Species Complex

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 as yet, usually, incomplete reproductive isolation. Ring species complexes, superspecies complexes and cryptic species complexes are some different examples of species complexes. Such groups of species with a complex-type relationship between species may occur in a line - undergoing rapid speciation - or where such speciaton has recently occurred, meaning species separation mechanisms or traits which distinguish species have yet to develop in full. Such cases may leave some species paraphyletic at the species level and lead to hybrid species, making phylogeny/phylogenetic analysis difficult.

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.