Wikipedia

Species complex

(Redirected fromCryptic speciation)
Several terms redirect here. For species-group names, seeInternational Code of Zoological Nomenclature.For individuals of different species grouping together, seeMutualism (biology)#Service-service relationships.For the principles of hiding, seeCrypsis.For supposed creatures, seeList of cryptids.For another use of physiologic race, seeRace (biology)#Physiological race.

In biology, aspecies complexis a group of closely relatedorganismsthat are so similar in appearance and other features that the boundaries between them are often unclear. Thetaxain the complex may be able to hybridize readily with each other, further blurring any distinctions. Terms that are sometimes used synonymously but have more precise meanings arecryptic speciesfor two or more species hidden under one species name,sibling speciesfor two (or more) species that are each other's closest relative, andspecies flockfor a group of closely related species that live in the same habitat. As informaltaxonomic ranks,species group,species aggregate,macrospecies,andsuperspeciesare also in use.

The butterfly genusHeliconiuscontains some species that are extremely difficult to tell apart.

Two or more taxa that were once consideredconspecific(of the same species) may later be subdivided intoinfraspecific taxa(taxa within a species, such as bacterialstrainsor plantvarieties), which may be a complex ranking but it is not a species complex. In most cases, a species complex is amonophyletic groupof species with a common ancestor, but there are exceptions. It may represent an early stage afterspeciationin which the species were separated for a long time period without evolvingmorphologicaldifferences.Hybrid speciationcan be a component in the evolution of a species complex.

Species complexes are ubiquitous and are identified by the rigorous study of differences between individual species that uses minute morphological details, tests ofreproductive isolation,orDNA-based methods, such asmolecular phylogeneticsandDNA barcoding.The existence of extremely similar species may cause local and globalspecies diversityto be underestimated. The recognition of similar-but-distinct species is important fordiseaseandpest controland inconservation biologyalthough the drawing of dividing lines between speciescan be inherently difficult.

Definition

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At least six treefrog species make up theHypsiboas calcaratus–fasciatusspecies complex.[1]
Thefly agariccomprises severalcryptic species,as is shown by genetic data.[2]
Theforest elephant(shown) is thebush elephant'ssibling species.[3]
Mbunacichlids form aspecies flockinLake Malawi.[4]

A species complex is typically considered as a group of close, but distinct species.[5]Obviously, the concept is closely tied to the definition of a species. Modern biology understands a species as "separately evolvingmetapopulationlineage"but acknowledges that thecriteria to delimit speciesmay depend on the group studied.[6]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 orreproductive isolation,are applied.[7]

A more restricted use applies the term to a group of species among whichhybridisationhas occurred or is occurring, which leads to intermediate forms and blurred species boundaries.[8]The informal classification, superspecies, can be exemplified by thegrizzled skipperbutterfly, which is a superspecies that is further divided into three subspecies.[9]

Some authors apply the term to a species withintraspecific variability,which might be a sign of ongoing or incipientspeciation.Examples arering species[10][11]or species withsubspecies,in which it is often unclear if they should be considered separate species.[12]

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The rubyspot damselflyHetaerina americanais 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 thenomenclature codesof zoology and bacteriology, notaxonomic ranksare defined at the level betweensubgenusand species,[13][14]but the botanical code defines four ranks below subgenus (section, subsection, series, and subseries).[15]Different informal taxonomic solutions have been used to indicate a species complex.

Cryptic species
Cryptic species are morphologically identical species. More generally, the term is often applied when species, even if they are known to be distinct, cannot be reliably distinguished by morphology.[16]
Sibling species
Also calledaphanic species.This term, introduced byErnst Mayrin 1942,[17]was initially used with the same meaning ascryptic species,[7]but later authors emphasized the common phylogenetic origin.[18]A recent article defines sibling species as "crypticsister species","two species that are the closest relative of each other and have not been distinguished from one another taxonomically ".[19]
Species flock
Also calledspecies swarm.This refers to "a monophyletic group of closely related species all living in the same ecosystem".[19]Conversely, the term has also been applied very broadly to a group of closely related species than can be variable and widespread.[20]Not to be confused with amixed-species foraging flock,a behavior in which birds of different species feed together.
Superspecies
Sometimes used as an informal rank for a species complex around one "representative" species.[21][22]Popularized byBernhard Renschand laterErnst Mayr,with the initial requirement that species forming a superspecies must haveallopatricdistributions.[23]For the component species of a superspecies,allospecieswas proposed.[23]
Species aggregate
Used for a species complex, especially in plant taxa wherepolyploidyandapomixisare common. Historical synonyms arespecies collectiva[la],introduced byAdolf Engler,conspecies,andgrex.[24]Components of a species aggregate have been calledsegregatesormicrospecies.[24]Used as abbreviationagg.after thebinomial species name.[8][25]
Sensu lato
A Latin phrase meaning 'in the broad sense', it is often used after abinomial species name,often abbreviated ass.l.,to indicate a species complex represented by that species.[26][27][28]

Identification

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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 asmicroscopy.However, distinct species sometimes have no morphological differences.[19]In those cases, other characters, such as in the species'life history,behavior,physiology,andkaryology,may be explored. For example,territorial songsare indicative of species in thetreecreepers,a bird genus with few morphological differences.[29]Mating tests are common in some groups such as fungi to confirm thereproductive isolationof two species.[27]

Analysis ofDNA sequencesis becoming increasingly standard for species recognition and may, in many cases, be the only useful method.[19]Different methods are used to analyse such genetic data, such asmolecular phylogeneticsorDNA barcoding.Such methods have greatly contributed to the discovery of cryptic species,[19][30]including such emblematic species as thefly agaric,[2]thewater fleas,[31]or theAfrican elephants.[3]

Salamandra corsica
Salamandra atra
Salamandra salamandra
Similarity can be misleading: theCorsican fire salamander(left) was previously considered asubspeciesof thefire salamander(right) but is in fact more closely related to the uniformly blackAlpine salamander(center).[32]

Evolution and ecology

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Speciation process

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A species complex typically forms amonophyletic groupthat has diversified rather recently, as is shown by the short branches between the species A–E (blue box) in thisphylogenetic tree.

Species forming a complex have typically diverged very recently from each other, which sometimes allows the retracing of the process ofspeciation.Species with differentiated populations, such asring 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".[19]For example, the Amazonian frogPristimantis ockendeniis actually at least three different species that diverged over 5 million years ago.[33]

Stabilizing selectionhas 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).[19]This may constrain possible directions of evolution; in such cases, strongly divergent selection is not to be expected.[19]Also, asexual reproduction, such as throughapomixisin plants, may separate lineages without producing a great degree of morphological differentiation.

Possible processes explaining similarity of species in a species complex:
a– morphological stasis
b–hybrid speciation

A species complex is usually a group that has one common ancestor (amonophyleticgroup), but closer examination can sometimes disprove that. For example, yellow-spotted "fire salamanders" in the genusSalamandra,formerly all classified as one speciesS. salamandra,are not monophyletic: theCorsican fire salamander's closest relative has been shown to be the entirely blackAlpine salamander.[32]In such cases, similarity has arisen fromconvergent evolution.

Hybrid speciationcan lead to unclear species boundaries through a process ofreticulate evolution,in which species have two parent species as theirmost recent common ancestors.In such cases, the hybrid species may have intermediate characters, such as inHeliconiusbutterflies.[34]Hybrid speciation has been observed in various species complexes, such as insects, fungi, and plants. In plants, hybridization often takes place throughpolyploidization,and hybrid plant species are callednothospecies.

Range and habitats

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Sources differ on whether or not members of a species group share arange.A source fromIowa State UniversityDepartment ofAgronomystates that members of a species group usually have partially overlapping ranges but do notinterbreedwith one another.[35]A Dictionary of Zoology(Oxford University Press1999) describes a species group as complex of related species that existallopatricallyand explains that the "grouping can often be supported by experimental crosses in which only certain pairs of species will producehybrids."[36]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 theSpanish sluginNorthern Europe,where interbreeding with the localblack slugandred slug,which were traditionally considered clearly separate species that did not interbreed, shows that they may be actually just subspecies of the same species.[37]

Where closely related species co-exist insympatry,it is often a particular challenge to understand how the similar species persist without outcompeting each other.Niche partitioningis 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.[citation needed] Similar methods also found that the Amazonian frogEleutherodactylus ockendeniis actually at least three different species that diverged over 5 million years ago.[33]

Aspecies flockmay arise when a species penetrates a new geographical area and diversifies to occupy a variety ofecological niches,a process known asadaptive radiation.The first species flock to be recognized as such was the 13 species ofDarwin's fincheson theGalápagos Islandsdescribed byCharles Darwin.

Practical implications

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Biodiversity estimates

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It has been suggested that cryptic species complexes are very common in the marine environment.[38]That suggestion came before the detailed analysis of many systems using DNA sequence data but has been proven to be correct.[39]The increased use of DNA sequence in the investigation of organismal diversity (also calledphylogeographyandDNA barcoding) has led to the discovery of a great many cryptic species complexes in all habitats. In the marine bryozoanCelleporella hyalina,[40]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.

Evidence from the identification of cryptic species has led some[who?]to conclude that current estimates of global species richness are too low.

Disease and pathogen control

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TheAnopheles gambiaemosquito complex contains both species that are a vector formalariaand species that are not.[41]

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.[citation needed]Examples are cryptic species in themalariavector genus of mosquito,Anopheles,the fungi causingcryptococcosis,and sister species ofBactrocera tryoni,or the Queensland fruit fly. That pest is indistinguishable from two sister species except thatB. tryoniinflicts widespread, devastating damage to Australian fruit crops, but the sister species do not.[42]

Conservation biology

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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.[citation needed]For example,giraffepopulations 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.[43]

See also

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References

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