Either of the two descendant species formed when one species splits during evolution. Hence, the sister species (or sister group) is the one most closely related to any given species (or group), since both share an ancestral species (or group) not shared by any other species (or group). In classification systems based on the principles of cladistics, sister species are always grouped together. Moreover the sister group is the prime choice for use in outgroup comparison.
Subjects: Biological Sciences.