INTERNODONS AS EQUIVALENCE CLASSES IN GENEALOGICAL NETWORKS - BUILDING-BLOCKS FOR A RIGOROUS SPECIES CONCEPT

Among the options suggested in phylogenetic systematics to solve the s pecies problem is the Hennigian or internodal species concept. This co ncept interprets species as parts of the genealogical network of indiv idual organisms between two successive permanent splits or between a p ermanent split and an extinction event. Though this option is at prese nt not favoured by phylogeneticists, we believe that, to solve the spe cies problem, there is no alternative to finding a satisfactory partit ion of the genealogical network. In previous work a formal definition has been developed of Hennigian or internodal species (called internod ons here), based on a logical relation between individual organisms. I n this paper, we prove that this definition indeed partitions genealog ical networks exhaustively into mutually exclusive entities, by showin g that the defining relation is an equivalence relation. Although inte rnodons should not themselves be seen as species, they are essential b uilding-blocks for any satisfying species concept.