To the editors:
Andrew Brower has written an insightful essay on Willi Hennig, the “quiet revolutionary.” Hennig was an entomologist, a dipterist, primarily known for his work on the relationships of flies and for his book Phylogenetic Systematics. He is said to have been the catalyst for a change in the way biological systematics, the study of the relationships among organisms, both living and dead, was to be undertaken.1 Brower claims that “biological systematics … has undergone a series of revolutions.” He traces them to the work of three men: Carl Linnaeus, Charles Darwin, and Hennig. The first of this trio, Linnaeus, was a revolutionary for enhancing and developing the naming system for organisms, specifically through the binomen, the unique two-part name given to each species once discovered. Most everyone is aware that the proper name for humans is Homo sapiens, meaning “wise man.” Darwin, of course, is known for introducing his theory of evolution, natural selection—a revolution often referred to by his own name, the Darwinian revolution. And then, of course, is the revolution instigated by Hennig, which Brower refers to as the Hennigian revolution. It became better known as cladistics. Even though both Darwin and Hennig produced impressive bodies of work during their careers, they are mostly known for one book apiece: On the Origin of Species for Darwin and Phylogenetic Systematics for Hennig. In some quarters, these two books are revered in the same way others of a different persuasion revere the Bible. And like the Bible, these two books are known, and probably owned, by many, but read by few.2 In the case of Darwin and Hennig’s books, any ensuing revolutions were based on their subsequent interpretation and the development of the ideas they contained.
In general, revolutions promise more than they deliver. One popular interpretation of the term revolution is the overthrow of something considered outdated, irrelevant, or so wrong that it needs replacing. An alternative concept of change is the notion of reform, meaning the improvement of something considered to have become corrupted or mutated in some way, something that at its core is sound, save for changes mistaken as progress.
Here it is instructive to return briefly to the first of Brower’s trio of revolutionaries. Linnaeus wrote a great deal during his life but is probably known best—at least by those who have an interest in such things—for his 1753 work Species Plantarum, the starting point for botanical nomenclature, and his Systema Naturae. In the tenth edition of the latter, dated to 1758, he introduced his system of zoological nomenclature. The full translated title of Systema Naturae is “System of nature through the three kingdoms of nature, according to classes, orders, genera, and species, with characters, differences, synonyms, places.” The first edition was published in 1735, the thirteenth and last, after Linnaeus’s death. In this series of works, Linnaeus set about classifying all known plants, animals, and minerals, with each revised edition being necessary to accommodate the many new organisms found and described by himself and others. With his characteristic lack of modesty, Linnaeus allegedly summed up his own achievements thus: “God created, Linnaeus ordered.”3 Even aside from the binomen, Linnaeus’s achievements were indeed impressive, classifying all animals and plants into a nested hierarchy of kingdom, class, order, genus, and species. His classifications were made possible by not just studying nature itself, but by studying the process by which we may understand it: the scientific classification of organisms. This study is better known as taxonomy, from the Greek words τάξις or taxis, meaning “arrangement,” and νομία or nomia, meaning “method.” Linnaeus may not have had any specific revolution named after him, but his influence is known through the everyday use of the taxonomic names, which guide more precisely our understanding of the living world. A relevant question then arises: did the study of classification, did taxonomy, require revolution or reform?
Brower notes that, “according to Darwin’s new ontological framework, homologous features were those that arose in a common ancestor.” Darwin’s influence on systematics, if he influenced it at all, came through introducing the concept of ancestors. It was an attempt to give meaning to the taxonomic groups discovered over the years, those captured in that hierarchy of kingdom, class, order, genus, and species. To some extent, taxonomy came under the influence of a search for the ancestor. The search was undertaken via the fossil record, its beginnings found in the work of the German biologist Ernst Haeckel.4 But how would one find those ancestors? As Gareth Nelson noted, “Looking for ancestors in the fossil record seems to be something like looking for honest men: in theory they must exist, but finding them in practice, alas, is another matter.”5 Haeckel began his search in 1866 with the publication of his Generelle Morphologie der Organismen.6 The usefulness of the search was challenged in 1966, partly because of Hennig’s book: one hundred years of endeavor coming to a close. The most significant change ushered in by cladistics was to replace the search for the ancestor with the search for the sister-group, meaning its nearest relative. Taxonomy now took concern with a more fundamental statement of relationship: which taxon, or group of organisms, would be considered more closely related to another, rather than any others. This could inform a direct statement of hierarchy. Those relationships would be founded directly on evidence from the organisms themselves rather simply an interpretation of ancestry. As Brower noted, “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.” It also implies that birds share a relationship exclusive of all other organisms. And more importantly, the manner of discovery was much like that of Linnaeus. Cladistics, in its most basic form, was about discovery, similar to, but not exactly, like that of Linnaeus. Cladistics was about reform rather than revolution.
Brower implied a fourth revolution, the matrix revolution. It consists of two parts: first, “the ever-increasing speed and sophistication of computation has enabled analyses of data sets that are much larger than Hennig could have contemplated.” And second, access to vast amounts of data has become available through the molecular revolution in systematics. But what is the net effect of those two additions?
Today, bio-informaticians conjure up enormous genealogical trees based on thousands of genomes and thousands of organisms. Brower offers an assessment: “Although these developments might appear to render Hennig’s contributions obsolete, they represent changes in scale but not in kind.” That is too generous by half. Their efforts, impressive though they are, are merely technology-driven, rather than concept-driven. This leaves their results almost devoid of any scientific content beyond a simplistic interpretation of the results.
It became generally understood that cladistics from the start was, and remains, an attempt to reform taxonomy, an attempt to understand past efforts and eliminate contemporary confusion, some of that confusion having been caused inadvertently by Darwin and Hennig. It is worth pausing to consider some words of William Whewell, written in 1840:
The basis of all Natural Systems of Classification is the Idea of Natural Affinity. The Principle which this Idea involves is this:—Natural arrangements, obtained from different sets of characters, must coincide with each other.7
If “Natural Systems of Classification” is replaced with “taxonomy/systematics” and “Natural Affinity” is equated with “natural relationships,” then classification returns to centerstage after the cladistic reform. As noted by Nelson and Norman Platnick, two of the principal architects of this reform,
If cladistics is merely a restatement of the principles of natural classification, why has cladistics been the subject of argument? I suspect that the argument is largely misplaced, and that the misplacement stems … from the confounding goals of artificial and natural systems.8
Revolutions to one side, a more fitting context to understand cladistics, rather than with Darwin, or even Hennig for that matter, can be found in the work of Linnaeus and the many others who studied what a natural classification might be and how it can be discovered.
For the last 50 years and more—even now continuing into the realm of nomenclature—in the name of the modern and the new, Visionaries aim, as it were, to confine the past to a dustbin of history, and to bolt and lock the lid upon it. As if without it, we be in some way better, even born again more whole-some; as if Carl Linnaeus really were among the last of the Ancients, and not, rightly, the first of the moderns, and so related to us—of a group inclusive of us.9
These words were spoken by Nelson after receiving the Linnean Gold Medal, as did Hennig before him. Linnaeus, the first of the moderns? Well, why not?
