Permanent Splits as Speciation Events: A Formal Reconstruction of the Internodal Species Concept

The internodal species concept, which interprets a species as a part of the genealogical network between two permanent splits, is a good prima facie candidate to satisfy the intuition that species should at least be mutually exclusive. This paper develops a formal definition of internodal species: such a species is defined as a set of organisms of which each pair satisfies a conspecificity relation INT. Species so defined are mutually exclusive since INT is an equivalence relation. The relation INT is constructed out of only two primitive relations, parenthood and priority of birth. An algorithm is offered to determine, from their positions in the genealogical network, whether two given organisms belong to the same internodal species. This algorithm deals with asexual as well as sexual organisms and is so sharp as to place a species boundary between close relatives. It also enables us to define monophyletic groups and genealogical networks as sets of organisms. From its reconstruction we may conclude that the internodal species concept forms a firm basis for further development of a satisfactory species concept.