To the editors:

Andrew Brower’s review is a timely reminder of Willi Hennig’s life and contributions. If, as he concludes, it is possible that Hennig and his work may fade into obscurity, then systematic biologists need to reexamine their mission, methods, and priorities. The revolution begun by Hennig is unfinished and, unless we abandon goals that have been central to the discipline for centuries, its completion ought to be a top priority. Do we continue taxonomy’s foundational quest to inventory earth’s species, describe their similarities and differences, discover the historic pattern of relationships among them, and reflect it in a natural classification?¹ Or, do we reduce the independent, fundamental science of taxonomy to little more than an identification service,² abandoning theory-rich systematic studies for barren branching diagrams stripped of the synapomorphies that make them interesting? For species determinations and skeletal representations of species relationships, it is conceivable that a single source of data, such as DNA, could be sufficient—assuming, that is, a comprehensive library of verified sequences for all species and evidence partitioned and analyzed as synapomorphies rather than raw phenetic similarity. But this robs systematic biology of its intellectual richness and deprives humanity of deep understanding of both the biosphere and phylogeny of which we are part. Basic, curiosity-driven species exploration demands that we follow the diversity and history of species where they lead us, pursuing comparative morphology, paleontology, developmental biology, and DNA with equal vigor.

Darwin’s theory of evolution by natural selection forever changed how we see ourselves in the natural world.³ This seismic shift in worldview from creationism to evolution had surprisingly little effect on how taxonomists do their work. They continued to compare, describe, name, and classify species much as they had before, even when published results were dressed in evolutionary language. And, while Darwin made clear the importance of learning the big picture of evolution—the phylogenetic history of life, expressed in cladograms and classifications—he provided no theories or methods sufficient to scientifically reconstruct it. Inference of relationships among species remained a matter of speculation. That is, until Hennig.⁴

Hennig is not much cited today, particularly in the molecular systematics literature. There is an assumption that he has little to say of relevance to the analysis of a class of data that did not exist when he wrote Phylogenetic Systematics. Yet his theories dealt with how synapomorphies are distinguished from other sources of similarity and analyzed, not where they come from. He did not ascribe a priori superiority to any particular kind of data. Technology remained in its proper place, in the background, as a tool enabling the gathering of data. Hennig’s concept of holomorphology, despite its word roots, expressly included any and all relevant sources of evidence, DNA and any other relevant source of evidence not yet envisioned included.

I was intrigued by Brower’s discussion of credit attribution in the history of science. Darwin was not alone, nor even the first, in deriving a theory of evolution. Nonetheless, he is an appropriate choice as the starting point and poster child for evolution. The same can be said of Hennig. Many had searched for the key to making phylogeny reconstruction rigorously scientific. The necessary groundwork had been laid. Richard Owen, for example, had developed the concept of homology.⁵ Hennig, like Darwin, had a retiring nature. Fortunately, Darwin had Thomas Huxley to argue his case. Hennig also had followers who trumpeted, refined, and extended his ideas. None better exemplify the clarification and advocacy of his ideas than Gareth Nelson and Norman Platnick.⁶ Theoretical and methodological fires were spread rapidly in many directions by numerous authors, but the spark was Hennig’s.

Given the number and urgency of environmental challenges, it is understandable that biologists simply want to identify species and know something of their relationships to other species. But meeting such practical necessities need not come at the expense of traditional taxonomic goals, morphological descriptions first among them, that are more relevant today than ever. Taxonomy, done well and not narrowly restricted to quick or easy methods, yields additional intellectual and practical benefits of immense importance to both science and society. As experience and common sense inform us, the most reliable species hypotheses and informative classifications come from taxonomy that integrates as much information as possible. And the more complete our descriptions of the attributes and natural history of species, the more useful knowledge exists to be retrieved when identifications are made. Matching DNA bar codes to known species in a library of sequences, only to retrieve little more than a name, is a rather hollow ambition. Given the rapidity with which poorly described and undescribed species are going extinct,⁷ this is an especially inopportune time to reduce the information content of taxonomy by focusing on molecular data alone.⁸

The belief that taxonomy is a service existing to provide identifications is a misconception. It is false premise used to justify shortcuts, such as estimating species with genetic distances rather than treating them as testable hypotheses based on predicted character distributions.⁹ Best practice is to integrate as many sources of evidence as possible.¹⁰ Multiple sources of evidence extend the checks and balances of what Hennig described as reciprocal illumination.

It is sometimes argued that a molecular approach is to be preferred because it avoids the cost and effort of years of scholarship typical of traditional taxonomy. But such expediency comes at a cost. Names are not backed up by detailed knowledge of species themselves, and we are deprived of the most fascinating details from the story of evolutionary history. This argument supposes that so-called descriptive taxonomy is inherently and necessarily inefficient. But revisionary and monographic studies, the gold standards in taxonomy, have built-in efficiencies. Their comprehensive comparative approach reviews the limits of all known species of a taxon simultaneously, while also recognizing and describing new species. Further, we have yet to make a serious effort to modernize monography by adapting information technologies to meet its needs and overcome traditional obstacles restricting access to museum collections, existing data, and colleagues distributed around the globe.¹¹ A modest investment in a modernized taxonomic research platform, including a specially designed cyberinfrastructure and innovative storage-and-retrieval systems for natural history museum specimens, could transform what can be done to accelerate species exploration.¹²

Morphology Matters

Taxonomists did not study morphology for centuries only because they lacked access to molecular data. Willie Sutton robbed banks because that is where the money is kept, and taxonomists study morphology because that is where many of the most interesting evolutionary novelties are found. It is a general rule that historical information about relationships among species and their attributes exist in proportion to the complexity of the evidence. Individual instances of nucleobases, such as guanine, have precisely zero historical information content. Any two guanine nucleobases are indistinguishable, entirely chemically identical anywhere in time or space. String them together with other nucleobases in a snippet of DNA, however, and a unique pattern emerges that includes evidence of history due to common ancestry. Sequence more DNA, get more historical information.

The marvel of morphology is that, appropriately interpreted, it efficiently summarizes information contained in multiple genes and thousands of base pairs. Even if we eventually learn enough about genes, developmental pathways, and epigenetic influences to read a complex morphological character from a sequenced genome, it is faster, easier, and a whole lot more fun to simply look at a giraffe to determine whether it has a long neck. Because so many molecular studies have reanalyzed relationships among already described species, it is easy to take for granted that knowledge of complex morphology exists. For the vast majority of species on earth, it does not. Working at a time of accelerated species extinction, we shortchange ourselves if we fail to complete detailed morphological descriptions alongside studies of other data sources.

DNA allows us to do some things difficult or impossible to do with morphology, such as identify individuals from a mere fragment of tissue, or associate wholly dissimilar life stages of a species. But the converse is true, too. Morphology gives us insights into species diversity and details of phylogeny impossible to glean from sequenced DNA:

• Anatomical features provide clues about the behavior and ecology of poorly known species. Insects with prognathous (forward-directed) mouthparts, for example, are reasonably inferred to be predators.
• Anatomical structures of species are frequently the result of natural selection, explaining, for example, elaborate forms and colors of orchids irresistible to their pollinators.
• Morphological characters can be directly compared with fossils.
• Morphological studies open a portal to a vast archive of biomimetic models at a time when society desperately needs ideas to inspire sustainable designs, materials, and products.¹³
• Knowledge of morphology, and the range of its variation within species, allows field biologists and amateur naturalists to identify, observe, and enjoy many species in the wild.
• Morphology is the primary driver of our curiosity to understand the story of evolution and the origins of species’ improbable, weird, and wondrous attributes.

Processes and Patterns

The logical progress of Hennig’s conceptual revolution has been derailed by an overemphasis on the importance of DNA data, disproportionate attention on identifying rather than truly knowing species, a misguided infatuation with technology for its own sake, and a kind of hubris that befalls many who control the lion’s share of funding in the field.

Nelson pointed out a disturbing similarity between the way paleontologists saw the fossil record in the early twentieth century and how molecular systematists treat sequence data today.¹⁴ William Matthew, for example, believed that fossils and their stratigraphic appearances reveal phylogeny directly, in a way unattainable by studies of living plants or animals.¹⁵ Today, many see similar phylogenetic revelations in sequence data, but both suffer from the same fallacy. Stratigraphic imperfections, differential fossilization, convergent evolution, and a host of other factors make it naive to think that cladogenetic history can simply be read from any data source. Molecular data is frequently interpreted based on overall similarity—as in the failed school of phenetics popular in the 1970s—or unjustified assumptions about evolutionary processes, as in the discredited evolutionary taxonomy of the same period. Did we learn nothing from the conceptual battles of that time?

Biologists have long suffered from physics envy. Biologists in general suffer from biomedical envy. And systematists in particular have fallen victim more than once to experimental envy. No less rigorous, the epistemology of taxonomy is widely misunderstood because it is non-experimental. Any approach with the veneer of experimentation, statistics, or high-tech is elevated in stature whether the science justifies it, or not. A leading biologist said to me that molecular studies do not receive more funding than morphological ones because their data is superior; instead, DNA is considered better data because it gets more money. In the case of phylogenetic systematics, the best evidence is in the form of synapomorphies, not any particular kind of data. Morphology is easily partitioned into characters of sufficient complexity as to be less subject to repeated convergent evolution. Of simple characters, such as color or a few DNA base pairs, one cannot say the same.

Taxonomists themselves share blame in derailing the advance of Hennig’s phylogenetic systematics. So much emphasis was placed on retrieving the best phylogenetic trees that we lost sight of the importance of describing the species themselves. People were looking for data sets to plug into the latest phylogenetic computer programs, and DNA provided data, lots of it. This resulted in a shift away from deep thought about individual characters, evidenced both by the paucity of monographs and the publication of trees devoid of the interesting attributes of species they purport to explain. Regrettably, technology is an easy sell in our culture that values the new over the tried-and-true, the technologically complex over the simple, and the costly over the inexpensive. I knew a program officer at the National Science Foundation who with a straight face told me that a particular investigator’s research was clearly the best in his field because he received the most funding. As well expressed by David Orr,

Unable to separate can do from should do, we suffer a kind of technological immune deficiency syndrome that renders us vulnerable to whatever can be done and too weak to question what it is that we should do.¹⁶

Ignorant of 80% of our world’s animals and plants, and facing the threat of a mass extinction event—the loss of 70% or more of earth’s species—within 300 years,¹⁷ what we should do is clear. We should complete a taxonomic inventory of life on earth with knowledge of species organized into a phylogenetic classification. Such an inventory, grounded in detailed species descriptions and backed up by museum specimens, would reestablish the progression from Linnaeus to Darwin to Hennig. It would result in a permanent record of the diversity of life as it exists in the early Anthropocene. It would assemble the material on which studies of the history of species and their attributes could continue indefinitely, regardless of how many or which species survive. It would open a treasure trove of ideas to inspire engineers, inventors, and entrepreneurs to conceive more sustainable ways to meet human needs through biomimicry. Coupled with geographic and ecological information about specimens, it would constitute a record of the organization of the biosphere prior to the ecosystem disruptions of species losses. And, by telling us what species exist and where, it would enable what may be our best hope for biodiversity conservation, a knowledge-informed selection of places to set aside to fulfill Edward O. Wilson’s Half-Earth vision.¹⁸

Meet a fellow biologist and you are likely asked, “What question do you study?” If you reply that you study instead a taxon, say dragonflies or composite flowers, don’t be surprised if they walk away. Asking how things work and relying on the experimental method are so ubiquitous in contemporary biology that any observational, non-experimental study is looked upon with suspicion. Yet, thanks to Hennig, taxonomy is replete with hypotheses as and often more rigorously scientific than well-designed experiments. This is because so many taxonomic hypotheses are all or nothing claims about the distribution of attributes among species and higher taxa.¹⁹ The observation of a single exception, one specimen, is sufficient to falsify such a hypothesis.

No single source of confusion has been more damaging to systematic biology than the failure to distinguish between pattern and process in general, and, specifically, between studies of species and speciation. Both are interested in species, of course, yet their epistemologies, theories, and methods are entirely separate. Population geneticists are interested in the processes by which species come to be, the generation and spread of mutations within populations, and the various forces of natural selection. This means that they do not study species, but populations of species-in-the-making. Systematists are interested in determining what species exist, understanding the pattern of similarities and differences among them, reconstructing their cladogenetic history, and summarizing it all in a predictive, informative, phylogenetic classification.

Referring to the course of human events, Søren Kierkegaard said that history must be lived forward, but can only be understood by looking back. It is the same with species. Geneticists study species formation as it happens, using a combination of experimentation and observation. Systematists are historians, thus limited to reconstructing observable patterns. It is intriguing to speculate about processes behind patterns, but given the many contingencies in evolution,²⁰ this can be no more than informed guesswork. Thanks to Hennig, however, patterns of common ancestry may be reconstructed in such a way as to be open to objective, critical testing.

Staying the Course

As George Santayana wrote, “Those who cannot remember the past are condemned to repeat it.”²¹ Not heeding the lessons from Hennig’s revolution,²² we have already begun to repeat mistakes of the past, such as grouping species based on overall similarity or naively believing we know enough about the evolutionary events of the past to ascribe a particular process to a specific speciation event. We have forgotten the important distinction between phylogenetic trees and cladograms. I have intentionally made the same mistake here in order to conform to current usage—better, misusage—of the terms. We have overlooked the importance of teasing apart traits and characters.²³ Failing to insist on synapomorphies rather than mere similarity, we foolishly accept data based on its source rather than being more critical of its analysis.

Reducing taxonomy to a single source of data or a mere identification service would be an intellectual and scientific mistake. Far from a service, fundamental taxonomy is one of the most audacious of all sciences. Astronomers have boldly set out to observe, describe, and map all the kinds of things in the heavens, from planets and stars to black holes and dark matter; cosmologists dare to explain it all, from the instant of the Big Bang to the universe as it is today. Taxonomists are on a mission to do for life on earth what astronomers and cosmologists are doing for the universe: discovering and describing its millions of kinds of animals, plants, and microbes, reconstructing the pattern and sequence of relationships among taxa and their synapomorphies, and understanding their origins and multi-billion-year history.

It was such patterns among species documented by taxonomists that led Darwin to seek an explanatory process. It is taxonomic knowledge that undergirds conservation, natural resource management, and the use of tens of thousands of species by human civilizations. And it will be taxonomy that paves the way to a sustainable future, enables us to set and achieve conservation goals, and continues to enrich our intellectual lives by increasing our knowledge of our place in the living world. We flatter ourselves with things uniquely human, but such arrogance is misplaced. As Platnick explained, all characters of an ancestral species are inherited by descendant species, either in their original or some subsequently modified form.²⁴ Thus, our impressive brains are no more than slightly larger and differently wired versions of earlier primate brains, and our agile arms and fingers mere modifications of quadruped forelimbs, along with bat wings and penguin flippers.

In both cosmology and taxonomy, we have only begun our journey of discovery. Most of what exists in the universe we have not seen and do not yet know about. Similarly, we are ignorant of the vast majority of living species and what makes each unique and interesting. In one of few truly unique human traits, we are curious about what exists and where it came from. It is our intellectual destiny to understand our world, selves, and universe. After hundreds of years of pondering the meaning of species diversity, Hennig has moved us to the cusp of breakthrough answers. If we ignore Hennig’s conceptual advances and systematic biology’s intellectual mission in order to meet immediate practical needs to identify species, or settle for one data source when we can have many, then we will condemn our own and future generations to uncorrectable ignorance about biodiversity and its origins. We owe it to ourselves and all curious humans who follow not to forget Hennig or allow his revolution to remain unfinished. We must not allow sirens of large research grants, or the popularity of the latest technology, to steer us off course.