The phylogenetic species concept defines a species as a group of organisms that shares a common ancestor and can be distinguished from other organisms that do not share that ancestor. As an analogy, the phylogenetic species concept asserts that on the tree of life, species are the distal twigs. This contrasts with the biological species concept, which asserts that organisms are of the same species if they can interbreed successfully.
Scientists have not yet devised a perfect definition for the term “species.” The phylogenetic species concept is one of the proposed alternatives to the current concept of choice, the biological species concept. While the biological species concept makes sense when trying to determine that an elephant and a mouse are different species, it fails to be a valid indicator among all species. Many reptiles can successfully breed with other groups of animals that are obviously different species. In some cases, snakes have bred with snakes from other families successfully.
The phylogenetic species concept places evolutionary relationships at the forefront of the discussion and sidesteps the problems associated with fertility. However, the phylogenetic species concept fails in other areas. For instance, ring species are collections of animals that have diverged from each other so far that they are incapable of interbreeding yet clearly share a common and unique ancestor. This makes them a single species phylogenetically, even though they represent clearly different animals.