Two or more taxa once considered conspecific (of the same species) may later be subdivided into infraspecific taxa (taxa within a species, such as bacterial strains or plant varieties), but this is not a species complex.
A species complex is in most cases a monophyletic group with a common ancestor, although there are exceptions. It may represent an early stage after speciation, but may also have been separated for a long time period without evolving morphological differences. Hybrid speciation can be a component in the evolution of a species complex.
Species complexes exist in all groups of organisms. They are identified by the rigorous study of differences between individual species, making use of minute morphological details, tests of reproductive isolation, or DNA-based methods such as molecular phylogenetics or DNA barcoding. The existence of extremely similar species may cause local and global species diversity to be underestimated. Recognizing similar but distinct species is important for disease and pest control, and in conservation biology, although drawing dividing lines between species can be inherently difficult.
A species complex is typically considered as a group of close, but distinct species. Obviously, the concept is closely tied to the definition of a species. Modern biology understands a species as "separately evolving metapopulation lineage" but acknowledges that the criteria to delimit species may depend on the group studied. Thus, many species defined traditionally, based only on morphological similarity, have been found to comprise several distinct species when other criteria, such as genetic differentiation or reproductive isolation were applied.
A more restricted use applies the term to close species between which hybridisation occurred or is occurring, leading to intermediate forms and blurred species boundaries. The informal classification, superspecies, can be exemplified by the grizzled skipper butterfly, a superspecies that is further divided into three subspecies.