Species complex

Stereum near Mörfelden-Walldorf, Hesse, Germany; in the S. hirsutum–S. ostrea species complex, only microscopic analysis can determine the species

A species complex is a group of related species, where the exact demarcation between species is often unclear or cryptic. The demarcation difficulty may be due to their recent and as yet incomplete reproductive isolation. 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.

Hybrid speciation may be a component of species complexes, when a reproductively isolated species arises from hybridization.

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

Animals

• 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.