Species complex

The butterfly genus Heliconius contains some species extremely difficult to tell apart.

In biology, a species complex is a group of closely related species that are very similar in appearance to the point that the boundaries between them are often unclear. Terms sometimes used synonymously but with more precise meanings are: cryptic species for two or more species hidden under one species name, sibling species for two cryptic species that are each other's closest relative, and species flock for a group of closely related species living in the same habitat. As informal taxonomic ranks, species group, species aggregate, and superspecies are also in use.

Two or more taxa once considered conspecific (of the same species) may later be subdivided into infraspecific taxa (taxa within a species, such as bacterial strains or plant varieties), but this is not a species complex.

A species complex is in most cases a monophyletic group with a common ancestor, although there are exceptions. It may represent an early stage after speciation, but may also have been separated for a long time period without evolving morphological differences. Hybrid speciation can be a component in the evolution of a species complex.

Species complexes exist in all groups of organisms. They are identified by the rigorous study of differences between individual species, making use of minute morphological details, tests of reproductive isolation, or DNA-based methods such as molecular phylogenetics or DNA barcoding. The existence of extremely similar species may cause local and global species diversity to be underestimated. Recognizing similar but distinct species is important for disease and pest control, and in conservation biology, although drawing dividing lines between species can be inherently difficult.

Definition

Six light brown treefrogs, labelled A to E
At least six treefrog species make up the Hypsiboas calcaratusfasciatus species complex.[1]
Picture showing two mushrooms with red caps on a meadow
The fly agaric comprises several cryptic species, as shown by genetic data.[2]
An adult and a young elephant bathing
The African forest elephant (shown) is the bush elephant's sibling species.[3]
A flock of differently coloured fish in a rocky setting
Mbuna cichlids form a species flock in Lake 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 evolving metapopulation lineage" but acknowledges that the criteria to delimit species may depend on the group studied.[6] Thus, many species defined traditionally, based only on morphological similarity, have been found to comprise several distinct species when other criteria, such as genetic differentiation or reproductive isolation were applied.[7]

A more restricted use applies the term to close species between which hybridisation occurred or is occurring, leading to intermediate forms and blurred species boundaries.[8]  The informal classification, superspecies, can be exemplified by the grizzled skipper butterfly, a superspecies that is further divided into three subspecies.[9]

Some authors apply the term also to a species with intraspecific variability, which might be a sign of ongoing or incipient speciation. Examples are ring species[10][11] or species with subspecies, where it is often unclear if these should be considered separate species.[12]

Included concepts

Several terms are used synonymously for a species complex, but some of them may also have slightly different, or more narrow meanings. In the nomenclature codes of zoology and bacteriology, no taxonomic ranks are defined at the level between subgenera and species,[13][14] while the botanical code defines four ranks below genera (section, subsections, series and subseries).[15] Different informal taxonomic solutions have been used to indicate a species complex.

Cryptic species
Also physiologic race[16] (uncommon). This describes "distinct species that are erroneously classified (and hidden) under one species name".[17] More generally, the term is often applied when species, even if known to be distinct, cannot be reliably distinguished based on their morphology. The usage physiologic race is not to be confused with physiological race.
Sibling species
Also aphanic species. This term, introduced by Ernst Mayr in 1942,[18] was initially used with the same meaning as cryptic species,[7] but later authors emphasized the common phylogenetic origin.[19] A recent article defines sibling species as "cryptic sister species", meaning "two species that are the closest relative of each other and have not been distinguished from one another taxonomically".[17]
Species flock
Also species swarm. This refers to "a monophyletic group of closely related species all living in the same ecosystem".[17] Conversely, the term has also been applied very broadly to a group of closely related species than can be variable and widespread.[20]
Superspecies
Sometimes used as an informal rank for a species complex around one "representative" species.[21][22] Popularized by Bernhard Rensch and later Ernst Mayr, with the initial requirement that species forming a superspecies must have allopatric distributions.[23] For the component species of a superspecies, allospecies was proposed.[23]
Species aggregate
Used for a species complex, especially in plant taxa where polypoidy and apomixis are common. Historical synonyms are species collectiva, introduced by Adolf Engler, conspecies, and grex.[24] Components of a species aggregate have been called segregates or microspecies.[24] Used as abbreviation agg. after the binomial species name.[8][25]
Sensu lato
Meaning "in the broad sense", this Latin phrase is often used after a binomial species name, often abbreviated as s.l., to indicate a species complex represented by that species.[26][27][28]
Other Languages
भोजपुरी: प्रजातिगण
Deutsch: Sammelart
Esperanto: Specikomplekso
српски / srpski: Kompleksna vrsta
Türkçe: Tür kompleksi