar

oligopeptides An oligopeptide, often just called peptide ('' oligo-'', "a few"), consists of two to twenty amino acids and can include dipeptides, tripeptides, tetrapeptides, and pentapeptides. Some of the major classes of naturally occurring oligopeptides incl ...

in the N-terminal tails of CenH3 have also been observed in humans and in mice. Many divergent

eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

lineages have recurrently evolved highly AT-rich genomes. GC-rich genomes are rarer among eukaryotes, but when they evolve independently in two different species the recurrent evolution of similar preferential codon usages will usually result. "Generally,

regulatory gene A regulator gene, regulator, or regulatory gene is a gene involved in controlling the expression of one or more other genes. Regulatory sequences, which encode regulatory genes, are often at the five prime end (5') to the start site of transcript ...

s occupying nodal position in

gene regulatory network A gene (or genetic) regulatory network (GRN) is a collection of molecular regulators that interact with each other and with other substances in the cell to govern the gene expression levels of mRNA and proteins which, in turn, determine the fun ...

s, and which function as morphogenetic switches, can be anticipated to be prime targets for evolutionary changes and therefore repeated evolution."

References