PHYLOGENETIC SYSTEMATICS IN BOTANY

Summary Phylogenetic systematics in the sense of W. Hennig has exerted a great influence on systematic zoology. Among botanists, however, it has been overlooked and the present paper is intended as a brief summary of the principles, results, and advantages of its application in botanical systematics. In phylogenetic reconstruction monophyletic groups are established by the use of the unique, derived features inherited by the members from the immediate ancestor of the group. Since species splitting has been mainly dichotomous, the search for sister species or sister groups is the main procedure in phylogenetic reconstruction. The effects of multiple splitting of species, parallelism, and reticulate evolution are also discussed. In traditional, “natural” or “evolutionary” classification the degree of similarity is a main criterion for relationship. Since similarity depends on both primitive and derived characters, the groups established will often be paraphyletic; some of the descendants of the common ancestor will nor be included in such a group. Paraphyletic groups have no reality in nature and their use in evolutionary and biogeographic discussion is a major obstacle to progress. In a strictly phylogenetic classification all taxa must be monophyletic and all sister groups given equal rank. If these principles are followed, the classification will reflect nature's own hierarchy and its groups will be directly useful in all branches of causal biology. The effects of phylogenetic classification on different levels are demonstrated by two examples. Takhtajan's angiosperm system is discussed and it is shown that the traditional division of the angiosperms into the two classes Magnoliatae (dicotyledons) and Liliatae (monocotyledons) will have to be abandoned, since Magnoliatae is paraphyletic. On the genus level it is shown that if one wishes to maintain a number of specialized genera within the Capparidaceae‐Cleomoideae, the genus Cleome becomes paraphyletic. As long as the phylogeny of the whole group is not worked out, the only solution is to include the specialized segregates within the genus Cleome. Similar examples could be found anywhere in contemporary botanical classification.