In biology, a species complex is a group of closely related species very similar in appearance, such that 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 others' 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 in use.
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.
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". Conversely, the term has also been applied very broadly to a group of closely related species than can be variable and widespread.
In the nomenclature codes of botany, zoology, and bacteriology, no taxonomic ranks are defined at the level between subgenera and species, although additional ranks are allowed in the botanical code as long as confusion is avoided. Defining species groups is sometimes a convenient, informal way of subdividing well-defined genera with a large number of species.
The term superspecies has sometimes been used as an informal rank to refer to a species complex around one "representative" species. It was popularized by Bernhard Rensch and later Ernst Mayr, with the initial requirement that species forming a superspecies must have allopatric distributions. For the component species of a superspecies, allospecies was proposed.
Species aggregate has likewise been used, with the previous species collectiva introduced by Adolf Engler at the beginning of the 20th century, and alternative terms including conspecies and grex. This has been applied especially to plant taxa in which polyploidy and apomixis are prevalent, an example being Ranunculus auricomus agg. The components of a species aggregate have been called segregates or microspecies.
When a species is assumed to actually represent a species complex, this is often indicated by the abbreviation "agg." (for "aggregate") after the binomial species name. Another possibility is the use of sensu lato, abbreviated as "s.l."
Distinguishing close species within a complex requires the study of often very small differences. Morphological differences may be minute and only visible using adapted methods, such as microscopy. However, distinct species may sometimes have no morphological differences. In these cases, other characters, e.g. in the species' life history, behavior, physiology, or karyology can be explored. As an example, territorial songs are indicative of species in the treecreepers, a bird genus with little morphological differences. Mating tests are common in some groups such as fungi to confirm the reproductive isolation 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, for example molecular phylogenetics 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
Species forming a complex have typically diverged very recently from each other, allowing in some cases to retrace the process of speciation. Species with differentiated populations such as ring species are sometimes seen as an example of early, ongoing speciation, i.e. a species complex in formation. Nevertheless, similar but distinct species have sometimes been isolated for a long time without evolving differences, a phenomenon called "morphological stasis".
Stabilizing selection has been invoked as a force maintaining similarity in species complexes, especially when adaptation to special environments, such as a host in the case of symbionts, or extreme environments, constrains possible directions of evolution: In such cases, strongly divergent selection is not to be expected. Also, asexual reproduction, such as through apomixis 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 group), although closer examination can sometimes disprove this. As an example, the yellow-spotted "fire salamanders" in the genus Salamandra, formerly all classified as one species S. salamandra, are not monophyletic: the Corsican fire salamander's closest relative was shown to be the entirely black Alpine salamander. In such cases, similarity has arisen from convergent evolution.
Hybrid speciation can lead to unclear species boundaries through a process of reticulate evolution, where species have two parent species as their most recent common ancestors. In such cases, the hybrid species may have intermediate characters, as demonstrated e.g. 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.
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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