Species complex

In biology, a species complex is a group of closely related organisms 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. In biology, a species complex is a group of closely related organisms 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. 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 lineage' but acknowledges that the criteria to delimit species may depend on the group studied. 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. 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.  The informal classification, superspecies, can be exemplified by the grizzled skipper butterfly, a superspecies that is further divided into three subspecies. 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. 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 ranks are defined at the level between subgenera and species, while 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. 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.