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In biology, a species complex is a group of closely related
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...
s that are so similar in appearance and other features that the boundaries between them are often unclear. The
taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
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 are cryptic 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, and species flock for a group of closely related species that live in the same habitat. As informal
taxonomic ranks In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in an ancestral or hereditary hierarchy. A common system consists of species, genus, family, order, class, phylum, kingdom, domain. While ...
, species group, species aggregate, macrospecies, and superspecies are also in use. Two or more taxa that were once considered conspecific (of the same species) may later be subdivided into
infraspecific taxa In botany, an infraspecific name is the scientific name for any taxon below the rank of species, i.e. an infraspecific taxon or infraspecies. (A "taxon", plural "taxa", is a group of organisms to be given a particular name.) The scientific names ...
(taxa within a species, such as bacterial strains or plant varieties), that is complex but it is not a species complex. A species complex is in most cases a monophyletic group with a common ancestor, but there are exceptions. It may represent an early stage after
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...
but may also have been separated for a long time period without evolving morphological differences.
Hybrid speciation Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was thou ...
can be a component in the evolution of a species complex. Species complexes exist in all groups of organisms and are identified by the rigorous study of differences between individual species that uses minute morphological details, tests of
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offsprin ...
, or DNA-based methods, such as
molecular phylogenetics Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
and DNA barcoding. The existence of extremely similar species may cause local and global species diversity to be underestimated. The recognition of similar-but-distinct species is important for
disease A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
and
pest control Pest control is the regulation or management of a species defined as a pest; any animal, plant or fungus that impacts adversely on human activities or environment. The human response depends on the importance of the damage done and will range ...
and in
conservation biology Conservation biology is the study of the conservation of nature and of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. It is an in ...
although the drawing of dividing lines between species can be inherently difficult.


Definition

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 A metapopulation consists of a group of spatially separated populations of the same species which interact at some level. The term metapopulation was coined by Richard Levins in 1969 to describe a model of population dynamics of insect pests in ...
lineage Lineage may refer to: Science * Lineage (anthropology), a group that can demonstrate its common descent from an apical ancestor or a direct line of descent from an ancestor * Lineage (evolution), a temporal sequence of individuals, populat ...
" but acknowledges that the criteria to delimit species may depend on the group studied. Thus, many traditionally defined species defined, based only on morphological similarity, have been found to be several distinct species when other criteria, such as genetic differentiation or
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offsprin ...
, are applied. A more restricted use applies the term to close species between which hybridisation occurred or is occurring, which leads to intermediate forms and blurred species boundaries. The informal classification, superspecies, can be exemplified by the
grizzled skipper ''Pyrgus'' is a genus in the skippers butterfly family, Hesperiidae, known as the grizzled skippers. The name "checkered" or "chequered skipper" may also be applied to some species, but also refers to species in the genera '' Burnsius'' and '' ...
butterfly, which is a superspecies that is further divided into three subspecies. Some authors apply the term to a species with intraspecific variability, which might be a sign of ongoing or incipient
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...
. Examples are ring species or species with
subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all specie ...
, in which it is often unclear if they should be considered separate species.


Related concepts

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 rank In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in an ancestral or hereditary hierarchy. A common system consists of species, genus, family, order, class, phylum, kingdom, domain. While ...
s are defined at the level between
subgenera In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between ...
and species, but the botanical code defines four ranks below genera (section, subsections, series and subseries). Different informal taxonomic solutions have been used to indicate a species complex.


Identification

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 Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...
. However, distinct species sometimes have no morphological differences. In those cases, other characters, such as in the species' life history,
behavior Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as we ...
,
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemic ...
, and karyology, may be explored. For example, territorial songs are indicative of species in the
treecreepers The treecreepers are a family, Certhiidae, of small passerine birds, widespread in wooded regions of the Northern Hemisphere and sub-Saharan Africa. The family contains eleven species in two genera, ''Certhia'' and ''Salpornis''. Their plumage i ...
, a bird genus with few morphological differences. Mating tests are common in some groups such as fungi to confirm the
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, behaviors and physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensure that any offsprin ...
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 Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
or DNA barcoding. Such methods have greatly contributed to the discovery of cryptic species, including such emblematic species as the fly agaric or the African elephants.


Evolution and ecology


Speciation process

Species forming a complex have typically diverged very recently from each other, which sometimes allows the retracing of the process of
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution withi ...
. 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 botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttin ...
in plants, may separate lineages without producing a great degree of morphological differentiation. A species complex is usually a group that has one common ancestor (a
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gr ...
group), but closer examination can sometimes disprove that. For example, yellow-spotted "fire salamanders" in the genus ''
Salamandra ''Salamandra'' is a genus of seven species of salamanders localized in central and southern Europe, Northern Africa, and western Asia. List of species References External links Salamandraat Fauna Europaea * Salamandraat Animal Diversity ...
'', 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 The alpine salamander (''Salamandra atra'') is a black salamander that can be found in the French Alps, and through the mountainous range in Europe. It is a member of the genus '' salamandra''. Their species name, ''atra'', may be derived from the ...
. In such cases, similarity has arisen from
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
.
Hybrid speciation Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. Previously, reproductive isolation between two species and their parents was thou ...
can lead to unclear species boundaries through a process of reticulate evolution, in which species have two parent species as their
most recent common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
s. 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 Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
, and hybrid plant species are called nothospecies.


Range and habitats

Sources differ on whether or not members of a species group share a range. A source from
Iowa State University Iowa State University of Science and Technology (Iowa State University, Iowa State, or ISU) is a public land-grant research university in Ames, Iowa. Founded in 1858 as the Iowa Agricultural College and Model Farm, Iowa State became one of th ...
Department of
Agronomy Agronomy is the science and technology of producing and using plants by agriculture for food, fuel, fiber, chemicals, recreation, or land conservation. Agronomy has come to include research of plant genetics, plant physiology, meteorology, and ...
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 Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print book ...
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 The Spanish slug (''Arion vulgaris'', but also widely known as ''Arion lusitanicus'' owing to a misidentification) is an air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs. Other vernac ...
in
Northern Europe The northern region of Europe has several definitions. A restrictive definition may describe Northern Europe as being roughly north of the southern coast of the Baltic Sea, which is about 54°N, or may be based on other geographical factors ...
, where interbreeding with the local
black slug The black slug (also known as black arion, European black slug, or large black slug) ''Arion ater'' L. is a large terrestrial gastropod mollusk in the family Arionidae, the round back slugs. Land slugs lack shells like other terrestrial mollusks ...
and
red slug The red slug (''Arion rufus''), also known as the large red slug, Where closely related species co-exist in
sympatry In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species s ...
, 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. 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 niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...
s, a process known as
adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic in ...
. The first species flock to be recognized as such was the 13 species of
Darwin's finches Darwin's finches (also known as the Galápagos finches) are a group of about 18 species of passerine birds. They are well known for their remarkable diversity in beak form and function. They are often classified as the subfamily Geospizinae or ...
on the
Galápagos Islands The Galápagos Islands ( Spanish: , , ) are an archipelago of volcanic islands. They are distributed on each side of the equator in the Pacific Ocean, surrounding the centre of the Western Hemisphere, and are part of the Republic of Ecuad ...
described by
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
.


Practical implications


Biodiversity estimates

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


Disease and pathogen control

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. Examples are cryptic species in the
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. ...
vector genus of mosquito, '' Anopheles'', the fungi causing
cryptococcosis Cryptococcosis is a potentially fatal fungal infection of mainly the lungs, presenting as a pneumonia, and brain, where it appears as a meningitis. Cough, difficulty breathing, chest pain and fever are seen when the lungs are infected. When the ...
, and sister species of ''
Bactrocera tryoni The Queensland fruit fly (''Bactrocera tryoni'') is a species of fly in the family Tephritidae in the insect order Diptera. ''B. tryoni'' is native to subtropical coastal Queensland and northern New South Wales. They are active during the day ...
,'' 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.


Conservation biology

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 The giraffe is a large African hoofed mammal belonging to the genus ''Giraffa''. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes were thought to be one species, '' Giraffa camelopardal ...
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.


See also

*
Glossary of scientific naming This is a list of terms and symbols used in scientific names for organisms, and in describing the names. For proper parts of the names themselves, see List of Latin and Greek words commonly used in systematic names. Note that many of the abbrevi ...
*
Alliance (taxonomy) An alliance is an informal grouping used in biological taxonomy. The term "alliance" is not a taxonomic rank defined in any of the nomenclature codes. It is used for any group of species, genera or tribes to which authors wish to refer, that have ...
* Association (ecology) *
Subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all specie ...
* Ring species


References

{{Authority control Evolutionary biology Species Speciation