ON SPECIES and OTHER TAXA.

Abstract- It is argued that taxa, whether Linnaean or phylogenetic, belong to Popper's worlds 2 and 3, the worlds of knowledge, but that they represent entities residing in world 1, the world of objects, namely, classes of living beings. The Linnaean taxa are concepts, and thus untestable, whereas phylogenetic taxa are statements, the monophyletic taxa being true, and the paraphyletic and polyphyletic ones false statements. The taxa are neither strictly nor numerically universal statements, but probabilistic ones which cannot be falsified by single observations. It is suggested that the classical "species problem" is due to the fact that "species" has been used in three different senses. First, traditionally it has been assumed that the specific "essence" of an organism is that by which it is what it is. When we know the species, we know the organism. Second, the species are terminal taxa in the phylogenetic hierarchy. This implies that it is only a very small part of the "essence" of the organism which distinguishes the species. The remaining part characterizes the succession of superior taxa in the phylogenetic lineage which ends with the species in question. Third, the species has been regarded to be the "evolutionary unit." This idea may be refuted for two reasons: (1) since concepts and statements cannot evolve, species cannot evolve either, and (2) it is generally in very small isolated populations that evolutionary innovations are first established. In Linnean systematics the superior taxa are allotted categorical rank. The fact that the classification is constrained by this conventional stipulation implies that the superior taxa are often man-made artifacts. In the phylogenetic hierarchy, composed of monophyletic taxa, the ontological states of the taxa is completely independent of their numerical rank; the kingdom is as "real" as the species.