The Development of Phylogenetic Concepts in Hennig's Early Theoretical Publications (1947-1966)

In this paper, we describe the development of Hennig's most important phylogenetic concepts, which culminated in the publication of the now famous Phylogenetic Systematics in 1966. Hennig proposed strict definitions of monophyly and phylogenetic relationships as early as 1950 in Grundziige einer Theorie der phylogenetischen Systematise. However, in this edition of the book, the terms "apomorphic" and "plesiomorphic" were introduced for taxa and not for characters. It was not until 1952 that these terms were consistently applied to characters, hereby for the first time allowing the reconstruction of phylogenetic relationships. But this is not Hennig's sole contribution to systematics, as revealed by a comparison of his Phylogenetic Systematics with publications of the "new systematics" school (e.g., Huxley, 1940, The new systematics, Oxford Univ. Press, Oxford, England; Mayr et al., 1953, Methods and principles of systematic zoology, McGraw-Hill, New York). Hennig was the person who redirected the interest of systematics to the study of supraspecific taxa after years of focusing on species and infraspecific taxa.