Since prehistory, humans have observed aspects of the living world around them and categorized organisms based on their salient features. Which berries are edible, and which ones are poisonous? Which animals are good to hunt, and which ones are hunting you? Knowing the practical benefits and dangers of plants and animals was critical to survival, as was the ability to generalize and predict these patterns. Since animals also learn and adapt to their surroundings in effectively the same manner, classifying biotic diversity in this broad, practical sense could even predate humanity.
It has long been evident that groups of organisms exhibit features unique to themselves which imply that they form groups, such as feathers on birds, or hair on mammals. Biological systematics, the work of inferring the natural system of relationships among living beings, has undergone a series of revolutions. The first was instigated by Carl Linnaeus, often described as the father of biological classification. In 1758 and 1767, he published books that established the foundations of modern animal and plant classification and taxonomic nomenclature: the binomen for each species, composed of a generic name plus a specific epithet, and the hierarchy of kingdom, phylum, class, order, family, genus, species.
In 1859, a second revolution occurred when Charles Darwin proposed his theory of evolution by natural selection. For the first time, rather than being interpreted as reflecting the Creator’s plan, homologies were explained through a material, mechanistic process. Darwin’s ideas were presaged by those of others. Alfred Russel Wallace independently conceived the process of natural selection, forcing Darwin to publish ahead of schedule. By weight of evidence and strength of sociopolitical connections, it was Darwin who emerged as the household name, and it is the Darwinian revolution that is hailed as the greatest intellectual achievement of the nineteenth century.
In the twentieth century, the systematic innovations of Willi Hennig may also have been foreshadowed in the work of his predecessors, but it is his name that will be forever associated with a third revolution in systematics.
Life and Work
Emil Hans Willi Hennig was born in 1913 in Dürrhennersdorf, a small town near Görlitz in the southern German state of Saxony.1 Hennig’s father was a railway worker and his mother, who had been born illegitimately, worked in a variety of menial jobs. The eldest of three brothers, Hennig grew up in humble circumstances. Despite their limited means, Hennig’s mother ensured that he gained access to competitive public schools, where he excelled in his studies and began developing an interest in natural history. Hennig’s gifts were obvious to his teachers, and introductions were arranged with the curators at Dresden’s natural history museum. It was the ideal environment to nurture Hennig’s growing fascination with entomology.
Hennig won a scholarship at Leipzig University, where he earned his doctorate in 1936. By the time he graduated, Hennig had already published eight monographs on flying reptiles and the order Diptera, or true flies, totaling more than 900 pages. In 1937, he obtained a postdoctoral position at the Deutsches Entomologisches Institut (DEI) in Berlin. Hennig was conscripted in 1938 and served in the infantry following the outbreak of World War II. In 1942, he was wounded by shrapnel while fighting on the eastern front.2 After a period spent recuperating, Hennig was deployed as a medical entomologist to combat malaria in Italy, where he was captured by the British. It was during his time as a prisoner of war that Hennig began drafting his treatise on theoretical systematics, Grundzüge einer Theorie der phylogenetischen Systematik, which was eventually published in 1950.3
In 1947, Hennig returned to the DEI as head of systematic entomology. Despite the partitioning of both Germany and Berlin, he was able to commute back and forth freely between his home in the American sector and his office in the Soviet sector. Hennig was not fond of communism and when the Berlin Wall was erected in 1961, he elected to remain in the west. He worked temporarily at the Technische Universität in West Berlin before becoming head of the department of phylogenetic investigation at the Staatliches Museum fur Naturkunde in Ludwigsburg, where he worked from 1963 until his death in 1976.
Over the course of his career, Hennig wrote 10 books, many of which were reissued or translated, and more than 100 peer-reviewed articles and monographs, mostly relating to systematic entomology, especially Diptera. He described 80 genera and more than 750 species of flies. If he had done nothing more, he would still be considered a major figure in his field.
Although a prolific scholar, Hennig was introverted and did not enjoy public speaking. His publications were written in a style that was rather convoluted and far from straight-forward. The influence of Grundzüge einer Theorie der phylogenetischen Systematik was limited to a small, German-reading audience, its accessibility hampered further by publication in the communist sector of the country. In 1966, Hennig published an extensively updated English translation as Phylogenetic Systematics.4 This new edition, along with a contemporaneous monograph on South temperate chironomid midges by Lars Brundin, finally brought Hennig’s ideas to the attention of the broader English-speaking world.5 His approach was soon enthusiastically adopted by systematists, particularly ichthyologists and entomologists, at the Natural History Museum in London and the American Museum of Natural History in New York.
The Hennigian Revolution
By the mid-twentieth century, the main focus of taxonomic research had shifted from inference of phylogenetic hypotheses, exemplified in the work of Ernst Haeckel during the latter part of the previous century, to the study of species and speciation.6 Systematic biologists were mostly focused on microevolutionary processes within populations, rather than patterns of relationship among taxa. Hennig rejected this perspective. He drew a distinction between the notions of tokogenetic relationship and phylogenetic relationship—the reticulate pattern of connections among individual organisms existing below the species level as a result of interbreeding or other factors, and the bifurcating hierarchical pattern of relationships among taxa.7 The real objective of phylogenetic systematics, Hennig asserted, was to address only the latter.8
The science of describing the pattern of the natural hierarchy among living things did not change significantly for almost a century following the publication of Darwin’s On the Origin of Species. Just as before, groups were recognized on the basis of homologous features, although the accepted explanation of homology became evolutionary. According to Darwin’s new ontological framework, homologous features were those that arose in a common ancestor. Monophyletic groups, following the terminology of Haeckel, are groups descended from that common ancestor. The model of common ancestry is a metaphysical framework that offers no empirical criteria for determining its validity. Evolution proceeds too slowly for individual humans to perceive the origin and differentiation of taxa, and we can never recognize a common ancestor or know with absolute certainty that a group of organisms is descended from a particular ancestor. The results of systematics provide the empirical pattern that descent with modification explains. In order to avoid circularity of reasoning, the methods of systematics should therefore avoid a priori assumptions about evolutionary processes.
In the absence of testable assertions about common ancestry, practical tools are needed for the recognition of homologous features. Once again illustrating the continuity of systematic thought spanning the Darwinian revolution, Étienne Geoffroy Saint-Hilaire described methods for recognizing homology, such as the principle of connections, in 1818.9 Substantially the same ideas were brought up to date by Adolf Remane in 1952.10 Features of organisms might be seen to represent a transformation series if they share similarity of form, composition, or relationships of parts, and connect rather different features through intermediate forms.11 Unlike most of his contemporaries, Hennig argued that biologists must differentiate among kinds of similarity, and sort out what is phylogenetically informative from what is not. The evidence for monophyly, he asserted, is not similarity in general, but a particular type of similarity. A synapomorphy is a shared, derived character state among all the members of a clade. Synapomorphies provide the evidence for monophyletic groups, and for hypotheses of common ancestry. This is one of Hennig’s most important and lasting insights.
A kind of similarity that Hennig considered to be phylogenetically uninformative is symplesiomorphy, or shared ancestral character state. In order to appreciate his perspective, consider the presence or absence of a uniquely derived feature, such as feathers. On the one hand, the presence of feathers is a synapomorphy that unites birds. An absence of feathers, on the other hand, is a symplesiomorphy that groups together mammals, snakes, lizards, frogs, and plants. Obviously, birds represent a more plausible natural group than nonbirds. Another kind of symplesiomorphic character state is one that is a synapomorphy for a more inclusive group. The presence of paired pectoral and pelvic fins in some marine animals is a symplesiomorphy since tetrapods have a similar construction of paired fore- and hindlimbs. But the presence of paired pectoral and pelvic appendages of any kind is a synapomorphy for a larger group of vertebrates including tetrapods plus most fishes.
Another kind of similarity that Hennig viewed as uninformative is homoplasy. Homoplastic character states seem to be homologies, yet evolved independently; they are either inferred independent derivations of a feature, or reversals from an inferred derived to an inferred ancestral state. Examples include the evolution of mimetic wing patterns in butterflies, or the loss of legs in snakes. The only way to recognize that a feature is homoplastic is to observe that its distribution among the taxa is incongruent with the more strongly supported pattern of other characters that implies the optimal hypothesis of relationships.
Grouping by synapomorphy alone means that some traditionally recognized assemblages of taxa, such as reptiles, invertebrates, protists, and gymnosperm plants are not groups in the Hennigian sense. In the 1960s and 1970s, evolutionary taxonomists, including Ernst Mayr, saw this view was a threat to their traditional classification schemes. They referred to the followers of Hennig’s tenets as cladists, and demanded that both monophyly and degree of difference be taken into account when forming classifications. They also argued for a definition of monophyly that encompassed paraphyletic groups—a group containing a hypothetical common ancestor and some, but not all, of its descendants—e.g., reptiles to the exclusion of birds. The followers of Hennig proudly adopted the cladist moniker, soundly debunked the incoherent arguments of the evolutionary systematists, and went on to found the Willi Hennig Society, publisher of the journal Cladistics, in the early 1980s.
Another principle espoused by Hennig is the idea that evidence should be interpreted parsimoniously. A given set of character data can be mapped onto any topology, but some of the trees will imply fewer character state transformations than others. The one that implies the fewest is the most parsimonious. Hennig’s preference for parsimonious solutions is indicated in his auxiliary principle, which states that the same character state shared between two or more taxa should always be assumed first to be homologous, rather than homoplastic. The most parsimonious tree is the one that minimizes hypotheses of homoplasy. For a given character, homoplasy is only invoked when it is more parsimonious to do so than to invoke additional homoplasy elsewhere.
Preference for the most parsimonious tree does not require evolution to occur parsimoniously. In an influential 1983 paper, James Farris argued that the methodological assumption of minimization is not the same as an ontological assumption of minimality.12 A regression line minimizes the variance in the data, regardless of how scattered the observations may be. Cladists, it should be noted, usually refer to their preferred tree as a hypothesis, rather than a confirmed fact. The endeavor of systematics can always accommodate new evidence that may overturn what seems to be an established fact.
In sum, the main components of the Hennigian revolution in systematics were:
- a clear demarcation of systematics as the study of phylogenetic relationships among taxa at the species level and above;
- a clear conception of monophyly as the relation among all taxa inferred to have descended from a common ancestor;
- grouping by synapomorphy as the most natural means to recover monophyletic taxa; and
- selection of the most parsimonious tree as the preferred hypothesis of relationships.
Hennig’s initial theoretical statements provided the impetus for his followers to expand and refine his ideas throughout the 1970s and into the twenty-first century.
Matrix Revolutions
Two major technological developments have drastically changed systematics since the Hennigian revolution. The ever-increasing speed and sophistication of computation has enabled analyses of data sets that are much larger than Hennig could have contemplated. The search for the most parsimonious trees is an NP-complete problem. It has no direct solution but must be conducted by trial and error examination of character distributions on all possible topologies. As the number of taxa in a data set grows, the number of possible trees grows exponentially. For matrices with more than 25 or so taxa, it is not feasible to evaluate all possible trees, even with very fast computers. Finding computational tricks to discover shorter trees and limit search time has become an objective for programmers.
The other technological advance is the availability of molecular data. Developed in the early 1980s, the polymerase chain reaction democratized the ability to generate a DNA sequence from just about any organism without the laborious process of cloning and isolating DNA fragments. As a result, the entire genome of an organism can be sequenced quickly at modest cost. Systematics is now awash with molecular data—millions or billions of potential characters that need to be sifted through to determine which are relevant to phylogenetics. In the great majority of instances, the data gleaned from genomes have corroborated relationships long recognized on the basis of traditional morphological studies.
Although these developments might appear to render Hennig’s contributions obsolete, they represent changes in scale but not in kind. The principles articulated and methods realized by Hennig and his followers are still entirely relevant to contemporary systematics. Before Hennig, the concept of monophyly was unclear. Now, almost all systematists agree that phylogenetic trees and resultant classifications ought to recognize monophyletic groups exclusively, and that groups held together by symplesiomorphies are invalid. Hennig’s work also led to clear and objective criteria for preferring one phylogenetic hypothesis over another. Although cladistic parsimony is currently out of fashion, the overarching principle of seeking a preferred tree under a consistent optimality criterion appears to remain at least a stated desideratum for most contemporary phylogenetic studies. Perhaps what has been lost in the flood of data and the tangle of workarounds developed in an effort to accommodate it is an appreciation for the more abstract and fundamental philosophical questions regarding the nature of parsimonious inference of general patterns. This is, after all, the aim of phylogenetic studies.
Cladistic Clarity
Phylogenetic inference based on statistical models of character change has become increasingly popular during the last few decades. Systematists have abandoned Hennig’s principles and regressed to an evolutionary taxonomist stance that forms groups through both synapomorphy and symplesiomorphy. In 1978, the statistician Joseph Felsenstein showed that cladistic parsimony could be statistically inconsistent. That is, under particular circumstances, it could produce the wrong hypothesis of relationships with greater and greater support as more data were sampled.13 To address this problem, he recommended the framework of maximum likelihood, a model-based approach that takes into account branch length and supposedly avoids inconsistency. In order for a particular model to be consistent, it has to accurately reflect the true pattern of evolutionary divergence. What Felsenstein failed to mention is that any method could be inconsistent, and without a priori knowledge of the true relationships, there is no way to know whether the method is consistent or not. Comparative analyses have shown that model- and parsimony-based analyses of empirical data sets almost always produce the same results.14 A preference for one over the other appears to be more a matter of taste than improved efficacy. To cladists, the practical advantages of methods that make fewer assumptions and are more epistemologically transparent as a result seem obvious. One can build a Rube Goldberg machine to swat a fly, or one can use a flyswatter.
The Linnean Society of London commemorated Hennig’s centenary with a symposium examining the impact of his work and a book, The Future of Phylogenetic Systematics: The Legacy of Willi Hennig.15 In a review of the latter, David Baum asked: “Does the future of phylogenetic systematics really rest on the legacy of a one mid-twentieth-century German entomologist?”16 I replied:
Does the future of evolutionary biology rely on the contributions of one 19th- century English biologist? Does the future of physics rely on the contributions of a 17th-century English physicist [or] astronomer? Probably not. But that does not mean that we do not honor the contributions of Darwin or [Isaac] Newton or deny that subsequent work “rests on their legacies.” As Newton himself said, “If I see further, it is because I stand on the shoulders of giants.”17
Hennig is not a household name like Darwin or Newton, and with time he may fade entirely into obscurity. But, for now at least, the principles he espoused provide a reasoned counterpoint to the all-encompassing technological fervor that has overtaken phylogenetics.18
