Cladists define a species as a group of organisms sharing a set of unique, derived features that sets them apart form all other such groups. this way of defining species exemplifies

For the answer to the question above, I believe the answer is the phylogenetic species concept.

The key to the phylogenetic species concept is that species must be "diagnosable." In other words, members of the species should share a combination of characteristics that other species don't have. To look for some unique features that define a  phylogenetic species, systematic comparisons must be done by paleontologists with other related fossils or living species

This post might have the info you need: