A species complex is a group of closely related species, where the exact demarcation between species is unclear or cryptic. Ring species, superspecies and cryptic species complex are example of of species complex. Such groups of species with complex relationship between species may occur in a line undergoing rapid speciation 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) 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 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 these may actually be subspecies of the same species.[1]

References

1. ^ (Danish) Engelke, S. (2006?): Til Snegleforeningen (Note to the Danish Slug-society). Article in Danish