In biology, a species complex, species group, or species aggregate is a group of species with very similar appearance, where the exact limits between species are often unclear or cryptic. Mostly, these species are closely related and form a clade.
The term has no defined meaning in taxonomy or in the nomenclature codes, but is often used in practice, for example to set apart a group of more closely related species within a genus, or to highlight that one suspects a species to actually conceal several species that one is currently unable to circumscribe precisely. The term species aggregate, abbreviated as "agg." after the binomial species name, is sometimes used for this latter purpose.
The demarcation difficulty may be due to recent and as yet incomplete reproductive isolation. Hybrid speciation may be a component of species complexes, when a reproductively isolated species arises from hybridization.
Species complexes exist in all groups of organisms, but are more known in conspicuous groups that have been studied more intensively by taxonomists, such as animals and plants. The use of DNA-based methods such as molecular phylogenetics or DNA barcoding has revealed "hidden", cryptic species even in taxa that were not thought of as species complex before.
Different definitions exist. Some authors use the term "species group" for species that share a geographic range and cannot interbreed. Others use the term for a group of related species with separate geographic ranges that might interbreed if opportunity arises. Others use the term for a group that has not yet been sufficiently studied, which might or might not have a monophyletic origin.
Superspecies and cryptic species complexes are examples of species complexes. A ring species is another situation where complex partial separation has developed between populations within a single species, but it is not considered to be a species complex.
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.
The use of the term 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 have enabled systematists to consolidate polytypic species into nominal species which in turn can be grouped into the larger array of the species group.
"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."
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.
Examples of known species complexes
- The fruit fly subgenus Sophophora contains the Drosophila melanogaster species group which itself contains 12 subgroups. The Drosophila obscura species group belongs to the same subgenus and contains 6 subgroups.
- In Vespula, a genus of wasps, only a few species have a scavenging habit (as opposed to a strictly predatory habit) and thus are considered major pests. The most abundant and bothersome of these are the three species belonging to the "Vespula vulgaris species group" which includes the "common wasp" or "yellowjacket" (Vespula vulgaris), the "German wasp" or "European wasp" (Vespula germanica), and the "western wasp" or "western yellowjacket" (Vespula pensylvanica).
- The Central American bark scorpions Centruroides limbatus and Centruroides bicolor belong to the "Gracilis species group". All of the species in this group are characterized by their long, narrow pedipalps and overall relatively large size.
- The arachnids to which the common name "Black widow spider" is given are in a species group that includes the "southern black widow" (Latrodectus mactans), the "northern black widow" (Latrodectus variolus), and the "western black widow" (Latrodectus hesperus').
- The Neotropical butterfly Morpho adonis is in a species group with Morpho eugenia and Morpho uraneis. Morpho marcus is also included in the group but might actually be the same species as Morpho adonis
- Members of the butterfly genus Charaxes are arranged in "species groups" (these are not superspecies, or clades but an informal phenetic arrangement). The groupings of morphologically similar species named as species groups in the literature may or may not be consistent with clades identified by molecular phylogenetic studies.
- The wolf-dog-coyote-dingo-jackal group, genus Canis
- The cobras, genus Naja
- Some species of the roundback slugs, genus Arion
- The jellyfish genus Cyanea, which contains anywhere from 1 to 14 species, depending on author.
- Brachygobius, a small genus of gobies which are popular as aquarium fish, are informally divided by taxonomists into two species groups. The dwarf "Brachygobius nunus species group" contains Brachygobius nunus, Brachygobius aggregatus, and Brachygobius mekongensis while the bigger "Brachygobius doriae species group" contains the bigger species of Brachygobius doriae, Brachygobius sabanus, and Brachygobius xanthomelas.
- The chameleon Brookesia minima has been characterized as belonging to a species group with other "Madagascan Dwarf Chameleons" such as Brookesia dentata, Brookesia tuberculata, and other new or unidentified species such as a recently described chameleon from Tsingy de Bemaraha Strict Nature Reserve.
- Peromyscus, a genus of deer mice, has been divided into subgenera Peromyscus and Haplomylomys and these subgenera are subdivided further into thirteen species groups.
- Recent cytogenetic studies have shown that the Middle East Blind Mole Rat (Spalax ehrenbergi) may actually be a species group containing several cryptic species that can be distinguished by chromosome numbers.
- The Ranunculus auricomus aggregate includes multiple diploid sexual and hybrid polyploid apomictic lineages.
- "Rubus fruticosus agg." has sometimes been used as a term of convenience for the entire European blackberry flora, although R. fruticosus itself is now considered to be a synonym of R. plicatus.
The term "species group" is also used in a different way so as to describe the manner in which individual organisms group together. In this non-taxonomic context one can refer to "same-species groups" and "mixed-species groups." While same-species groups are the norm, examples of mixed-species groups abound. For example, zebra (Equus burchelli) and wildebeest (Connochaetes taurinus) can remain in association during periods of long distance migration across the Serengeti as a strategy for thwarting predators. Cercopithecus mitis and Cercopithecus ascanius, species of monkey in the Kakamega Forest of Kenya, can stay in close proximity and travel along exactly the same routes through the forest for periods of up to 12 hours. These mixed-species groups are cannot be explained by the coincidence of sharing the same habitat. Rather, they are created by the active behavioural choice of at least one of the species in question.
- Circumscription (taxonomy)
- Cryptic species
- Parapatric speciation
- Ring species
- Sensu lato, meaning "in the broad sense"; often used with a species name
- Sibling species
- Species flock, a species group/complex living in sympatry
- Michael L. Arnold (1993). "Iris nelsonii (Iridaceae): Origin and Genetic Composition of a Homoploid Hybrid Species". American Journal of Botany 80 (5). pp. 577–583.
- Iowa State University Department of Agronomy
- Michael Allaby. "species group." A Dictionary of Zoology (Oxford University Press 1999)
- Tiemann-Boege I, Kilpatrick CW, Schmidly DJ, Bradley RD (2000). "Molecular systematics of the Peromyscus boylii species group (Rodentia: muridae) based on mitochondrial cytochrome b sequences". Molecular Phylogenetics and Evolution 16 (3): 366–378. doi:10.1006/mpev.2000.0806. PMID 10991790.
- Heywood, V.H. (1962). "The "species aggregate" in theory and practice". In V.H. Heywood; Á. Löve. Symposium on Biosystematics, organized by the International Organization of Biosystematists, Montreal, October 1962. pp. 26–36.
- (Danish) Engelke, S. (2006?): Til Snegleforeningen (Note to the Danish Slug-society). Article in Danish[dead link]
- Ranz JM, Maurin D, Chan YS, et al. (June 2007). "Principles of genome evolution in the Drosophila melanogaster species group". PLoS Biol. 5 (6): e152. doi:10.1371/journal.pbio.0050152. PMC 1885836. PMID 17550304.
- World- Wide Distribution of Pestiferous Social Wasps(Vespidae)
- Walter Reed Biosystematics Unit "Scorpion of the Day":Centruroides limbatus
- Kaston, B. J. (1970). "Comparative biology of American black widow spiders". Transactions of the San Diego Society of Natural History 16 (3): 33–82.
- Anderson Tully Worldwide
- Le Moult (E.) & Réal (P.), 1962-1963. Les Morpho d'Amérique du Sud et Centrale, Editions du cabinet entomologique E. Le Moult, Paris
- Schäfer F (2005). Brackish Water Fishes. Aqualog. pp. 49–51. ISBN 3-936027-82-X.
- AdCham.com: Brookesia minima by E. Pollak
- Sözen M, Matur F, Çolak E, Özkurt Ş, Karataş A (2006). "Some karyological records and a new chromosomal form for Spalax (Mammalia: Rodentia) in Turkey" (PDF). Folia Zool. 55 (3): 247–256.
- Horandl, E.; Greilhuber, J.; Klimova, K.; Paun, O.; Temsch, E.; Emadzade, K.; Hodalova, I. (2009). "Reticulate evolution and taxonomic concepts in the Ranunculus auricomus complex (Ranunculaceae): insights from analysis of morphological, karyological and molecular data". Taxon 58 (4): 1194–1215. PMC 2855680.
- Tosh CR, Jackson AL, Ruxton GD (March 2007). "Individuals from different-looking animal species may group together to confuse shared predators: simulations with artificial neural networks". Proc. Biol. Sci. 274 (1611): 827–32. doi:10.1098/rspb.2006.3760. PMC 2093981. PMID 17251090.