Species complex

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| caption1 = At least six treefrog species make up the Hypsiboas calcaratus–fasciatus species complex.

| alt1 = Six light brown treefrogs, labeled A to E

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| caption2 = The fly agaric comprises several cryptic species, as is shown by genetic data.

| alt2 = Picture showing two mushrooms with red caps on a meadow

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| caption3 = The forest elephant (shown) is the bush elephant's sibling species.

| alt3 = An adult and a young elephant bathing

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| caption4 = Mbuna cichlids form a species flock in Lake Malawi.

| alt4 = A flock of differently coloured fish in a rocky setting

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A species complex is typically considered as a group of close, but distinct species. Obviously, the concept is closely tied to the definition of a species. Modern biology understands a species as "separately evolving metapopulation lineage" but acknowledges that the criteria to delimit species may depend on the group studied. Thus, many traditionally defined species, based only on morphological similarity, have been found to be several distinct species when other criteria, such as genetic differentiation or reproductive isolation, are applied.

A more restricted use applies the term to a group of species among which hybridisation has occurred or is occurring, which leads to intermediate forms and blurred species boundaries. The informal classification, superspecies, can be exemplified by the grizzled skipper butterfly, which is a superspecies that is further divided into three subspecies.{{Cite journal|last=JONG |first=R. |date=December 1987 |title=Superspecies Pyrgus malvae (Lepidoptera: Hesperiidae) in the East Mediterranean, with notes on phylogenetic and biological relationships |journal=Rijksmuseum van Natuurlijke Historie – Via Naturalis Repository}}

Some authors apply the term to a species with intraspecific variability, which might be a sign of ongoing or incipient speciation. Examples are ring species or species with subspecies, in which it is often unclear if they should be considered separate species.

File:Sparganium eurycarpum-Hetaerina americana-male perching.jpg is suspected to be a cryptic complex with at least one other species of rubyspot.]]

Several terms are used synonymously for a species complex, but some of them may also have slightly different or narrower meanings. In the nomenclature codes of zoology and bacteriology, no taxonomic ranks are defined at the level between subgenus and species, but the botanical code defines four ranks below subgenus (section, subsection, series, and subseries). Different informal taxonomic solutions have been used to indicate a species complex.

; {{Anchor|Cryptic species}}

; Cryptic species

:Cryptic species, also known as sibling species, are morphologically identical lineages of a species that are genetically distinct. More generally, the term is often applied when species, even if they are known to be distinct, cannot be reliably distinguished by morphology. Rather, these lineages can be distinguished by use of DNA barcoding and meta-barcoding which sequence a particular region of the genome. {{Cite journal |last1=Vega-Sánchez |first1=Yesenia Margarita |last2=Mendoza-Cuenca |first2=Luis |last3=González-Rodríguez |first3=Antonio |date=2022-06-28 |title=Morphological variation and reproductive isolation in the Hetaerina americana species complex |journal=Scientific Reports |language=en |volume=12 |issue=1 |pages=10888 |doi=10.1038/s41598-022-14866-8 |issn=2045-2322 |pmc=9240019 |pmid=35764791|bibcode=2022NatSR..1210888V }}

: Cryptic species are often sexually isolated; less so because they are unable to mate with one another but rather due to geography and slight differences in breeding behavior or chemical signals.

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; {{Anchor|Species flock}}

; Species flock

: A species flock--also known as species swarm--occurs when, in a limited geographic area, a single species evolves into multiple distinct species which each fill their own ecological niche. Similarly, a superspecies can be described as a species that diverges into specific species in isolation and then remains geographically or reproductively isolated.{{Cite journal |last1=Hodges |first1=S A |last2=Arnold |first2=M L |date=1994-05-24 |title=Columbines: a geographically widespread species flock. |journal=Proceedings of the National Academy of Sciences |language=en |volume=91 |issue=11 |pages=5129–5132 |doi=10.1073/pnas.91.11.5129 |doi-access=free |issn=0027-8424 |pmc=43945 |pmid=8197196|bibcode=1994PNAS...91.5129H }}{{Cite journal |last=Amadon |first=Dean |date=1966-09-01 |title=The Superspecies Concept |url=https://academic.oup.com/sysbio/article-abstract/15/3/245/1704294?redirectedFrom=fulltext |journal=Systematic Biology |volume=15 |issue=3 |pages=245–249 |doi=10.2307/sysbio/15.3.245 |issn=1063-5157|url-access=subscription }} The main difference between a cryptic or sibling species and a species flock or superspecies is that while the former is very nearly indistinguishable, the latter can be identified morphologically. A species flock should not be confused with a mixed-species foraging flock, a behavior in which birds of different species feed together.

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; Species aggregate

: Used for a species complex, especially in plant taxa where polyploidy and apomixis are common. Historical synonyms are species collectiva [la], introduced by Adolf Engler, conspecies, and grex. Components of a species aggregate have been called segregates or microspecies. Used as abbreviation agg. after the binomial species name.

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: A species aggregate is very similar in definition to that of a species complex, a term to describe a group of organisms in the stages of speciation, where the species involved may be morphologically identical, much like a cryptic species, or distinct, much like a species flock.{{Cite journal |last1=Hörandl |first1=Elvira |last2=Greilhuber |first2=Johann |last3=Klímová |first3=Katarina |last4=Paun |first4=Ovidiu |last5=Temsch |first5=Eva |last6=Emadzade |first6=Khatere |last7=Hodálová |first7=Iva |date=2009 |title=Reticulate evolution and taxonomic concepts in the Ranunculus auricomus complex (Ranunculaceae): insights from analysis of morphological, karyological and molecular data |journal=Taxon |language=en |volume=58 |issue=4 |pages=1194–1216 |doi=10.1002/tax.584012 |pmid=20401184 |issn=1996-8175|pmc=2855680|bibcode=2009Taxon..58.1194H }}{{Cite journal |last1=Kankare |first1=Maaria |last2=Van Nouhuys |first2=Saskya |last3=Hanski |first3=Ilkka |date=2005-05-01 |title=Genetic Divergence Among Host-Specific Cryptic Species in Cotesia melitaearum Aggregate (Hymenoptera: Braconidae), Parasitoids of Checkerspot Butterflies |url=https://academic.oup.com/aesa/article-abstract/98/3/382/85533?redirectedFrom=fulltext |journal=Annals of the Entomological Society of America |volume=98 |issue=3 |pages=382–394 |doi=10.1603/0013-8746(2005)098[0382:GDAHCS]2.0.CO;2 |issn=0013-8746|url-access=subscription }} The term is most used in plant biology, and is a synonym for the more utilized species flock.

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; Sensu lato

: A Latin phrase meaning 'in the broad sense', it is often used after a binomial species name, often abbreviated as s.l., to indicate a species complex represented by that species.

Distinguishing close species within a complex requires the study of often very small differences. Morphological differences may be minute and visible only by the use of adapted methods, such as microscopy. However, distinct species sometimes have no morphological differences. In those cases, other characters, such as in the species' life history, behavior, physiology, and karyology, may be explored. For example, territorial songs are indicative of species in the treecreepers, a bird genus with few morphological differences. Mating tests are common in some groups such as fungi to confirm the reproductive isolation of two species.

Analysis of DNA sequences is becoming increasingly standard for species recognition and may, in many cases, be the only useful method. Different methods are used to analyse such genetic data, such as molecular phylogenetics or DNA barcoding. Such methods have greatly contributed to the discovery of cryptic species,{{Cite journal|last1=Marques |first1=Isabel|last2=Montgomery |first2=Sean A. |last3=Barker |first3=Michael S. |last4=Macfarlane |first4=Terry D. |last5=Conran |first5=John G. |last6=Catalán |first6=Pilar |last7=Rieseberg |first7=Loren H. |last8=Rudall |first8=Paula J.|last9=Graham |first9=Sean W. |date=2016-04-01 |title=Transcriptome-derived evidence supports recent polyploidization and a major phylogeographic division in Trithuria submersa (Hydatellaceae, Nymphaeales) |journal=New Phytologist|language=en |volume=210 |issue=1 |pages=310–323 |doi=10.1111/nph.13755 |pmid=26612464 |issn=1469-8137|doi-access=free|bibcode=2016NewPh.210..310M}} including such emblematic species as the fly agaric, the water fleas,{{Cite journal|last1=Kotov |first1=Alexey A. |last2=Garibian |first2=Petr G. |last3=Bekker |first3=Eugeniya I. |last4=Taylor |first4=Derek J. |last5=Karabanov |first5=Dmitry P.|date=2020-06-17 |title=A new species group from the Daphnia curvirostris species complex (Cladocera: Anomopoda) from the eastern Palaearctic: taxonomy, phylogeny and phylogeography |journal=Zoological Journal of the Linnean Society|volume=191 |issue=3 |pages=772–822 |doi=10.1093/zoolinnean/zlaa046 |issn=0024-4082 |doi-access=}} or the African elephants.

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| caption1 = Salamandra atra

| alt1 = An individual of a uniformly black salamander.

| image2 = Salamandra_corsica.jpg

| caption2 = Salamandra corsica

| alt2 = An individual of a yellow-spotted salamander

| image3 = Salamandra salamandra (Marek Szczepanek).jpg

| caption3 = Salamandra salamandra

| alt3 = An individual of a fire salamander

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| footer = Similarity can be misleading: the Corsican fire salamander (center) was previously considered a subspecies of the fire salamander (right) but is in fact more closely related to the uniformly black Alpine salamander (left).

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File:Phylogeny species complex.svg that has diversified rather recently, as is shown by the short branches between the species A–E (blue box) in this phylogenetic tree.]]

Species forming a complex have typically diverged very recently from each other, which sometimes allows the retracing of the process of speciation. Species with differentiated populations, such as ring species, are sometimes seen as an example of early, ongoing speciation: a species complex in formation. Nevertheless, similar but distinct species have sometimes been isolated for a long time without evolving differences, a phenomenon known as "morphological stasis". For example, the Amazonian frog Pristimantis ockendeni is actually at least three different species that diverged over 5 million years ago.

Stabilizing selection has been invoked as a force maintaining similarity in species complexes, especially when they adapted to special environments (such as a host in the case of symbionts or extreme environments). This may constrain possible directions of evolution; in such cases, strongly divergent selection is not to be expected. Also, asexual reproduction, such as through apomixis in plants, may separate lineages without producing a great degree of morphological differentiation.

File:Cryptic speciation.svg]]

A species complex is usually a group that has one common ancestor (a monophyletic group), but closer examination can sometimes disprove that. For example, yellow-spotted "fire salamanders" in the genus Salamandra, formerly all classified as one species S. salamandra, are not monophyletic: the Corsican fire salamander's closest relative has been shown to be the entirely black Alpine salamander. In such cases, similarity has arisen from convergent evolution.

Hybrid speciation can lead to unclear species boundaries through a process of reticulate evolution, in which species have two parent species as their most recent common ancestors. In such cases, the hybrid species may have intermediate characters, such as in Heliconius butterflies. Hybrid speciation has been observed in various species complexes, such as insects, fungi, and plants. In plants, hybridization often takes place through polyploidization, and hybrid plant species are called nothospecies.

Sources differ on whether or not members of a species group share a range. A source from Iowa State University Department of Agronomy states that members of a species group usually have partially overlapping ranges but do not interbreed with one another. A Dictionary of Zoology (Oxford University Press 1999) describes a species group as complex of related species that exist allopatrically and explains that the "grouping can often be supported by experimental crosses in which only certain pairs of species will produce hybrids." The examples given below may support both uses of the term "species group."

Often, such complexes do not become evident until a new species is introduced into the system, which breaks down existing species barriers. An example is the introduction of the Spanish slug in Northern Europe, where interbreeding with the local black slug and red slug, which were traditionally considered clearly separate species that did not interbreed, shows that they may be actually just subspecies of the same species.{{Cite journal |last1=Reise |first1=Heike |last2=Schwarzer |first2=Anne-Katrin |last3=Hutchinson |first3=John M. C. |last4=Schlitt |first4=Bettina |date=2020 |title=Genital morphology differentiates three subspecies of the terrestrial slug Arion ater (Linnæus, 1758) s.l. and reveals a continuum of intermediates with the invasive A. vulgaris Moquin-Tandon, 1855 |url=http://www.foliamalacologica.com/Genital-morphology-differentiates-three-subspecies-of-the-terrestrial-slug-Arion,119132,0,2.html |journal=Folia Malacologica |volume=28 |issue=1 |pages=1–34 |doi=10.12657/folmal.028.001}}{{Cite journal |last1=Hutchinson |first1=John M C |last2=Schlitt |first2=Bettina |last3=Reise |first3=Heike |date=2021-11-18 |title=One town's invasion by the pest slug Arion vulgaris (Gastropoda: Arionidae): microsatellites reveal little introgression from Arion ater and limited gene flow between infraspecific races in both species |url=https://academic.oup.com/biolinnean/article/134/4/835/6364936 |journal=Biological Journal of the Linnean Society |language=en |volume=134 |issue=4 |pages=835–850 |doi=10.1093/biolinnean/blab082 |issn=0024-4066|url-access=subscription }}

Where closely related species co-exist in sympatry, it is often a particular challenge to understand how the similar species persist without outcompeting each other. Niche partitioning is one mechanism invoked to explain that. Indeed, studies in some species complexes suggest that species divergence have gone in par with ecological differentiation, with species now preferring different microhabitats.{{Cite journal |last1=Scriven |first1=Jessica J. |last2=Whitehorn |first2=Penelope R. |last3=Goulson |first3=Dave |last4=Tinsley |first4=Matthew. C. |date=2016 |title=Niche partitioning in a sympatric cryptic species complex |journal=Ecology and Evolution |language=en |volume=6 |issue=5 |pages=1328–1339 |doi=10.1002/ece3.1965 |issn=2045-7758 |pmc=4730923 |pmid=26848386|bibcode=2016EcoEv...6.1328S}} Similar methods also found that the Amazonian frog Eleutherodactylus ockendeni is actually at least three different species that diverged over 5 million years ago.

A species flock may arise when a species penetrates a new geographical area and diversifies to occupy a variety of ecological niches, a process known as adaptive radiation. The first species flock to be recognized as such was the 13 species of Darwin's finches on the Galápagos Islands described by Charles Darwin.

It has been suggested that cryptic species complexes are very common in the marine environment. That suggestion came before the detailed analysis of many systems using DNA sequence data but has been proven to be correct. The increased use of DNA sequence in the investigation of organismal diversity (also called phylogeography and DNA barcoding) has led to the discovery of a great many cryptic species complexes in all habitats. In the marine bryozoan Celleporella hyalina, detailed morphological analyses and mating compatibility tests between the isolates identified by DNA sequence analysis were used to confirm that these groups consisted of more than 10 ecologically distinct species, which had been diverging for many millions of years.

File:AnophelesGambiaemosquito.jpg mosquito complex contains both species that are a vector for malaria and species that are not.]]

Pests, species that cause diseases and their vectors, have direct importance for humans. When they are found to be cryptic species complexes, the ecology and the virulence of each of these species need to be re-evaluated to devise appropriate control strategies as their diversity increases the capacity for more dangerous strains to develop. Examples are cryptic species in the malaria vector genus of mosquito, Anopheles, the fungi causing cryptococcosis, and sister species of Bactrocera tryoni, or the Queensland fruit fly. That pest is indistinguishable from two sister species except that B. tryoni inflicts widespread, devastating damage to Australian fruit crops, but the sister species do not.{{Cite journal|last1=Clarke |first1=A.R. |last2=Powell |first2=K.S. |last3=Weldon |first3=C.W. |last4=Taylor |first4=P.W. |date=2010-11-02 |title=The ecology of Bactrocera tryoni (Diptera: Tephritidae): what do we know to assist pest management?|journal=Annals of Applied Biology |volume=158 |issue=1 |pages=26–54 |doi=10.1111/j.1744-7348.2010.00448.x |issn=0003-4746 |url=https://eprints.qut.edu.au/40107/1/c40107.pdf|hdl=10019.1/122744|hdl-access=free}}

When a species is found to be several phylogenetically distinct species, each typically has smaller distribution ranges and population sizes than had been reckoned. The different species can also differ in their ecology, such as by having different breeding strategies or habitat requirements, which must be taken into account for appropriate management. For example, giraffe populations and subspecies differ genetically to such an extent that they may be considered species. Although the giraffe, as a whole, is not considered to be threatened, if each cryptic species is considered separately, there is a much higher level of threat.

{{Portal|Evolutionary biology}}

• Glossary of scientific naming
• Alliance (taxonomy)
• Association (ecology)
• Subspecies
• Ring species

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{{cite journal|author1=Muñoz MM |author2=Crawford NG |author3=McGreevy Jr. TJ |author4=Messana NJ |author5=Tarvin RD |author6=Revell LJ |author7=Zandvliet RM |author8=Hopwood JM |author9=Mock E|author10=Schneider AL |author11=Schneider CJ. |title=Divergence in coloration and ecological speciation in the Anolis marmoratus species complex |journal=Molecular Ecology |volume=22 |issue=10 |pages=2668–2682|date=2013 |doi=10.1111/mec.12295 |pmid=23611648 |bibcode=2013MolEc..22.2668M |s2cid=16832446 |url=http://faculty.umb.edu/liam.revell/pdfs/Munoz_etal_2013.MolEcol.pdf}}

{{Cite web|url=https://www.encyclopedia.com/earth-and-environment/ecology-and-environmentalism/environmental-studies/species-group#speciesgroup |title=Species Group | Encyclopedia.com|website=www.encyclopedia.com|access-date=18 October 2019}}

{{cite journal|vauthors=Roca AL, Georgiadis N, Pecon-Slattery J, O'Brien SJ |title=Genetic evidence for two species of elephant in Africa |year=2001 |journal=Science |volume=293 |issue=5534|pages=1473–1477 |doi=10.1126/science.1059936 |pmid=11520983 |url=https://zenodo.org/record/1230804 |bibcode=2001Sci...293.1473R |s2cid=38285623}}

{{cite journal|vauthors=Steinfartz S, Veith M, Tautz D |title=Mitochondrial sequence analysis of Salamandra taxa suggests old splits of major lineages and postglacial recolonizations of Central Europe from distinct source populations of Salamandra salamandra|year=2000 |journal=Molecular Ecology |volume=9 |issue=4 |pages=397–410 |doi=10.1046/j.1365-294x.2000.00870.x |pmid=10736043|bibcode=2000MolEc...9..397S |s2cid=10952136}}

{{cite journal|vauthors=Tietze DT, Martens J, Sun YH |year=2006 |title=Molecular phylogeny of treecreepers (Certhia) detects hidden diversity |journal=Ibis |volume=148 |issue=3 |pages=477–488|doi=10.1111/j.1474-919X.2006.00547.x|doi-access=free}} {{open access}}

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{{Evolution}}

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Category:Evolutionary biology

Category:Species

Category:Speciation