In biology, a species complex is a group of two or more species very similar in appearance, where exact boundaries between them are often unclear. More narrow terms sometimes used synonymously are cryptic species, sibling species, or species flock. As informal taxonomic ranks, species group, species aggregate, and superspecies are in use.
The circumscription of a complex and its species depends on the species concept applied. In most cases, a species complex is a monophyletic group, although similar species are sometimes found to be less closely related than estimated. A species complex may represent an early stage during or shortly after speciation, but may also have been separated for a long period without evolving morphological differences. Hybrid speciation can be a component in the evolution of a species complex, leading to a pattern of reticulate evolution.
Species complexes exist in all groups of organisms. They are identified by the rigorous study of differences between 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 lead to underestimation of local and global species diversity. Recognizing similar but distinct species is important for disease and pest control, and in conservation biology.
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, and thus to the species concept one adheres to. Following a morphological species concept, the members of a species complex although being very similar overall, will show some slight differences in appearance, while the biological species concept requires reproductive isolation to accept species as distinct.
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 or species with subspecies, where it is often unclear if these should be considered separate species.
The following terms are sometimes used as synonyms for a species complex, but apply in general to more narrow concepts:
- Cryptic species: This describes "distinct species that are erroneously classified (and hidden) under one species name". More generally, the term is often applied when species, even if known to be distinct, cannot be reliably distinguished based on their morphology.
- Sibling species: Also "aphanic species", this term was initially used with the same meaning as cryptic species, but later authors emphasized the common phylogenetic origin. 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".
- Species flock: Also "species swarm", this refers to "a monophyletic group of closely related species all living in the same ecosystem", an example being the Mbuna cichlids of Lake Malawi. Conversely, the term has also been applied very broadly to a group of closely related species than can be variable and widespread.
In the botanical, zoological, and microbiological nomenclature codes, no specifications exist for ranks at the level between species and subgenera. The use of the terms "species complex", "species group", or "species aggregate" reduces the need to use a higher taxonomic category in cases with taxa that exhibit sufficient differentiation to be recognized as separate species but possess inadequate variation to be recognized as subgenera. Defining species groups is a convenient way of subdividing well-defined genera with a large number of recognized species. The use of species groups has enabled systematists to consolidate polytypic species into nominal species which in turn can be grouped into the larger array of the species group.
The term superspecies is sometimes used as an informal taxonomic rank to refer to a species complex around one "representative" species. The rank was popularized by Bernhard Rensch and later by Ernst Mayr. These authors specified that the species involved must have allopatric distributions. The superspecies rank is neither a primary nor secondary rank under the International Code of Nomenclature for algae, fungi, and plants, but is permitted under article 4.3. The component species of a superspecies are called semispecies.
An earlier term was species collectiva (plural: species collectivae), popularized by Adolf Engler in his book series Das Pflanzenreich: regni vegetabilis conspectus at the beginning of the 20th century. Other related terms include conspecies (=aggregate). Grex was at one time used similarly, but a different meaning has taken hold.
The components of a species aggregate have been called segregates or microspecies, contrasting with the semi-species components of a superspecies.
"It will be quite evident that the aggregate is no more easy to define than the species itself [see species problem], yet like the latter it is a phenomenon which most taxonomists can and do recognize. ... Often it is a confession of ignorance (and sometimes still is), in the sense that some of the binomials involved in the aggregate have been satisfactorily accounted for later as taxa of other rank ... Often, however, the species collectiva was a straightforward expression of taxonomic opinion that here was a group of small-scale, very closely related species which had more in common with one another than with other species or with other similar groups of species, yet they were felt to be on too small a scale to warrant separate recognition."
A species aggregate is often indicated by the abbreviation "agg." after the binomical species name. Another possibility is the use of "s.l." for sensu lato.
Usually, when a cryptic species complex has been discovered, the individual species within the complex are separated by analysing data from multiple sources, such as by comparing polytene chromosomes, DNA sequence analyses, bioacoustics and thorough life history studies.
Evolution and ecology
Species forming a complex have typically diverged very recently from each other, allowing in some cases to retrace the process of speciation. They typically from a monophyletic group, although closer examination can sometimes disprove this, especially when similarity of species has resulted from parallel evolution.
In regards to whether or not members of a species group share a range, sources differ. A source from Iowa State University Department of Agronomy says that members of a species group usually have partially overlapping ranges but do not interbreed with each other. A Dictionary of Zoology (Oxford University Press 1999) describes a species group as complex of related species that exist allopatrically and explains that this "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 only become evident when a new species is introduced into the system, breaking 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, traditionally considered clearly separate species that did not interbreed, shows they may be actually just subspecies of the same species.
Where closely related species coexist in sympatry, it is often a particular challenge to understand how these similar species persist without outcompeting each other. Niche partitioning is one mechanism invoked to explain this. Studies in some species complexes indeed suggest that species divergence went in par with ecological differentiation, with species now preferring different microhabitats.
A species flock may arise when a species penetrates a new geographical area and diversifies to occupy a variety of ecological niches; this process is 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. Although this suggestion came before the detailed analysis of many systems using DNA sequence data, it has been proven 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 that had been diverging for many million years.
Evidence from the identification of cryptic species has led some[who?] to conclude that current estimates of global species richness are too low. For example, mitochondrial DNA research published in January 2008 suggests that there are at least 11 genetically distinct populations of giraffes. Similar methods also found that the Amazonian frog Eleutherodactylus ockendeni is actually at least 3 different species that diverged over 5 million years ago.
Disease and pathogen control
Pests, species causing diseases, and their vectors, have direct importance for humans. When they are found to be cryptic species complexes, the ecology and virulence of each of these species needs to be reevaluated to devise appopriate control strategies. An example are cryptic species in the malaria vector Anopheles, or the fungi causing cryptococcosis.
When a species is found to comprise in fact several phylogenetically distinct species, each of these typically have smaller distribution ranges and population sizes than reckoned before. These different species can also differ in their ecology, e.g. having different breeding strategies or habitat requirements, which has to be taken into account for appropriate management.
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