The butterfly genusHeliconius contains some species that are extremely difficult to tell apart.
In biology, aspecies complex is a group of closely relatedorganisms that are so similar in appearance and other features that the boundaries between them are often unclear. Thetaxa in 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 species for two or more species hidden under one species name,sibling species for two (or more) species that are each other's closest relative, andspecies flock for a group of closely related species that live in the same habitat. As informaltaxonomic ranks,species group,species aggregate,macrospecies, andsuperspecies are also in use.
Two or more taxa that were once consideredconspecific (of the same species) may later be subdivided intoinfraspecific taxa (taxa within a species, such as plantvarieties), which may be a complex ranking but it is not a species complex. In most cases, a species complex is amonophyletic group of species with a common ancestor, but there are exceptions. It may represent an early stage afterspeciation in which the species were separated for a long time period without evolvingmorphological differences.Hybrid speciation can be a component in the evolution of a species complex.
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 species may 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 whichhybridisation has occurred or is occurring, which leads to intermediate forms and blurred species boundaries.[8] The informal classification, superspecies, can be exemplified by thegrizzled skipper butterfly, which is a superspecies that is further divided into three subspecies.[9]
The rubyspot damselflyHetaerina americana 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 thenomenclature codes of zoology and bacteriology, notaxonomic ranks are defined at the level betweensubgenus and 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, 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.[2] Rather, these lineages can be distinguished by use of DNA barcoding and meta-barcoding which sequence a particular region of the genome.[16]
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.[16]
Species flock
A species flock--also known asspecies 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.[17][18] 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 amixed-species foraging flock, a behavior in which birds of different species feed together.
Species aggregate
Used for a species complex, especially in plant taxa wherepolyploidy andapomixis are common. Historical synonyms arespecies collectiva [la], introduced byAdolf Engler,conspecies, andgrex. Components of a species aggregate have been calledsegregates ormicrospecies. Used as abbreviationagg. after thebinomial species name.
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.[19][20] The term is most used in plant biology, and is a synonym for the more utilized species flock.
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.
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.[21] In those cases, other characters, such as in the species'life history,behavior,physiology, andkaryology, may be explored. For example,territorial songs are indicative of species in thetreecreepers, a bird genus with few morphological differences.[22] Mating tests are common in some groups such as fungi to confirm thereproductive isolation of two species.[23]
A species complex typically forms amonophyletic group that 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".[21] For example, the Amazonian frogPristimantis ockendeni is actually at least three different species that diverged over 5 million years ago.[27]
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).[21] This may constrain possible directions of evolution; in such cases, strongly divergent selection is not to be expected.[21] Also, asexual reproduction, such as throughapomixis in 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 (amonophyletic group), 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.[26] In such cases, similarity has arisen fromconvergent evolution.
Hybrid speciation can 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 inHeliconius butterflies.[28] 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.
Sources differ on whether or not members of a species group share arange. A source fromIowa State University Department ofAgronomy states that members of a species group usually have partially overlapping ranges but do notinterbreed with one another.[29]A Dictionary of Zoology (Oxford University Press 1999) describes a species group as complex of related species that existallopatrically and explains that the "grouping can often be supported by experimental crosses in which only certain pairs of species will producehybrids."[30] 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 slug inNorthern Europe, where interbreeding with the localblack slug andred slug, which were traditionally considered clearly separate species that did not interbreed, shows that they may be actually just subspecies of the same species.[31][32]
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 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.[33] Similar methods also found that the Amazonian frogEleutherodactylus ockendeni is actually at least three different species that diverged over 5 million years ago.[27]
It has been suggested that cryptic species complexes are very common in the marine environment.[34] That suggestion came before the detailed analysis of many systems using DNA sequence data but has been proven to be correct.[35] The increased use of DNA sequence in the investigation of organismal diversity (also calledphylogeography andDNA barcoding) has led to the discovery of a great many cryptic species complexes in all habitats. In the marine bryozoanCelleporella hyalina,[36] 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.
TheAnopheles gambiae mosquito complex contains both species that are a vector formalaria and species that are not.[37]
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 themalaria vector 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. tryoni inflicts widespread, devastating damage to Australian fruit crops, but the sister species do not.[38]
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.[39]
^Brown JK, Frohlich DR, Rosell RC (1995). "The sweetpotato or silverleaf whiteflies: biotypes ofBemisia tabaci or a species complex?".Annual Review of Entomology.40 (1):511–534.doi:10.1146/annurev.en.40.010195.002455.
^JONG, R. (December 1987). "Superspecies Pyrgus malvae (Lepidoptera: Hesperiidae) in the East Mediterranean, with notes on phylogenetic and biological relationships".Rijksmuseum van Natuurlijke Historie – Via Naturalis Repository.
^Moritz C, Schneider CJ, Wake DB (1992). "Evolutionary relationships within theEnsatina eschscholtzii complex confirm the ring species interpretation".Systematic Biology.41 (3):273–291.doi:10.1093/sysbio/41.3.273.S2CID49267121.
^Dai Y-C, Vainio EJ, Hantula J, Niemelä, Korhonen K (2003). "Investigations onHeterobasidion annosum s.lat. in central and eastern Asia with the aid of mating tests and DNA fingerprinting".Forest Pathology.33 (5):269–286.doi:10.1046/j.1439-0329.2003.00328.x.
^Kotov, Alexey A.; Garibian, Petr G.; Bekker, Eugeniya I.; Taylor, Derek J.; Karabanov, Dmitry P. (2020-06-17). "A new species group from theDaphnia curvirostris species complex (Cladocera: Anomopoda) from the eastern Palaearctic: taxonomy, phylogeny and phylogeography".Zoological Journal of the Linnean Society.191 (3):772–822.doi:10.1093/zoolinnean/zlaa046.ISSN0024-4082.
^abSteinfartz S, Veith M, Tautz D (2000). "Mitochondrial sequence analysis ofSalamandra taxa suggests old splits of major lineages and postglacial recolonizations of Central Europe from distinct source populations ofSalamandra salamandra".Molecular Ecology.9 (4):397–410.Bibcode:2000MolEc...9..397S.doi:10.1046/j.1365-294x.2000.00870.x.PMID10736043.S2CID10952136.
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