Hennig, (Emil Hans) Willi
HENNIG, (EMIL HANS) WILLI
(b. Dürrhennersdorf, Germany, 20 April 1913; d. Ludwigsburg, Germany, 5 November 1976),
evolutionary biology, systematics, taxonomy, phylogenetic systematics, entomology, education. For the original article on Hennig, see DSB, vol. 3.
Hennig was a German entomologist specializing in the systematics of flies and fossil insects. He became the most influential systematist and taxonomist in the twentieth century by developing the philosophy and methodology of phylogenetic systematics (cladistics). His basic philosophy and approach to the analysis of evolutionary relationships of organisms are the dominant paradigms of systematics in the twenty-first century.
Hennig the Biologist . Hennig began his career while still in Gymnasium (high school) in Dresden, when he worked as a volunteer at the Zoological Museum. His early interest in systematics dates to an essay written in 1931 on the “state of systematics in zoology.” Early papers include works on snakes (at age nineteen, with Wilhelm Meise) and Draco, the flying lizards of Asia. His interest in flies (order Diptera) grew from his work with Dresden entomologists Fritz van Emden and Emden’s successor, Klaus Günther. After finishing his doctorate in the reproductive anatomy of flies at the University of Leipzig in 1936, he joined the German Entomological Institute (GEI) of the Kaiser Wilhelm Society (later the Max Planck Society) in Berlin. By 1939 he had already published some forty-one papers, many dealing with the morphology of the copula-tory organs of flies, but also including papers on classifying higher taxa (groups of many species; genera, families, etc.) and geographic variation.
Hennig served in the German army between 1939 and 1945. In spite of being wounded on the Russian front and later serving in Greece and Italy as a mosquito control specialist, he managed to publish some twenty-five papers dealing with fly systematics as well as mosquito control. As a prisoner of war, he was pressed into mosquito control work by his British captors and also began working on his magnum opus, Grundzüge einer Theorie der Phylogenetis-chen Systematik (Fundamentals of the theory of phylogenetic systematics), published in 1950.
After release, Hennig first served as acting director of the Zoological Institute at the University of Leipzig (1945–1947) and then returned to the GEI in 1947, where he finished his major work on the larvae of flies (1948) and began his studies on fossil insects. Many works followed, including papers on dipteran systematics, phylogenetics, biogeography, and chapters on invertebrates for the Textbook of Zoology. By 1961 Hennig had finished a revision of his phylogenetics book, which was sent to the United States to be translated into English, appearing as Phylogenetic Systematics in 1966.
Hennig was nominated for the directorship of the GEI. His appointment was delayed due to his open opposition of the East German government. After the erection of the Berlin Wall in 1961, he decided to terminate his appointment because he felt he might be in political danger. This suspicion proved to be well founded, as the documents of the East German Secret Service (Stasi) later confirmed that proceedings had been initiated to imprison him. Hennig briefly held a professorship at the Technical University (West) Berlin (TU Berlin), but moved in 1963 to the State Museum of Natural History, Stuttgart, where he turned his attention to fossil insects preserved in amber (authoring some twenty papers). This renewed interest in fossil insects, coupled with his extensive knowledge of insect diversity and systematics, culminated with the publication of Die Stammesgeschichte der Insekten (1969), with an authorized English translation, Insect Phylogeny, published in 1981. This monumental work included both the most comprehensive review of Paleozoic and Mesozoic insects to date, and also contributed to his development of phylogenetic systematics.
Hennig’s Phylogenetic Systematics . Although Hennig is remembered by entomologists for his fundamental works in dipteran systematics and fossil insects, his influence reached all aspects of evolutionary biology through his works on phylogenetic systematics. Hennig considered himself a reformer rather than a revolutionary, and his goal was to bring studies of the evolutionary relationships and classification of organisms fully into the Darwinian paradigm. Before Hennig’s work there were no generally acceptable protocols for reconstructing the evolutionary histories of species, the “Tree of Life.”
Hennig provided the basic methods for such protocols, enabling biologists to present their hypotheses of relationship in a rigorous and testable framework. Hennig’s proposal was fairly simple. First, relationship means “genealogical relationship,” the relationship among ancestors and descendants (alternate concepts equated relationship with “similarity”). Second, only certain kinds of characters can be used to test a proposed genealogical relationship. Homologous characters (characters whose similarity is due to common ancestry), Hennig argued, may be of two kinds: more ancient (plesiomorphies) and more recent (apomorphies). Ancient homologues may make two species appear similar, but cannot form the basis for uniting these species into a unique common ancestry group.
For example, humans and lizards share the homology of having five toes on each limb, while living horses have a single toe. Is this evidence that humans and lizards share a common ancestor not shared by horses? No, states Hennig; having multiple toes is an ancient evolutionary innovation found in the ancestor of all legged vertebrates, horses included. More recent homologies are evidence of unique common ancestry. For example, humans and horses share the homology of having fur-covered bodies while lizards have scales. Is this evidence that horses share a common ancestor with humans that is not shared with lizards? Yes, states Hennig; having fur is a relatively recent homology not found in the common ancestor of lizards, humans, and horses, but only in the common ancestor of humans and horses (and other mammals). Thus, it is a valid test of the hypothesis that humans are more closely related to horses than humans are to lizards.
The basis for Hennig’s methods for testing common ancestry relationships rests on the ability of systematists to distinguish between different kinds of homologous similarities and to distinguish between homologous and nonhomologous similarities (convergent similarities, independently evolved two or more times). The method works for analysis of the evolution of morphology, behavior, DNA sequences, ontogeny, and indeed any heritable characters that evolve. The phylogenetic trees produced from these analyses form the basis for many other studies, including speciation and extinction dynamics, historical biogeography, genomic research, and even disease tracking.
In addition to his method of testing alternative phylogenetic trees, Hennig provided the rationale for modern biological classifications by rigorously defining what constitutes a natural group of organisms (termed a clade or monophyletic group) from unnatural groups, some of which had previously been considered natural. This created the possibility of a taxonomic system that was objective and logical relative to the phylogenies being proposed by the existing, qualitative analytical protocols. It is also a source of continuing controversy, as many familiar groups are identified as unnatural because they formed paraphyletic groups.
Before Hennig, all systematists had agreed that polyphyletic groups were unnatural because the common ancestor was not included in the group. An example is the polyphyletic group Homeothermia, composed of birds and mammals. Paraphyletic groups included the ancestor, but only some of the descendants. An example is Reptilia, which includes the common ancestor but excludes birds and mammals. Hennig argued that natural groups (his monohyletic groups) must include both the ancestor and all descendants. For example, the family of great apes, Pongidae, excludes one of the descendants of the common ancestor, humans, who are placed in another family, Hominidae. By Hennigian logic, the Pongidae, as a paraphyletic group, is unnatural. Humans should either be placed in Pongidae or the great apes in Hominidae to form a natural group that includes the ancestor and all descendants. And, if Reptilia was to be natural, it should include birds and mammals as well as crocodiles and lizards.
It should be noted that Hennig did not work in a vacuum: he was influenced by earlier workers, especially Walter Zimmermann. But his synthesis was the work that convinced others that a rigorous science could be developed for discovering the Tree of Life. Although phylogenetic systematics has undergone many refinements, Hennig’s basic philosophy persists as the basis of modern evolutionary analysis of relationships among species and higher groups.
Hennig’s contributions to entomology and systematics were widely recognized. He was an elected member of several scientific academies, including the German Academy of Natural Scientists Leopoldina (1963) and the Swedish Royal Academy of Science (1972). He received many honors. He received the Fabricius Medal from the German Entomological Society (1953), and gold medals from the Linnaean Society, London (1974), and the American Museum of Natural History, New York (1975). He was awarded an honorary doctorate from the Free University of Berlin in 1968.
Hennig the Man . Hennig was one of three sons born to Karl (1873–1947) and Marie (1885–1965) Hennig in Dürrhennersdorf, near Dresden. His father was a railroad official. One brother, Karl Herbert (1917–1943?) went missing at Volgograd (then Stalingrad) and his fate remains unknown; the other, Fritz Rudolf (1915–1990), was a minister. Hennig married fellow student Irma Wehnert (1910–1990) in 1939. Her contributions to his career were many. During World War II, Irma Hennig acted as research assistant, editor, and librarian for her husband, enabling him to publish numerous papers even while serving in the military. Hennig served in the German army from 1939 to1945, serving in the infantry in Poland, France, and Russia until wounded in 1942, and thereafter as a medical entomologist. (It should be noted that Hennig was never associated with the Nazi Party of Germany or the Communist Party of East Germany.) Willi and Irma had three sons, Wolfgang (b. 1941, geneticist; four children), Bernd (b. 1943, geneticist; two children), and Gerd (b. 1945, teacher; one child). Hennig’s grandchildren adored him. He was a lover of classical music, especially Mozart, and an avid reader of literature as well as science. He eschewed personality cults and refused to the end of his life to use his personal influence to promote phylogenetic systematics, preferring to let the discipline grow or die on its merits. History has proven him correct.
The author would like to thank Dr. Wolfgang Hennig for reviewing this essay and providing helpful comments and critical revisions.
SUPPLEMENTARY BIBLIOGRAPHY
WORKS BY HENNIG
Grundzüge einer Theorie der Phylogenetischen Systematik. Berlin: Deutscher Zentralverlag, 1950.
“Phylogenetic Systematics.” Annual Review of Entomology 10 (1965): 97–116. A short exposition of Hennig’s methods and goals.
Phylogenetic Systematics. Translated by D. Dwight Davis and Rainer Zangerl. Urbana: University of Illinois Press, 1966. Translation of the revised Theorie der Phylogenetischen Systematik; a Spanish-language and a Chinese translation also were published.
Die Stammesgeschichte der Insekten. Senckenberg-Buch 49. Frankfurt, Germany: Kramer, 1969. Translated by Adrian C. Pont as Insect Phylogeny. Chichester, U.K.: Wiley, 1981.
OTHER SOURCES
Ax, Peter. “Professor Dr. H. C. Willi Hennig.” Zoomorphology 87(1977): 1–2.
Hennig, Wolfgang. “In Memoriam: Willi Hennig.” Beiträge zur Entomologie 28 (1978): 169–177. Written by the son of Willi Hennig. The volume also includes a complete list of Willi Hennig’s publications.
Hull, David L. Science as a Process. An Evolutionary Account of the Social and Conceptual Development of Science. Chicago: University of Chicago Press, 1988. Places Hennig into the larger context of debates over systematics in the twentieth century.
Schlee, D. “In Memoriam Willi Hennig 1913–1976.” Eine biographische Skizze. Entomologica Germanica 4 (1978): 377–391. Shortened translation in English available from http://zoo.bio.ufpr.br/diptera/bz023/willi_hennig.htm.
E. O. Wiley