from types to individuals: hennig’s ontology and the development of phylogenetic systematics

From types to individuals: Hennig�s ontology and the development ofphylogenetic systematics

Andrew Hamilton*

School of Life Sciences, Arizona State University, Tempe, Arizona, USA

Accepted 10 July 2011

Abstract

Contemporary phylogenetic systematics was framed, in part, as a response to a resurgent idealistic morphology in the German-speaking world in the first half of the 20th century. There were also conceptual and methodological challenges from Anglo-American researchers who were sceptical about whether a phylogenetic approach to systematics could be made to work. This paperdescribes these challenges as a way of providing context for some ontological innovations made first by Walter Zimmermann andthen by Willi Hennig. The principal argument of this paper is that what has become known as the individuality thesis played a muchmore important role in the conceptual foundations of Hennig�s version of phylogenetic systematics than has been widelyappreciated. Understanding Hennig�s ontology illuminates his responses to objections to phylogenetic systematics from both sides ofthe Atlantic and sheds substantial light on the extinction part of the dichotomy rule. Although many have taken Hennig�s claim thatparent species go extinct at speciation to be an arbitrary and biologically unrealistic rule, extinction of the parent follows directlyfrom the way Hennig understands species and how they are individuated.

� The Willi Hennig Society 2011.

In recent years scholars from several fields havecontributed to a nuanced understanding of the contextand details of Willi Hennig�s revolutionary contribu-tions to phylogenetic thinking, making clear the extentto which Hennig was arguing against the idealisticmorphologists in his 1950 book (Hull, 1988), howheavily he drew on the work of Walter Zimmermann(Donoghue and Kadereit, 1992), his epistemology(Dupuis, 1984), and how theoreticians and philosophersincluding Rudolf Carnap, Theodor Ziehen, and Ludwigvon Bertalanffy influenced his views on metaphysics andexplanation (Rieppel, 2007).

This paper builds on an emerging focus on thehistorical and conceptual foundations of 20th centurysystematics, exploring Hennig�s views on the individu-ality and objective reality of both species and mono-phyletic groups of species (clades). The main aim of thispaper is to make clear just how foundational the notion

of individuality for species and clades was to Hennig�sthinking and further to show how he used this idea toformulate a theory of systematics that he took to be freeof the conceptual problems raised by early critics. Thesecritics argued that even if phylogenetics made concep-tual sense as a basis for systematics and even if speciesand higher groups are real, there were still technicalhurdles about how to rank ancestral groups, the extentto which evolution has been convergent, and whetherphylogenetic information could inform systematicswithout being disruptive of existing classifications.

A focus on Hennig�s thinking about individuals alsosheds light on one of his more controversial ideas.Hennig (1950, 1966) famously argued that parent speciesshould be treated as having gone extinct when they gaverise to new species in a speciation ‘‘event’’. Manycommentators have taken this idea to be biologicallyunrealistic and therefore to be a methodological con-cession on Hennig�s part because such a ‘‘rule’’ leads toneat branchings that make lineages easy to track (Hull,1988; Schmitt, 2011). The extinction of the parent atbranching, however, is not only consistent with Hennig�s

*Corresponding author:E-mail address: [email protected]

� The Willi Hennig Society 2011

Cladistics 28 (2012) 130–140

Cladistics

10.1111/j.1096-0031.2011.00372.x

thinking about species and clades as individuals, but isrequired by it. This rule, methodologically helpful as itmay be, is built into the basic structure of Hennig�sapproach to individuating phylogenetic groups and hecannot do without it, at least as his views wereformulated between 1947 and 1966.

Before turning to the set of concerns that form thebackground for Hennig�s ontological thinking, it may beworthwhile to note the separateness of two lineages ofideas about individuality for species. It is not the thesisof this paper that Hennig subscribed to a version ofGhiselin�s individuality thesis. On the contrary, Hennig�sthinking about individuality seems to have been moti-vated by quite different concerns than the ‘‘speciesproblem’’ issues Ghiselin addressed in his 1974 paperand the related ‘‘metaphysics of species’’ issuesaddressed in his 1997 book (Ghiselin, 1974, 1997).

Hennig�s motivations and innovations with respect toideas about individuality are discussed in detail belowas a means for better understanding the theoreticalunderpinnings of Hennigian systematics as well as forgetting a more complete grasp on the shape ofcontemporary controversies about the ontology ofspecies. It is not the argument of this paper thatcladists do or should embrace the individuality ofspecies, but rather that individuality was central toHennig�s thinking.

Early criticisms of phylogenetic systematics

The idea of a phylogenetic systematics can be dated toCharles Darwin, who predicted in 1859 that ‘‘ourclassifications will come to be, as far as they can be somade, genealogies’’ (Darwin, 1859, p. 486). ErnstHaeckel advanced this idea, coining the terms ‘‘phylog-eny’’, ‘‘phylogenesis’’, and ‘‘monophyly’’ in 1866 (Haec-kel, 1866). He also drew what he took be lineages intotree forms, but he seems not to have seriously faced anyof the difficulties that arise when one tries to offer aprincipled defence of phylogeny as a basis for system-atics (Naef, 1919). Neither did he attempt to articulate anew method to go with the phylogenetic approach hefavoured (Reif, 1986; Breidbach, 2006).

Between Haeckel�s initial attempts and Hennig�soffering of a mature theory in the 1960s, many theoristswere not enthusiastic about the possibility of a phylo-genetic systematics. This was especially true of thosewhose understanding of systematics was informed bypalaeontology. Although many saw the value of phy-logenetics for understanding evolutionary history, therewere widespread concerns on both sides of the Atlanticabout using it as a basis for systematics. These were oftwo broad kinds. First, there were objections about thenature of systematics and the things it studies, with somearguing that classification is artificial, that there are no

such things as species or especially higher taxa, and thatideal types could best inform classification. Second,there were more detailed sets of objections based on theperceived incompatibility of phylogenetics and system-atics.

In the German-speaking world, where idealistic mor-phology was enjoying a renaissance in the first half ofthe 20th century and palaeontologists were influential indirecting the course of biological science (Reif, 1986),the idea that systematics could or should be phyloge-netic was particularly unwelcome. The idealistic mor-phologists, who were committed to various forms of theidea that the archetype, not history or descent, should bethe basis for understanding fossils as well as forsystematic categories, objected to phylogenetic system-atics in principle, but also offered particular challengesthat were cogent even if one did not subscribe to theirgeneral philosophy.

The Austrian palaeontologist Othenio Abel, a foun-der of palaeobiology, for instance, argued in 1914 thatsystematics and phylogenetics are incommensurable onthe grounds that the Linnaean classificatory system iscapable only of expressing a relatively recent andsynchronic view of the biological world—one thatfocuses on extant species. ‘‘It really seems,’’ he wrote,‘‘as if the system and phylogeny cannot be matched,either in the form of a �phylogenetic systematics� or inthe form of a �systematic phylogenetics� ’’ (Abel, 1914, p.34). A little more than a decade earlier, AlexanderGoette, a German zoologist and developmental bio-logist, offered arguments to a similar conclusion,premised on the claim that the units of analysis anddescription in phylogenetics and in systematics cannotmap onto one another because all proper taxa arediscrete, but lineages are continuous (Goette, 1898).

Concerns about how to rank ancestral forms werealso a consideration. Adolf Naef, for instance, arguedthat ‘‘a natural (phylogenetic) system’’ that includesstem forms is ‘‘impossible’’ because ‘‘when it was extant,the ancestral kind of a descendant kind was certainlyitself a kind, as are the family, order, and class’’ (Naef,1919, p. 48). The implication is that the system does nothave a way to rank distantly extinct ancestral kinds thatmarks them as both ancestral and of higher rank thanthe species in their lineage, as well as at the appropriatelower rank for their time.

Naef is here substantially agreeing with Abel, whosearguments about what the system can and cannotexpress were driven in part by the way he understoodthe history of systematics and the way he understoodstem groups:

Originally, the ‘‘system’’ had been merely the presentation of

the upper cross-section laid in the present by the genealogy of

the animal kingdom. At the moment, though, that we managed

to trace back larger groups to their root and to detect that two,

131A. Hamilton / Cladistics 28 (2012) 130–140

three, or more families, two, three, or more suborders, etc.

merge into one stem group, it had to become clearly obvious

that the ‘‘system’’ is never able to give a clear overview of the

phylogenetic processes in the course of the history of a stem

group. (Abel, 1914, p. 33)

While Abel�s argument is somewhat difficult to track,he seems to have thought that tracing phylogenies leadsto the identification of stem groups of species that gaverise to families. These he sometimes also called ‘‘rootgenera’’ (Abel, 1914, p. 3). According to Abel, thesegroups of species do not themselves form a genus orfamily, and in fact they do not correspond to anytaxonomic rank, so there is no way to identify them inthe classificatory system developed by Linnaeus (in thisconnection see Willmann, 2003).

The phrase ‘‘idealistic morphology’’ captures a com-plex and varied movement. The suggestion here is notthat Abel and Naef represent a single historical orintellectual thread, but rather that these two importantleaders in the movement shared a scepticism about thepossibility of phylogenetic systematics that was based onthe work they thought systematics should and could do.

One did not have to be an idealistic morphologist toraise concerns over whether the Linnaean system hadthe means to rank ancestral forms appropriately,whether it could be used to give structure to somethingdeeper than the recent diversity of life, or even,independently of issues related to the Linnaean hierar-chy, whether phylogenetics should be the basis forsystematics. Questions like these were raised in theEnglish-speaking world as well, and often by those whowere quite a lot more sympathetic to a phylogeneticapproach than were most of the idealistic morpholo-gists.

The English malacologist and palaeontologist FrancisArthur Bather, who was happy to call himself a‘‘phylogenist’’ made concern over the relationshipbetween the Linnaean system and phylogenetics thesubject of his 1927 Presidential address to the GeologicalSociety of London (Bather, 1927). He argued againstphylogenetic systematics on three grounds: (i) becauseno taxa are real, the Linnaean system is not and shouldnot be a natural one—it should not attempt to capturereal lineages, (ii) convergent evolution, reticulation, and‘‘polyphyly’’ mean that phylogeny cannot provide asuitable basis for classification, and (iii) adopting aphylogenetic approach and encoding it in the Linnaeansystem would mean splitting existing taxa, most espe-cially genera, into so many groups that the genus andperhaps other higher taxa would become useless asclassificatory tools.

Bather�s practical concerns and his worries aboutconvergent evolution were fairly widespread, and therewas also a lot of controversy over the reality ofbiological taxa, including species. The Dutch botanistCornelis Eliza Bertus Bremekamp, who was an active

and prolific taxonomist as well as the author of a well-read general introduction to biology (Bremekamp,1963), rejected the possibility of a phylogenetic system-atics entirely, arguing that names for taxon categoriesindicated relative group inclusiveness but did notcorrespond to anything objective in the world. Lin-naeus�s arguments to the contrary, he wrote, were‘‘based on a quaint mysticism’’ and are of ‘‘historicalvalue only’’ (Bremekamp, 1931, p. 9).

Others held versions of this view as well. By themiddle of the 20th century, Hennig (1966, p. 78) wouldsurvey the situation and describe the long-standingconventional wisdom as being that organisms andspecies might be taken to be real, but that authoritiesincluding Plate (1914), Kinsey (1936, 1937), Simpson(1951) and Claus et al. (1932) were of the opinion thatnothing at the genus level or above corresponds to anyreal object. Although this view may have been the mostprominent one, there was no shortage of theorists whoalso denied the reality of species (Martini, 1929;Thompson, 1952; Blackwelder, 1959).

Bather�s concerns about evolution, convergence, andthe distinction between naturalness and artificiality werealso expressed by several other prominent authors. Inhis introduction to The New Systematics, J. S. Huxleypointed to a dilemma that arises when species areconsidered over time, ‘‘as palaeontologists perforcemust do’’:

When, as often happens, more or less parallel evolution occurs

in undoubtedly distinct lineages, should generic names be given

to the horizontal stages—in which case the genus is not

monophyletic—or to the lineages—when extreme practical

inconvenience will result? (Huxley, 1940, p. 18)

Here, Huxley was essentially echoing Bather: generabecome problematic if we insist that they have to bemonophyletic. He was hinting at the discussion, carriedout in detail elsewhere in the volume (Arkell and Moy-Thomas, 1940) and also by Otto Schindewolf (1928), ofthe practical problems of giving generic standing to eachparallel lineage of Devonian ammonites detectable asvertical traces through rock strata, as opposed tonaming genera as they present themselves horizontallyin the strata. Huxley was also concerned about what todo in what he thought would be the very many cases inwhich convergence and reticulation would make itimpossible to determine phylogenies.

While he was broadly supportive of phylogenetics,these considerations led him to offer a tentative argu-ment to the conclusion that phylogeny might not be thebest basis for classification on the grounds that it is not‘‘natural’’ in the right way:

A natural system is then one which enables us to make the

maximum number of prophecies and deductions. It also in the

majority of cases follows the lines of phylogenetic descent,

though these are not always discernible; but there are a certain

132 A. Hamilton / Cladistics 28 (2012) 130–140

number of exceptions where a phylogenetic interpretation is

meaningless, and others where taxonomy and phylogeny cannot

be made to square with one another (Huxley, 1940, p. 20).

Arguments to the same conclusion were offered inthe same volume by the British botanist John ScottLennox Gilmour, who would later become Director ofthe Cambridge University Botanic Garden. Gilmourargued against a phylogenetic basis for systematics onthe grounds that the correlation of a large number ofattributes gives the most natural set of groups, and thatphylogenetic classification is just one among manyspecial-purpose classificatory schemes (Gilmour, 1940,p. 473). Gilmour had offered a version of this argumentin Nature three years prior (Gilmour, 1937), and helater made it a centrepiece of his theoretical thinkingabout systematics (Gilmour, 1951, 1961). In the handsof Sokal and Sneath (1963), Gilmour�s understandingof naturalness and grouping would later play animportant role in the foundations of numerical taxon-omy (Sokal and Sneath 1963, 12 ff; Winsor Mary,1994).

By the time of the Second World War, it was not at allclear that there could be a phylogenetic systematics thatwould be consistent with the aims, goals, and methodsof systematics as it was then understood, or even thatone was desirable. For those who prioritized compar-ative morphology and were concerned about its auton-omy from phylogenetic interpretations of morphologicalstudies, a phylogenetic systematics was at best asecondary pursuit. For those who thought that system-atics is and should be artificial and should not attemptto name or order real things, phylogeny could only be adistraction, or worse, could disrupt 100 of years ofcareful arranging. Others who thought species andperhaps even some higher taxa are real but thatphylogeny is only one among many ways to find naturalgroups sought what they took to be a broader basis forsystematics.

The birth of modern phylogenetic systematics: Walter

Zimmermann

Although there is a history of movement towardphylogenetic systematics in the first two decades of the20th century (Craw, 1984; Willmann, 2003), the first realsteps toward a new theory were taken by Zimmermann(1931, 1937) in an important but long-neglected paperentitled ‘‘Arbeitsweise der botanischen Phylogenetik undanderer Gruppierungswissenschaften’’ (Methods ofBotanical Phylogenetics and other Grouping Sciences).Zimmermann was then very much championing aphylogenetic and anti-metaphysical approach to sys-tematics through a very polemical debate with thebotanist Wilhelm Troll (Zimmermann, 1930, 1937;Troll, 1928, 1937, 1951; Claßen-Bockhoff, 2001). Zim-

mermann made several innovations that would becomeimportant to Hennig�s understanding of the relationshipbetween systematics and phylogenetics, including aninsistence on the objective reality of stem groups,phylogenetic groups as naturally given, a careful expla-nation of what phylogenetic relationships are, and a newsense of the ‘‘task’’ of systematics.

Part of the force of Zimmermann�s thinking lies in awholesale reorientation of systematic and phylogeneticstudies away from ideal types and toward history as abasis for classification. For Zimmermann, ‘‘the task ofhistorical phylogenetics is to find out �how it [really]was� ’’ (Zimmermann, 1931, p. 981; also quoted inDonoghue and Kadereit, 1992), and the implication isthat ‘‘it’’ was just one way—that there is a unique fact ofthe matter about the history of life on Earth. Given thisapproach it is not surprising that Zimmermann arguedthat systematists should ‘‘group phylogenetically, thatis, following naturally given relationships’’ (Zimmer-mann, 1931, p. 949). For Zimmermann, lineages werenatural and objective as opposed to artificial andsubjective, and systematists should regard them as suchand build classifications accordingly (Zimmermann,1937).

Grouping phylogenetically, of course, requires anunderstanding of what phylogenetic relationships are.Zimmermann�s definition reads as a very contemporaryone: a dendrogram with three terminal taxa A, B, and C,where B and C share a recent ancestor not also sharedby A, should be read as ‘‘B and C are more closelyrelated to one another than to A’’ (Zimmermann, 1931,p. 989). This way of thinking represents a fundamentalshift because it defines the systematists� relation of studyas the relation between sister descendants of a commonancestor, and because it points to hierarchy as theappropriate general structure for a phylogenetic system.Species B and C combine to form higher group B + C;Species A, B and C combine to form a still moreinclusive higher group A + B + C, and so on forgroups in which there are more taxa. As we will see,Hennig made much of both of these innovations as wellas of Zimmermann�s understanding of what phyloge-netic relations are.

Hennig on individuality

Hennig adopted and adapted Zimmermann�s argu-ments about the importance of history and the reality ofphylogenetic groups and used them to develop whatHennig explicitly thought of as a new theory forsystematics (Hennig, 1950; Rieppel, 2007), whichincluded not only the new vocabulary and method withwhich contemporary systematists are familiar, but also aset of ontological arguments that he used to justify thismethod, inform his new vocabulary, and direct attention


toward sister groups and their relations as topics ofspecial interest for the systematist.

By the time his 1966 book was published, Hennig haddeveloped quite sophisticated views about biologicalindividuality as a way of dealing with species as well aswith higher taxonomic categories and their reality, andas a way of providing a justification for phylogenetics asthe appropriate basis for systematics. In contrast tomost of his peers, Hennig was quite willing to acceptwhat Michael Ghiselin later approvingly called the‘‘radical’’ view that anything—including species andgroups of species—that is held together by the appro-priate causal, spatial, and temporal connections isappropriately recognized as an individual (Ghiselin,1974).

This line of Hennig�s thinking has not been widelyappreciated in the literature (but see Richter and Meier1994; Rieppel, 2007), and this has been the case fromearly on: Lars Brundin, from whose 1966 monographmany English-speaking systematists learned of Hennig�swork (Williams and Ebach, 2007), made no mention ofHennig�s commitment to individuality for species andclades, although he did offer an otherwise thoroughintroduction to Hennig�s version of phylogenetic sys-tematics. This lacuna in Brundin and others is surprisinggiven that Hennig�s attention to ontology dates to hisfirst theoretical paper, ‘‘Probleme der BiologischenSystematik’’ (Problems of Biological Systematics), pub-lished in 1947. There Hennig argued for the reality ofsuper-specific groups on the grounds that if species arereal, and in speciating they give rise to a new group ofspecies, the new group is no less real, so long as all thespecies in it stand in the sort of relation that Zimmer-mann had defined as ‘‘phylogenetic’’ (Hennig, 1947, p.279). Hennig�s Grundzuge contains arguments along thesame lines in which Zimmermann is discussed and oneof his dendrograms is reproduced (Hennig, 1950, p.115).

In arguing for the reality of higher groups, Hennigpoints out that groups of individuals can themselves beindividuals. His examples of higher-level individualsinclude metazoans that are individuals made up of cellsthat are also individuals, bee colonies that are made upof organisms (bees) that are also individuals, and groupsof species: ‘‘there can be no doubt that all the supra-individual categories from the species to the highestcategory rank, have individuality and reality’’ (Hennig,1966, p. 81). The argument is that lineages are real andthey can be divided into segments that correspond toranks in the Linnaean system, although Hennig alsosuggested other, non-Linnaean ways of ranking taxa(Hennig, 1969, p. 30).

Hennig pressed this view of ontology into service forunderstanding both what species are and why theyshould be taken to be the basic unit of phylogeneticsystematics. He viewed species as what might be called

scattered individuals, individuals that are held togetherby something other than skin or a membrane, and heviewed clades as groups as individuals made up ofappropriately related species. He wrote:

… all categories of the phylogenetic system are characterized by

individuality and reality, in contrast to the abstract and timeless

categories of the morphological system. This does not mean

that they are all ‘‘individuals’’ in the same sense as the

individual organisms that are the units of life, and which we

ordinarily call individuals (Hennig, 1966, p. 83).

Drawing on the philosophical work of Nicolai Hart-mann (1942), the conceptual work of Max Hartmann(1947), Ziehen (1934 [1939]) and the logicians Woodger(1952) and Gregg (1954), Hennig understood the cate-gories of the phylogenetic system to be individuals in thesense that ‘‘they are all … segments of the temporalstream of successive breeding populations. As such theyhave a beginning and end in time.’’ (Hennig, 1966, p. 81;emphasis added). But what does this mean?

For Hennig, species are held together by tokogeneticrelationships, which he thought of primarily asinterbreeding relationships. Hennig can be read as offer-ing a reinterpretation of Mayr�s understanding of speciesin terms of individuality: species begin when the appro-priate interbreeding relationships obtain and end whenthey cease to hold. Species-hood, for Hennig, is arelationship between organisms over time, and notdirectly about the persistence of member organismsthrough time. Similarly, Hennig individuated groups ofrelated species—clades—by their beginning and endingpoints: a clade begins when a species branches and endswhen there is a further branching in one of the daughterspecies (Haber and Hamilton, 2005; Hamilton andHaber, 2006). What is important to notice here is thatHennig realized that individuals must have a beginningand end in time, and that this is more important than thefunctional or bodily coherence that we normally think ofas constitutive of individuals. Also, he is quite clear thatideal types have neither individuality nor reality preciselybecause they are ‘‘timeless’’ (Hennig, 1966, p. 81).

This way of understanding organisms, species, indi-viduals, and the relationships between them pushes backagainst idealistic morphology in some obvious ways: ittakes species and groups of species to be real and pointsto a method for tracking them that requires paying closeattention to lineages and their histories—that is, to theirbeginning and end points. What is a less obvious is theway Hennig�s ontology informs the details of his theoryof phylogenetic systematics, and precisely how he getsfrom arguing for the reality and individuality of theobjects studied by systematics to the view that aphylogenetic basis for systematics is not only to bepreferred for philosophical reasons, but also can bemade to cohere well with the methods, goals, andconceptual structure of systematics.


Individuality again: phylogenetics as the basis for

systematics

Hennig seems to have appreciated that pointing outthat lineages and parts of lineages are both real andindividuals would probably not be enough, all by itself,to move his sceptical colleagues toward acceptingphylogeny as the best or most appropriate basis forsystematics. He readily acknowledged that there couldwell be more than one basis, and that he had to providegood reasons for his approach.

In a section of the first chapter of the 1966 volumeentitled ‘‘The Phylogenetic System and its PositionAmong the Possible and Necessary Systems in Biology’’Hennig concluded ‘‘that the choice of a general referencesystem for biological systematics is not at all free, butfor intrinsic reasons must be the phylogenetic system’’(p. 23). This claim comes after the articulation of a set ofontological arguments to the conclusion that genetic andgenealogical relationships between individual organismsform a ‘‘complex of individuals’’ that have a reticularstructure. These complexes, he argued, are species, andgaps in this structure are species boundaries (Hennig,1966, p. 18).

Hennig offered four numbered arguments for aphylogenetic basis for systematics (and for phylogenetic

systematics as the general reference system for all ofbiology), but he took the first of them to be ‘‘decisive’’:

Making the phylogenetic system the general reference system

for special systematics has the inestimable advantage that the

relations to all other conceivable biological systems can be most

easily represented through it. This is because the historical

development of organisms must necessarily be reflected in some

way in all relationships between organisms. Consequently,

direct relations extend from the phylogenetic system to all other

possible systems, whereas there are often no such direct

relations between these other systems. (Hennig, 1966, p. 22)

This passage is a good example of Hennig�s famouslydifficult prose, and not the best work of his translators,but it is somewhat clearer in context what Hennig meantby these claims. His view of the ontological structure ofthe objects studied by phylogenetics, captured in Fig. 1(Hennig�s well-known fig. 6), was that phylogeneticsstudies nested groups: inclusive or higher clades aregroups of clades, clades are groups of species, species aregroups of organisms, and organisms are groups ofsemaphoronts. Semaphoronts—organisms at particularperiods in their development—are, Hennig argued, ‘‘theelement of all biological systematics’’ (1966, p. 6),because the semaphoront is all the properties (morpho-logical, physiological, and psychological) of an organ-ism at a particular developmental stage. Arrows in the

Fig. 1. Hennig�s (1966) figure 6, showing the relationship between temporal and developmental parts of organisms, organisms, species, and groupsof species. See text for further explanation.


circle at the bottom right of the diagram indicate thatthe ‘‘individual’’, by which Hennig means ‘‘organism’’,is a group of semaphoronts.

Arrows in the main body of the diagram point fromorganism to organism (open and closed circles, whichrepresent females and males, respectively) and weave a‘‘fabric of tokogenetic relationships’’. These are rela-tionships that hold between organisms such that theyform species (Hennig, 1966, p. 30). On Hennig�s view,new species arise when tokogenetic relationshipsbetween groups of organisms break down and new onesare formed. This process is represented by the wedge atthe fork of the main diagram. Notice that at the terminalends Hennig has circled two daughter species. Therelation between them is phylogenetic, not tokogenetic,although tokogenetic relations are the ‘‘glue’’ that holdseach species together. The phylogenetic relationshipbetween these two species is represented by a branchingshape on the right of the diagram.

The argument for the primacy of phylogenetics as thebasis of the system is ultimately that in representingphylogenies (lineage relations between groups of spe-cies), phylogenetics also captures and expresses non-phylogenetic relations between organisms (tokogeneticrelations) and between developmental stages of organ-isms (ontogenetic relations) in ways that other bases forthe classificatory system do not. This is a virtue, Hennigargued, because alternative ways of approaching thesystem can be expressed through a phylogenetic system,but not vice versa. The ideological and morphological(overall similarity) approaches leave one or another levelof ontology out. The other three arguments Hennigoffered in favour or phylogenetic systematics are largelypractical, and it is telling that Hennig thought that theargument about what phylogenetic systematics canrepresent, as well as how ontogenetic, organismal,tokogenetic, and phylogenetic relationships should berepresented turned importantly on his understanding ofwhat these relationships and entities are. As thisargument has been little noticed, there is room to doubtwhether it was as decisive as Hennig thought, but it isclear that Hennig�s ontology of nested individuals driveshis reasoning.

Answering objections: convergence and ranking stem

groups

Hennig�s ontology also has several important impli-cations for meeting the concerns discussed above aboutreticulation and convergent evolution on the one handand the problem of ranking stem groups on the other.The problem of reticulation and convergence is partlyaddressed by the hierarchical way in which firstZimmermann and then Hennig understood phylogeneticrelationships. It has been well appreciated that in

Hennig�s system, tokogenetic relations can be reticulate,but phylogenetic relations cannot; the splitting ofmonophyletic groups into groups that are themselvesmonophyletic necessarily describes a hierarchy of nestedgroups. Zimmermann hinted at this point while discuss-ing the definition of ‘‘phylogeny’’ in his 1931 paper,Hennig developed it in his 1950, 1957 and 1966 works,and it has been discussed by many others since (Plat-nick, 1977; Hull, 1988; Williams, 1992; Rieppel, 2009).

It follows from the reading of Hennig offered herethat reticulation at the population or species level cansafely be ignored by the systematist because it does notbear on phylogeny. Phylogenetic reticulation is impos-sible not just because of the way phylogeny is definedand represented—a trick of the logic—but becauseconceiving of species as individuals made up of organ-isms that are unified into a higher unit by tokogenymeans that a breakdown in a particular tokogeneticrelationship is, ontologically speaking, a speciationevent. On this view, a re-constitution of a previoustokogenetic relationship, inasmuch as it makes sense totalk of such a thing, is not the collapsing of two sistergroups into one, but rather a further branching—themaking of a new individual from old parts, as it were. Itshould be noted that this line of thinking applies wellonly to sexually reproducing species of organisms thatdo not have reticulate inheritance through hybridizationor lateral gene transfer (Hennig, 1965).

Against this reading of the logic of Hennig�s thinkingand his ontological commitments, it might be pointedout that Hennig (and Brundin) also subscribed toDobzhansky�s population-based way of thinking ofspeciation (Hennig, 1966, p. 20; Brundin, 1966, p. 14),and that this approach, as Dobzhansky and others haveexplicated it, does not require extinction of the ancestorat speciation. The thesis that species are individuals andtherefore have a beginning and end in time and the viewthat species are ‘‘syngameons’’ (Dobzhansky, 1935) andare therefore bounded by their gene-flow patterns arenot in conflict. Hennig is free to accept both, and I havebeen arguing that he did so partly because taking thisview allowed him to address certain problems that arisefrom uniting phylogenetics and systematics. Dobzhan-sky, of course, felt these problems much less acutely.

Noticing that phylogenetic relations cannot be retic-ulate does not, of course, answer all of Bather�s andHuxley�s concerns about whether convergent evolutionor ‘‘polyphyly’’ will confound attempts at reconstructingphylogenies, but Hennig�s shift toward individualitydoes change the conversation by providing a newunderstanding of precisely what is being tracked, whattools will be necessary to do so, and the relativeimportance of various objects and the relationshipsbetween them. It is something of an accident of historythat at the time Hennig was prepared to offer a maturetheory of phylogenetic systematics, along with an


accompanying method for hypothesis formation andtesting that was based on it, many of the debates fromearlier in the century about the possibility of doingphylogenetics at all had subsided. Early scepticismamong palaeontologists who thought they detectedmany parallel lines of evolution in the fossil recordand therefore were sceptical about finding good evidencefor phylogenies gave way to more positive views as themodern synthesis pointed to a tidier evolutionaryhistory than many had feared (Bowler, 1996).

A second consequence of Hennig�s ontological com-mitments is the conclusion that stem groups are neces-sarily of species rank, because new clades are createdwhen species branch. Hennig argued that the ancestorfor a given taxon will always be a species, based on hisdefinition, adopted from Zimmermann, of what (mono-phyletic) phylogenetic relations are:

… to every taxon in the phylogenetic system there corresponds

a ‘‘stem species’’ from which all of the species included in the

taxon have arisen. It is also evident that in the phylogenetic

system, the species included in each higher taxon must be

derivable from a common stem species, and that no species

having arisen from this stem species can be placed outside this

taxon (Hennig, 1966, p. 71).

From understanding the relevant parts of the biolog-ical world as hierarchically nested individuals that makeup other, higher-level individuals, Hennig was able toderive an answer, for free, as it were, to the question ofhow to rank stem groups. For him, higher groups arejust branched species lineages, and ultimately, the stemgroup must always be a species. Here again, theontology is helping to drive his conclusions in asubstantial way. Groups of species are the smallest unitamong which phylogenetic relationships obtain becauserelationships between organisms are tokogenetic; cladesgiving rise to other clades is perhaps not precluded byHennig�s logic (see Hamilton and Haber, 2006; Rieppel,2009; Haber and Hamilton, 2010), but it is true that inhis system each will ultimately have an ancestor species.

Individuality, hierarchy, and the dichotomy rule

Beginning in his 1950 book, Hennig posited what hascome to be known as the dichotomy rule, which saysthat parent species go extinct when their lineages splitinto exactly two daughter species, regardless of the fateof the individual organisms that make up the parentspecies and whether some or all of them persist acrossthe speciation boundary. This is counterintuitive, somehave argued, as there is no good biological reason thatbranchings cannot give rise to more than two daughterspecies and because it is odd to think of living organismsas belonging to an extinct group. Some early commen-tators reacted quite strongly to what they took to be

arbitrary rules that do not reflect biological reality(Darlington, 1970; Mayr, 1982).

Hennig justified the rule in 1966 as ‘‘primarily no morethan a methodological principle’’ (1966, p. 211), havingsaid similar things already in 1950 (p. 102). The rule is nowwidely regarded by systematists to be a marking conven-tion (Hull, 1988; Rieppel, 2010; Schmitt, 2011). Othershave also argued at length that Hennig meant to offersomething more than a convention, including those whowere not friendly to his work (in this connection see Hull,1988, p. 147; Darlington, 1970). As Olivier Rieppel (2009,p. 2) has recently pointed out, some of Hennig�s ownwritings seem, at least, to point to the conclusion thatHennig first formulated the extinction part of the dichot-omy rule as an ontological, rather than methodological,principle.

Support for this view is also to be found in the wayHennig understood the objects of his study. As we sawabove, Hennig�s view of species has it that species beginwith one lineage-splitting event and end with anotherone. This necessarily implies the extinction of the parentspecies—where extinction means that the relevanttokogenetic relations cease to hold—even if it does notensure a dichotomous split or point to the death of allthe organisms in the parent population. Hennig wasvery well aware that part of what it means to be anindividual is to have a beginning and end in time.Indeed, following Hartmann (1942), he argued thatduration in time is the most important feature forindividuating spatially scattered individuals: ‘‘temporal-ity is the only characteristic of reality and individuality’’(Hennig, 1966, p. 81). The dichotomy rule, for better orworse, is not only a methodological principle intendedto keep the diagrams tidy. Rather, it is an importantpart of the conceptual underpinnings of Hennig�sversion of phylogenetic systematics, and, as we haveseen, is part of his justification for a phylogeneticapproach, part of his argument for a particular kind ofhierarchy, part of his argument for the reality of highergroups, and part of his argument that phylogeneticrelationships only obtain at the species level and above.

Hennig�s claim that parent species go extinct at splittingevents is not quite so counterintuitive as itmight appear atfirst blush. When species are understood as compoundindividuals in Hennig�s sense, they are not only orprimarily groups of organisms that share commonfeatures that are not shared by other groups. On thecontrary, as I argued above, species-hood for Hennig hastwo components: organisms and tokogenetic relation-ships between organisms. On this way of understandingspecies, it is possible that the very same organisms persistacross a speciation boundary but do so by engaging innew patterns of tokogenetic relations. Because Hennig istracking the organisms and the relevant relations betweenthem, he will mark the former relation pattern as havingconcluded when the new one began. As he put it:


When some of the relationships among the individuals [read:

organisms] cease to exist, the species disintegrates into two

species and itself ceases to exist as a species. It is the common

ancestor of the two daughter species. Both daughter species

share a first-order phylogenetic relationship. They constitute a

group category of a higher order. (Hennig, 1950, p. 102)

This breakdown in relationships is the force of thewedge in Fig. 1. When old patterns of interbreeding arereplaced by new ones, the old pattern—the old com-pound individual—goes ‘‘extinct’’ even if many or all ofthe organisms that made up the former species-individ-ual go on to become part of the new one. Extinction ofthe parent species at splitting is counterintuitive biolog-ically only if one understands extinction to mean that allthe organisms die, rather than that a new compoundindividual is formed.

Conclusion

I have argued here that Hennig was more committed tothe individuality of species and clades than has beenwidely appreciated, and that this commitment was animportant part of his attempts to offer a new theory ofphylogenetic systematics while addressing importantobjections that had been raised from several quartersabout the reality of species and higher taxa, the course ofevolution, and how systematics could be made to workwithin a phylogenetic framework, including, in particu-lar, how to rank the distant ancestors of more recentgroups. I have also argued that Hennig�s commitment tospecies as compound individuals goes at least somedistance toward explaining part of the dichotomy rule.On this reading of Hennig, the extinction part of thedichotomy rule is as expected and as necessary as hisconclusion that groups cannot be paraphyletic. Bothideas are surprising and counterintuitive until the onto-logical motivations behind them are understood.

It is not the case, of course, that Hennig�s focus onindividuality and the emphasis on hierarchy that itengenders settled the issue of what the appropriaterelationship is between phylogenetics and systematics.Hennig has been criticized, correctly, for not having atheory of ranking (Mayr and Ashlock, 1991) (although itmust be pointed out that the degree-of-divergenceapproach championed by Mayr and Ashlock is not wellmotivated theoretically either). The question of rank-ing—the nature of the relationship between phylogeneticgrouping and systematic classification—is exactly what isat issue in the debates over the PhyloCode. Argumentsabout natural groups versus artificial classificationsremain, and even if they are no longer informed directlyby Hennig�s ontology, his commitment to the idea thatspecies and clades are individuals is a crucial part of thehistory of the way these debates have been framed.

In addition to being of historical importance, there are,perhaps, some lessons here for contemporary workers.One reason to understand the underpinnings of Hennig�sthinking is to be clearer about our own. There are a host ofcontemporary controversies over species ontology,including whether species ought to be regarded asindividuals. Individuality, however, is a very thin notion;it onlymeans that the thing under discussion is concrete, islocated in space and time, and coheres in some way as aunit (Hamilton et al., 2009). Countenancing something asan individual leaves a lot of ground uncovered, includingwhich particular species concept or set of concepts oneought to accept. Hennig subscribed to the individualityconcept for species and for higher taxa, as well as to thebiological species concept, and to Dobzhanky�s relatedviews about the process of speciation. Hewas free to do sopartly because of a focus on higher taxa (Nelson, 1989),but also because it does not much matter from thephylogenetic perspective he adopted which species con-cept one subscribes to: what matters is that there arecohesions between organisms (tokogenetic relationships)that sometimes break down and lead to the formation ofdaughter species (phylogenetic relationships).

This way of understanding Hennig�s logic—the logicunderlying at least some of modern phylogeneticsystematics—may point to a useful way to reframecontemporary controversies over ontology. To say thatspecies and higher taxa are individuals is really to claimonly that they are real and that they participate as unitsin causal processes. How they do so, what makes themcohere, and what strengths these cohesive forces haveare topics not addressed by pointing to their individu-ality. Individuality does very little work in indicatingwhich species concept one ought to choose, but in thepresent context adopting the individuality perspectivedoes ask that one specify how one thinks tokogeneticrelationships are formed, maintained, and eventuallybreak down. The individuality framing might be a stepin the right direction if it forces clarity about disagree-ments over tokogeny and about the relationshipbetween tokogeny and phylogeny.

Acknowledgements

Julia Damerow helped with several translations fromGerman, Steve Elliott was invaluable in tracking downobscure references, and Manfred Laubichler gave goodadvice and information about 19th century Germanbiology. Two anonymous reviewers did me the favour ofpushing for clarity on several major points in the paper.I have also benefited from conversations with OlivierRieppel and Jurgen Gadau. This work was supported bythe National Science Foundation (SES-09083935) andby the Center for Biology and Society in the School ofLife Sciences at Arizona State University.


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from types to individuals: hennig’s ontology and the development of phylogenetic systematics

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