Species complex in the context of Biological species

A species (pl. species) is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. It can be defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour, or ecological niche. In addition, palaeontologists use the concept of the chronospecies since fossil reproduction cannot be examined. The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. About 14% of these had been described by 2011. All species (except viruses) are given a two-part name, a "binomen". The first part of a binomen is the name of a genus to which the species belongs. The second part is called the specific name or the specific epithet (in botanical nomenclature, also sometimes in zoological nomenclature). For example, Boa constrictor is one of the species of the genus Boa, with constrictor being the specific name.

While the definitions given above may seem adequate at first glance, when looked at more closely they represent problematic species concepts. For example, the boundaries between closely related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, and in a ring species. Also, among organisms that reproduce only asexually, the concept of a reproductive species breaks down, and each clonal lineage is potentially a microspecies. Although none of these are entirely satisfactory definitions, and while the concept of species may not be a perfect model of life, it is still a useful tool to scientists and conservationists for studying life on Earth, regardless of the theoretical difficulties. If species were fixed and distinct from one another, there would be no problem, but evolutionary processes cause species to change. This obliges taxonomists to decide, for example, when enough change has occurred to declare that a fossil lineage should be divided into multiple chronospecies, or when populations have diverged to have enough distinct character states to be described as cladistic species.

The king cobra (Ophiophagus hannah) is a species complex of snakes endemic to Asia. With an average length of 3.18 to 4 m (10.4 to 13.1 ft) and a record length of 5.85 m (19.2 ft), it is the world's longest venomous snake and among the heaviest. Under the genus Ophiophagus, it is not phylogenetically a true cobra despite its common name and some resemblance. Spanning from the Indian subcontinent through Southeastern Asia to Southern China, the king cobra is widely distributed albeit not commonly seen.

Individuals have diversified colouration across its habitats, from black with white strips to unbroken brownish grey, although after taxonomic re-evaluation, it is no longer the sole member of its genus but is now a species complex; these differences in pattern and other aspects may cause the genus to be split into at least four species, spread across its large geographic range.

The common toad, European toad, or in Anglophone parts of Europe, simply the toad (Bufo bufo, from Latin bufo "toad"), is a toad found throughout most of Europe (with the exception of Ireland, Iceland, parts of Scandinavia, and some Mediterranean islands), in the western part of North Asia, and in a small portion of Northwest Africa. It is one of a group of closely related animals that are descended from a common ancestral line of toads and which form a species complex. The toad is an inconspicuous animal as it usually lies hidden during the day. It becomes active at dusk and spends the night hunting for the invertebrates on which it feeds. It moves with a slow, ungainly walk or short jumps, and has greyish-brown skin covered with wart-like lumps.

Although toads are usually solitary animals, in the breeding season, large numbers of toads converge on certain breeding ponds, where the males compete to mate with the females. Eggs are laid in gelatinous strings in the water and later hatch out into tadpoles. After several months of growth and development, these sprout limbs and undergo metamorphosis into tiny toads. The juveniles emerge from the water and remain largely terrestrial for the rest of their lives.

There are over 80 genera of flowering plants estimated to contain at least 500 described species. The largest of these is currently the legume genus Astragalus (milk-vetches), with over 3,000 species.

The sizes of plant genera vary widely from those containing a single species to genera containing thousands of species, and this disparity became clear early in the history of plant classification. The largest genus in Carl Linnaeus' seminal Species Plantarum was Euphorbia, with 56 species; Linnaeus believed that no genus should contain more than 100 species.

Iguana (/ɪˈɡwɑːnə/, Spanish: [iˈɣwana]) is a genus of herbivorous lizards that are native to tropical areas of Mexico, Central America, South America, and the Caribbean. The genus was first described by Austrian naturalist J.N. Laurenti in 1768. Two species are placed in the genus: The green iguana, which is widespread throughout its range and a popular pet; and the Lesser Antillean iguana, which is native to the Lesser Antilles. Genetic analysis indicates that the green iguana may comprise a complex of multiple species, some of which have been recently described, but the Reptile Database considers all of these as subspecies of the green iguana.

The word "iguana" is derived from the original Taino name for the species, iwana.In addition to the two species in the genus Iguana, several other related genera in the same family have common names of the species including the word "iguana".

The bald eagle (Haliaeetus leucocephalus) is a bird of prey found in North America. A sea eagle, it has two known subspecies and forms a species pair with the white-tailed eagle (Haliaeetus albicilla), which occupies the same niche as the bald eagle in the Palearctic. Its range includes most of Canada and Alaska, all of the contiguous United States, and northern Mexico. It is found near large bodies of open water with an abundant food supply and old-growth trees for nesting.

The bald eagle is an opportunistic feeder that subsists mainly on fish, upon which it swoops down and snatches from the water with its talons. It builds the largest nest of any North American bird and the largest tree nests ever recorded for any animal species, up to 4 m (13 ft) deep, 2.5 m (8.2 ft) wide, and 1 metric ton (1.1 short tons) in weight. Sexual maturity is attained at the age of four to five years.

Juncus bufonius, known commonly as toad rush, is a widespread flowering plant species complex in the rush family Juncaceae.

The radish (Raphanus sativus) is a flowering plant in the mustard family, Brassicaceae. Its large taproot is commonly used as a root vegetable, although the entire plant is edible and its leaves are sometimes used as a leaf vegetable. Originally domesticated in Asia, radishes are now grown and consumed globally. The radish is sometimes considered to form a species complex with the wild radish and instead treated as the subspecies Raphanus raphanistrum subsp. sativus.

Radishes are often used raw as a crunchy salad vegetable with a pungent, slightly spicy flavor, varying in intensity depending on its growing environment. There are numerous varieties varying in size, flavor, color, and length of time they take to mature. Radishes owe their sharp flavor to the various chemical compounds produced by the plants, including glucosinolate, myrosinase, and isothiocyanate. They are sometimes grown as companion plants and suffer from few pests and diseases. They germinate quickly and grow rapidly, common smaller varieties being ready for consumption within a month, while larger daikon varieties take several weeks. Being relatively easy to grow and quick to harvest, radishes are often planted by novice gardeners. Another use of radish is as a cover or catch crop in winter, or as a forage crop. Some radishes are grown for their seeds; others, such as daikon, may be grown for oil production. Others are used for sprouting.

Amanita muscaria, commonly known as the fly agaric or fly amanita, is a basidiomycete fungus of the genus Amanita. Its common name derives from its traditional use as an insecticide. It is a distinctive, large white-gilled mushroom typically featuring a bright red cap covered with white warts. The complex genetic diversity of A. muscaria suggests that it is a species complex. It is a widely distributed mushroom native to temperate and boreal forests of the Northern Hemisphere, now also naturalised in the Southern Hemisphere, forming symbiotic relationships with various trees and spreading invasively in some regions.

Ingestion of the mushroom can cause poisoning, especially in children and those seeking its hallucinogenic effects, due to psychoactive compounds like muscimol and the ibotenic acid; however, fatal poisonings are extremely rare. Parboiling reduces toxicity, though drying converts ibotenic acid into muscimol, retaining psychoactive effects. Some cultures use it as food after preparation. Indigenous peoples of Siberia used A. muscaria as an inebriant and entheogen. It has been controversially linked to Santa Claus, Viking berserkers, Vedic soma, and early Christianity, though evidence is sparse and disputed. Its rise in the 2020s as a legal hallucinogen alternative has led to Food and Drug Administration scrutiny.

The black-backed dwarf kingfisher (Ceyx erithaca), also known as the three-toed kingfisher, is a pocket-sized bird in the family Alcedinidae. It was formerly considered as conspecific with the rufous-backed dwarf kingfisher and together the species complex was known by the English name "oriental dwarf kingfisher".

This tropical kingfisher is a partial migrant that is endemic to parts of the Indian Subcontinent and Mainland Southeast Asia. It resides in lowland forests, typically near streams or ponds, where it feeds upon insects, spiders, worms, crabs, fish, frogs, and lizards. It is easily distinguishable from other birds in its range due to its red bill, yellow-orange underparts, lilac-rufous upperparts, and blue-black back.

Horseshoe bats are bats in the family Rhinolophidae. In addition to the single living genus, Rhinolophus, which has about 106 species, the extinct genus Palaeonycteris has been recognized. Horseshoe bats are closely related to the Old World leaf-nosed bats, family Hipposideridae, which have sometimes been included in Rhinolophidae. The horseshoe bats are divided into six subgenera and many species groups. The most recent common ancestor of all horseshoe bats lived 34–40 million years ago, though it is unclear where the geographic roots of the family are, and attempts to determine its biogeography have been indecisive. Their taxonomy is complex, as genetic evidence shows the likely existence of many cryptic species, as well as species recognized as distinct that may have little genetic divergence from previously recognized taxa. They are found in the Old World, mostly in tropical or subtropical areas, including Africa, Asia, Europe, and Oceania.

Horseshoe bats are considered small or medium-sized microbats, weighing 4–28 g (0.14–0.99 oz), with forearm lengths of 30–75 mm (1.2–3.0 in) and combined lengths of head and body of 35–110 mm (1.4–4.3 in). The fur, long and smooth in most species, can be reddish-brown, blackish, or bright orange-red. They get their common name from their large nose-leafs, which are shaped like horseshoes. The nose-leafs aid in echolocation; horseshoe bats have highly sophisticated echolocation, using constant frequency calls at high-duty cycles to detect prey in areas of high environmental clutters. They hunt insects and spiders, swooping down on prey from a perch, or gleaning from foliage. Little is known about their mating systems, but at least one species is monogamous, while another is polygynous. Gestation is approximately seven weeks and one offspring is produced at a time. A typical lifespan is six or seven years, but one greater horseshoe bat lived more than thirty years.

The wildcat is a species complex comprising two small wild cat species: the European wildcat (Felis silvestris) and the African wildcat (F. lybica). The European wildcat inhabits forests in Europe, Anatolia and the Caucasus, while the African wildcat inhabits semi-arid landscapes and steppes in Africa, the Arabian Peninsula, Central Asia, into western India and western China.The wildcat species differ in fur pattern, tail, and size: the European wildcat has long fur and a bushy tail with a rounded tip; the smaller African wildcat is more faintly striped, has short sandy-gray fur and a tapering tail; the Asiatic wildcat (F. lybica ornata) is spotted.

The wildcat and the other members of the cat family had a common ancestor about 10–15 million years ago. The European wildcat evolved during the Cromerian Stage about 866,000 to 478,000 years ago; its direct ancestor was Felis lunensis. The silvestris and lybica lineages probably diverged about 173,000 years ago.

Poecilia sphenops, called the Mexican molly or simply the molly, is a species of poeciliid fish from Central America. It was once understood as a widespread species with numerous local variants ranging from Mexico to Venezuela, but these variants are today considered distinct species belonging to the P. sphenops complex and P. sphenops itself as being native to Mexico, Guatemala, and Honduras. Due in part to its popularity as an aquarium fish, the species has been introduced outside of its native range, but many records may in fact refer to P. mexicana or other species from the complex. P. sphenops has been crossbred with other mollies, notably P. latipinna and P. velifera, to produce fancy mollies for the ornamental fish trade.

Channa striata, the striped snakehead, is a species of snakehead fish. It is also known as the common snakehead, chevron snakehead, or snakehead murrel and generally referred simply as mudfish. It is native to South and Southeast Asia, and has been introduced to some Pacific Islands. Reports from Madagascar and Hawaii are misidentifications of C. maculata.

A genetic study published in 2017 indicates that C. striata is a species complex.

The bronze corydoras (Osteogaster aenea), also known as the green corydoras, bronze catfish, lightspot corydoras or wavy catfish, is a species of freshwater fish in the armored catfish family, Callichthyidae, often kept as an aquarium fish. It is widely distributed in South America on the eastern side of the Andes, from Colombia and Trinidad to the Río de la Plata basin; however, as presently defined it is a species complex and a taxonomic review is necessary. It was originally described as Hoplosoma aeneum (based on a specimen from Trinidad) by Theodore Gill in 1858 and has historically also been referred to as Corydoras aeneus.