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Evolutionary biology
Darwin's finches by John Gould
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Natural history Origin of life Common descent History of life Timeline of evolution Human evolution Phylogeny Biodiversity Biogeography Classification Evolutionary taxonomy Cladistics Transitional fossil Extinction event
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Social implications Evolution as fact and theory Social responses Creation–evolution controversy Theistic evolution Objections to evolution Level of support
v t e

Reticulate evolution is a way that living things evolve. Different lines of descent exchange genes. This happens through hybridization, horizontal gene transfer, gene reassortment, gene fusion, symbiosis, and symbiogenesis. Because of this, their family history looks like a phylogenetic network, not a strict branching tree. It works together with the traditional "tree of life", which describes vertical inheritance from parent to child. It is common in viruses, prokaryotes, fungi, plants, and animals, including humans. It affects speciation, adaptive evolution, and the spread of disease.

The idea of reticulate evolution is older than the modern synthesis. In 1867, Swiss botanist Simon Schwendener said lichens are not one organism. He said they are a union of fungi and algae. This idea was attacked because it changed the idea of an "individual".^[1]

In 1878, German mycologist Heinrich Anton de Bary introduced the word "symbiosis". He defined it as "different living things living together". Later, Russian scientists such as Konstantin Mereschkowski and Andrey Famintsyn developed these ideas. Between 1905 and 1910, Mereschkowski used the word "symbiogenesis". He drew a two-root tree of life. He said life began from two independent kinds of protoplasm and later merged.^[2]

In 1915, Hermann Reinheimer used the term "web of life". In the 1940s, the modern synthesis said evolution is mainly vertical descent with natural selection. Hybridization and gene transfer were seen as rare and not useful. This was partly because there were few discrete molecular markers.^[3]

Joshua Lederberg introduced the idea of plasmids in 1952. He called bacterial conjugation and transduction "infective heredity" and "hereditary symbiosis". At that time, these ideas were mostly seen as special cases in microbiology or medicine.^[4]

In 1967, Lynn Margulis published a famous paper on the origin of mitotic cells. She used molecular and cell biology evidence to argue that mitochondria and chloroplasts came from endosymbiosis. Her ideas were strongly resisted for more than ten years.^[5]

In the 1990s, Carl Woese used ribosomal RNA to propose the three-domain system. He drew a molecular "tree of life". Later, whole genome sequencing showed that many genes do not fit this tree. Scientists such as Ford Doolittle and Peter Gogarten said horizontal gene transfer was needed to explain this. In 1999, Doolittle used the idea of a "web of life" for prokaryotes. In 2009, New Scientist had a cover saying "Darwin was wrong". This caused strong reactions from Richard Dawkins, Daniel Dennett, and Jerry Coyne. Later, whole genome data from Canis and ancient humans such as Neanderthals and Denisovans showed that reticulate evolution is common in mammals.^[3]

Modern evolutionary biology sees "tree" and "net" as complementary. It tries to build one framework for reticulate events at different levels, from genes to lineages.

McShea and Brandon proposed the "Zero-Force Evolutionary Law" (ZFEL). It says that without selection, drift, or constraints, diversity and complexity tend to increase. Reticulate evolution can be seen as a second law. By exchanging genes and merging lineages, it can create new variation for natural selection and drift.^[6]

In hierarchy theory, evolutionary entities are nested at levels such as gene, chromosome, genome, cell, individual, population, species, and clade. Reticulate events can give three kinds of results:^[6]

1. Full fusion (1+1=1): for example, chromosome fusion or species merger. This raises diversity and complexity at that level, but lowers diversity at the next higher level.
2. Partial exchange (1+1=2): for example, gene introgression or organelle exchange. This raises diversity at that level, but the next higher level stays the same.
3. New entity (1+1=3): for example, hybrid speciation or endosymbiosis. This raises diversity and complexity at both levels.

This framework says natural selection and genetic drift balance the complexity added by divergence and reticulation. This is like a "sloshing bucket" model. When the environment changes greatly, reticulate events may become much more common. This can lead to rapid evolutionary jumps. The idea of "Goldilocks Zones" says there is a best range between full fusion and full isolation for reticulate events.^[6]

Koonin and others separate living things into "reproducers" and "replicators". Reproducers keep physical continuity. Replicators only pass on sequence information. This helps explain why viruses have no universal genes and why the virosphere looks like a net.^[7]

Reticulate evolution also asks scientists to study "communities", not only individuals or species. A community is a group of many living things that interact, including symbiotic microbes. At this level, shared traits, such as human–gut microbe interactions, can become units of selection.^[8] Reticulate evolution also makes species boundaries less clear. In conservation, hybrids should not simply be seen as "pollution". Scientists should study their unique ancestry and adaptive value.^[9][10]

The main mechanisms of reticulate evolution are hybridization, horizontal gene transfer, gene reassortment, and gene fusion and duplication.

Hybridization is sexual reproduction between different species or very different groups. It makes fertile or infertile offspring. In plants and fungi, hybridization often comes with polyploidy. This can make infertile hybrids fertile again and form new species.^[11][12]

Horizontal gene transfer (HGT) is the movement of DNA between organisms that are not parent and offspring. It is very common in prokaryotes. It happens by transformation, conjugation, and transduction.^[13] In eukaryotes, HGT is less common, but it still matters. For example, several key genes in the nitrate assimilation pathway came from bacteria and moved between eukaryote groups.^[14]

Gene reassortment is special to viruses with segmented genomes, such as influenza viruses. When two strains infect the same host cell, they can swap genome segments. This makes new virus strains. It helps viruses adapt to new hosts.^[15]

Gene fusion and whole genome duplication can make new gene functions. For example, eukaryotic nitrate reductase (EUKNR) is made from three gene pieces from different sources.^[14] Whole genome duplication in plants such as grasses is often linked to polyploid hybridization.^[16]

Symbiosis and symbiogenesis are also important. Symbiosis means different living things live together for a long time. Symbiogenesis means that long-term hereditary symbiosis makes a new individual or organelle. Mitochondria and chloroplasts came from endosymbiosis. Symbiosis can also bring new metabolic abilities. Genes from the symbiont can move into the host genome.^[5][1]

Infectious heredity means viruses, phages, and other movable genetic elements insert their genes into a host genome. These genes can be passed to offspring through germ cells. This can affect host evolution.^[2] Bacteria such as Wolbachia can control host reproduction and help make new species quickly.^[17] Non-transferable plasmids can also stay in prokaryotic cells for a long time.^[18]

Reticulate evolution is found in many groups. This section gives only a few core examples.

Viruses have no single universal gene. So scientists cannot build one "virus tree". Viruses exchange genes by reassortment, recombination, and capturing host genes. Influenza reassortment is more common when viruses jump between species. SARS-CoV-2 variant graphs show many ancestors, not a simple tree.^[7][15][19]

In prokaryotes, horizontal gene transfer is very common. A study of about 7,000 gene trees found that "near-universal genes", such as ribosomal protein genes, still have about 63% tree signal. But the whole set of genes has more net signal, about 61%. HGT often moves metabolic genes and information-processing genes, though the latter move less often.^[13] Movable elements such as plasmids, transposons, and integrative conjugative elements help this movement. Even non-transferable plasmids can stay stable in hosts.^[18]

Hybridization and polyploidy are important in plant evolution. About 15% of angiosperm species and 31% of fern species formed with polyploidy.^[12] Lichens are a classic symbiosis between fungi and algae or cyanobacteria.^[5] Bread wheat is a good example. It formed by two rounds of hybridization and polyploidy. First, AA and BB genomes came together. Later, this wheat mixed with DD genome Aegilops tauschii. After that, bread wheat received large gene flow from wild relatives. This helped it regain diversity lost during polyploidy and domestication.^[20]

In mammals, gene flow between species is common. Canis genomes show a complex hybrid network among gray wolves, coyotes, African golden wolves, and Ethiopian wolves.^[21] Polar bears and brown bears also had ancient mitochondrial capture. This may have given polar bears useful variation for ice-age environments.^[3]

Darwin's finches are a classic example of reticulate evolution. Lamichhaney and others (2015) sequenced all species in the Galápagos Islands. They found widespread gene flow between species. The phylogenetic network showed conflicts inside ground finches and tree finches. They also found a 240-kb haplotype of the ALX1 gene linked to beak shape.^[22]

Grant and Grant (2016) studied Daphne Major. They found that the medium ground finch (Geospiza fortis) hybridized with the small ground finch (G. fuliginosa) and the cactus ground finch (G. scandens). The hybrid rates were about 10.5% and 3.4%. This made the medium ground finch beak smaller and pointier. The drought of 2003–2005 then selected small-beak birds. These birds became very similar to the small ground finch.^[23]

Grant and Grant (2025) used museum specimens from Darwin Island from 1897–1906. They found extreme convergence between the large ground finch (G. magnirostris) and the Genovesa ground finch (G. propinqua). On Darwin Island, the large ground finch had smaller beak depth and width. The Genovesa ground finch had larger beaks. Their beaks now overlap strongly. This suggests the two species may be fusing through hybridization.^[24]

Human evolution also shows reticulate patterns. Neanderthal and Denisovan genes make up about 1–4% and 4–6% of modern non-African genomes.^[25] These genes affect skin color, immune response, and lipid metabolism. EPAS1 is linked to high-altitude adaptation. More than half of human HLA alleles came from ancient human groups. This gave modern humans immune advantages.^[3] Human gut microbes and pathogens also form cross-species networks with humans.^[8]

Early detection of reticulate evolution used conflict between organelle DNA, such as chloroplast DNA, and nuclear genes.^[11][26]

With more genome data, scientists now use network methods. TriLoNet builds networks from small "trinets". ABC-RF can compare different reticulate models, such as hybrid origin versus vertical origin.^[27][9]

Quartet analysis and the TNT score can measure tree and net signals in a genome. rIBD and chromosome painting can find introgressed regions.^[10][13][9]

• Hybridization
• Introgression
• Speciation