The cladistic or phylogenetic species concept is that a species is the smallest lineage which is distinguished by a unique set of either genetic or morphological traits. No claim is made about reproductive isolation, making the concept useful also in palaeontology where only fossil evidence is available.

With the rise of online databases, codes have been devised to provide identifiers for species that are already defined, including:

A simple textbook definition, following Mayr's concept, works well for most multi-celled organisms, but breaks down in several situations: