Two closely related, similar-looking species, but often with ecological and behavioural differences.

two or more species derived from a common parental stock

species which don't cross-reproduce

Closely-related species that are so similar that they are nearly indistinguishable morphologically.

Species so similar to each other as to be difficult to distinguish by human observers.

Species that are indistinguishable morphologically but are reproductively isolated.

morphologically similar or identical species (or 'taxa') that are reproductively isolated and can be distinguished using a variety of techniques (e.g. cytotaxonomy). Also referred to as 'cryptic' species.

species that are not morphologically distinguishable.