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Recurrent evolution is the repeated evolution of a particular character. Most evolution, or changes in allele frequencies from one generation to the next, is the result of drift, or random chance of some alleles getting passed down and others not. Recurrent evolution is when patterns emerge from this stochastic process when looking across populations. These patterns are of particular interest to evolutionary biologists as they can teach people about the underlying forces of evolution.
Recurrent evolution is a broad term, but is usually used to describe recurring regimes of selection within or across lineages. 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. It encompasses both convergent evolution and parallel evolution and 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. The changes can 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 changes 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 represent convergent and recurrent evolution toward increased evolvability. In organisms with longer generation times, any potential recurrent genomic evolution within a lineage would be difficult to detect. Recurrent evolution has been studied most extensively at the organismic level but with cheaper and faster sequencing technologies more attention is being paid to recurrent genomic evolution. Recurrent evolution can also be described as recurring or repeated evolution.
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 there are still important distinctions. These debates are important when considering recurrent evolution as their basis is in the degree of phylogenetic relatedness among the organisms being considered and convergent and parallel evolution are the major sources of recurrent evolution. While convergent evolution and parallel evolution are both forms of recurrent evolution they involve multiple lineages whereas recurrent evolution can also take place in a single lineage. As mentioned before, recurrent evolution within a lineage can be difficult to detect in organisms with longer 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 the natural fluctuations in the climate or a pathogenic bacteria 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 of phenotypic recurrent evolution
On the island of Bermuda, the shell size of the land snail poecilozonites has increased during glacial periods and shrunk again during warmer periods. This is due to the increased size of the island during glacial periods resulting in more large vertebrate predators creating selection for larger shell size in the snails.
In eusocial insects, new colonies are usually formed by a solitary queen; however 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 the colonial invertebrate animal cheilostomata bryozoans has given rise to zooid polymorphs and some skeletal structures several times in evolutionary history.
There is evidence for at least 133 transitions between dioecy and hermaphroditism in the sexual systems of bryophytes. Additionally the transition rate from hermaphroditism to dioecy was approximately twice the reverse rate suggesting greater diversification among hermphrodites and demonstrating the recurrent evolution of dioecy in mosses.
C4 photosynthesis has evolved over 60 times in different plants. This is has occurred through using genes present in the C3 photosynthetic ancestor, altering levels and patterns of gene expression, and adaptive changes in the protein coding region. Recurrent lateral gene transfer has also played a role in optimizing the C4 pathway by providing better adapted C4 genes to the plant.
Examples of genomic recurrent evolution
Certain mutations occur with measurable and consistent frequencies. Deleterious and neutral alleles can increase in frequency if the mutation rate to this phenotype is sufficiently higher than the reverse mutation rate, however this appears to be rare. Beyond creating new 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 purify selection 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 in mammals and this can be attributed to recurrent mutations and recurrent evolution.
The existence of mutational hotspots within the genome gives rise to recurrent evolution. Hotspots can arise at certain nucleotide sequences because of interactions between the DNA and DNA repair, replication, and modification enzymes. These sequences can act like fingerprints to 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 species highly conserved blocks not only in the histone fold domain but also in N-terminal tail of centromeric histone H3 (CenH3) demonstrate recurrent evolution by purifying selection. In fact very similar oligopeptides in the N-terminal tails of CenH3 have also been observed in humans and in mice.
Many divergent eukaryotic 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 genes occupying nodal position in gene regulatory networks, and which function as morphogenetic switches, can be anticipated to be prime targets for evolutionary changes and therefore repeated evolution."
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