A species complex is a group of closely related species, where the exact demarcation between species is often unclear or cryptic owing to their recent and usually still incomplete reproductive isolation

Reproductive isolation

The mechanisms of reproductive isolation or hybridization barriers are a collection of mechanisms, behaviors and physiological processes that prevent the members of two different species that cross or mate from producing offspring, or which ensure that any offspring that may be produced is not...

. Ring species

Ring species

In biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene...

, superspecies

Superspecies

A superspecies is a group of at least two more or less distinctive species with approximately parapatric distributions. Not all species complexes, whether cryptices or ring species are superspecies, and vice versa, but many are...

 and cryptic species complex

Cryptic species complex

In biology, a cryptic species complex is a group of species which satisfy the biological definition of species—that is, they are reproductively isolated from each other—but whose morphology is very similar ....

 are example of species complex. Such groups of species with complex relationship between species may occur in a line undergoing rapid speciation

Speciation

Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

 or where such speciaton recently have occurred, so that species separation mechanisms has yet to be fully developed. In such cases speciation may leave some species paraphyletic at the species level.

Species complexes are more common among plants, but animal examples exist, such as the dog-wolf-coyote complex (the genus Canis

Canis

Canis is a genus containing 7 to 10 extant species, including dogs, wolves, coyotes, and jackals, and many extinct species.-Wolves, dogs and dingos:Wolves, dogs and dingos are subspecies of Canis lupus...

) and the cobras (genus Naja

Naja

Naja is a genus of venomous elapid snakes. Although there are several other genera that share the common name, Naja are the most recognized and most widespread group of snakes commonly known as cobras. The genus Naja consists of 20 to 22 species, but has undergone several taxonomic revisions in...

). 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 Spanish slug

Spanish slug

The Spanish slug, scientific name Arion vulgaris, more commonly known in English-speaking countries under the incorrectly applied Latin name, Arion lusitanicus, is a species of air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs.This is a...

 in Northern Europe

Northern Europe

Northern Europe is the northern part or region of Europe. Northern Europe typically refers to the seven countries in the northern part of the European subcontinent which includes Denmark, Estonia, Latvia, Lithuania, Norway, Finland and Sweden...

, where interbreeding with the local black slug

Black slug

The black slug also known as black arion, European black slug, or large black slug, scientific name Arion ater, is a species of large land slug, a terrestrial slug in the family Arionidae, the roundback slugs.-Description:...

 and red slug

Red Slug

The red slug, also known as the chocolate arion or the European red slug, Arion rufus, is a large land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs....

, traditionally considered clearly separate species that did not interbreed, shows these may actually be subspecies of the same species.

Examples of known species complexes

Animals

• The wolf-dog-coyote-dingo group, genus Canis
  Canis
  Canis is a genus containing 7 to 10 extant species, including dogs, wolves, coyotes, and jackals, and many extinct species.-Wolves, dogs and dingos:Wolves, dogs and dingos are subspecies of Canis lupus...
  
  
• The cobras, genus Naja
  Naja
  Naja is a genus of venomous elapid snakes. Although there are several other genera that share the common name, Naja are the most recognized and most widespread group of snakes commonly known as cobras. The genus Naja consists of 20 to 22 species, but has undergone several taxonomic revisions in...
  
  
• Some species of the roundback slugs, genus Arion
• The jellyfish genus Cyanea
  Cyanea (jellyfish)
  Cyanea is a cosmopolitan genus of stinging jellyfish, primarily found in northern waters of the Atlantic and Pacific Oceans. The same genus name has been given to a genus of plants of the Hawaiian lobelioids, an example of a parahomonym .-Species:The taxonomy of Cyanea species is not fully agreed...
  
  , with from 1 to 14 species, depending on author.