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Hennigian Thoughts. . .

I just finished Willi Hennig's modern classic on systematics theory and methodology, Phylogenentic Systematics (1966). I made some notes as I was reading, which I thought I would annotate further and share with the web community. This isn't a book review per se; those of you who are professional systematists probably already have a strong opinion about Hennig one way or another. I just figured that my notes might make interesting points of discussion. Feel free to e-mail me with your own views on these matters, be they congruent with mine or not. Page references are from the recent reissue from the University of Illinois Press (yea!).


p. 55 - bottom - "ultimate aim of taxonomy at the species level must be to recognize and distinguish as subspecies all vicarying reproductive communities, at least insofar as . . . these are not merely externally limited but also have definite distinguishing characters"; needless to say, this definition of a legitimate subspecies is very Mayrian; Mayr's 1969 definition of subspecies included the notions of "taxonomically distinct" geographic variants. While I think Hennig places a stronger emphasis on genetic kinship than reproductive compatibility in general, the discussion of species as communities of sexually-reproducing organisms on pp. 43-48 preceding the section cited above clearly shows that Hennig clung to a notion of species as defined by reproductive compatibility. Of course, I'm not the first to recognize the nonhistorical nature of Hennig's species concept; de Queiroz and Donoghue (1988), for example, briefly discuss this point. I'd like to point out, regarding this particular quote, however, that what Hennig describes as legitimate subspecies would be considered species under alternative species concepts.


pp. 59-61 - this discussion of gall wasps appears to attack the notion of chronospecies (an arbitrary delimitation of multiple species along an evolutionary "time continuum"); it's interesting to contrast Figs. 14 and 15 here with Fig. 4 on page 19, where Hennig discusses species splitting. Strange that Hennig did not note the self-contradictory argument here, on the one hand stating that the newer gall wasps colonizing only Alopecurus and lacking a distinguishing characteristic from the older wasps should not be recognized as different species, while on the other hand arguing that in a branching event, the species B must necessarily be different from A (while in an ontological sense this may be true, if B has no distinguishing characters, it's going to look a lot like A to you and me).


page 121 - this is the home of the oft-discussed "Hennig's auxiliary principle," which in Hennig's own words, goes like this: "the presence of apomorphous characters in different species is always reason for suspecting kinship." Hennig's quoting himself on this, from Hennig (1953). This principle is less ambiguously translated by Wiley et al. (1991) as "always assume homology in the absence of contradictory evidence." Although this is a somewhat unsatisfactory formulation (I would prefer to argue that we postulate homoplasy as needed, rather than generally assuming homology), it nonetheless makes the critical point that, barring evidence of homoplasy, one should not toss out potentially useful apomorphies.


pp. 130-132 - I think the text here and the accompanying figures speak for themselves; the analogy between the road maps and character congruence in phylogenetics is elegant in its simplicity and brilliant in its conception. This is one of several places in the book where I had to stop and marvel at the ingenuity of the author and his ability to explain complex concepts in pithy ways.


pp. 144-145 - Here, as well as in several other places (e.g., page 121), Hennig seems to conflate the notions of synapomorphy and homology; he describes the rigid coronas as homoplasies, which therefore cannot be synapomorphies. Well, the distinction here is clear--homology is a character state present in two taxa as a result of common descent, while a synapomorphy is a character state derived from a more primitive state and shared between or among multiple taxa. This is a particularly peculiar error for the founder of cladistics to make, although, in fairness, Hennig clearly intended for these two terms to be used simultaneously. Again, I am far from the first to recognize this, and in fact, this topic has been the subject of substantial debate over the years (a somewhat meandering and decidedly partisan review can be found in Nelson 1994). Some authors (e.g., Patterson 1982) favor the equating the concepts of synapomorphy and homology, while others (like Kluge 1993), reject this equation.


p. 145, second paragraph - as a point of interest, this discussion of the relative ages of groups and the use of Protorhyphus as an age marker for the two sister groups is the antecedent of the concept that Norell (1992) called "ghost lineages," undiscovered yet theoretically necessary lineages of taxa that connect the oldest taxon in a group to its sister in geologic time.


p. 146, bottom - In this description of kinds of nonmonophyly (say that fast ten times), Hennig delineates between paraphyly as a grouping based on symplesiomorphy and polyphyly as a grouping based on convergence; it took me a few readings to get this, but it makes perfect sense in the context of more recent definitions of these nonmonophyletic groupings.


p. 148 - Hennig uses the term "reciprocal clarification" to describe the relationship between phylogenetics and biogeography; although I disagree with his emphasis on dispersalism, this seems to foreshadow (to a certain degree) the development of its counterpart, vicariance biogeography.

pp. 159-160 - This is an interesting discussion that (inadvertently, I think) answers a frequent creationist criticism of evolutionary biology--why do no new phyla appear? The answer lies in the definition of a taxon. As Hennig says, absolute rank is not independent of group age, and sister taxa must have the same rank in a phylogenetic system; therefore, by definition, no new phyla can appear in the future. (This issue is somewhat more complicated, but I nonetheless viewed this as an intriguing and elegant argument.)


pp. 161-163 - I found Hennig's explication of the distinction between the age of origin and the age of differentiation to be rather confusing and somewhat misguided. First, these ages may not be readily distinguished (as he points out on page 163), and, therefore, the distinction, as a purely theoretical one, is less than useful. This is seen also on page 162 when he defines age of origin and age of differentiation; when the "last common stem species. . . ceased to exist as such" (emphasis mine) is a determination that is difficult, if not impossible in many cases, to make. Finally, I submit that the controversies regarding the "so-called obligatory categories of the system" which arise as a result of failing to make this origin-differentiation distinction, could be circumvented completely with the employment of an alternative taxonomic approach.


p. 163 - Hennig notes here that fossils used as an indicator of the relative age of a group can only provide minimal age estimates.


pp. 172ff. - In this discussion of biogeographic principles, Hennig seems to occasionally conflate the terms "vicariance" and "speciation," which I find a bit peculiar. Moreover, he is clearly firmly rooted in a dispersalist framework for his biogeographic explanations.


pp. 185-190 - Hennig proposes "normalizing" ranks for absolute ages by using insect groups as a standard. Unfortunately, he cannot provide a nonarbitrary justification for his choice of the insects. This is another weakness of relying on Linnean ranks.


p. 192 - I found this brief comment on the relationship between phylogenetic systematics and paleontology quite interesting. Although some of the so-called "pattern cladists" (e.g., Forey, 1982) claimed that Hennig was opposed to the use of fossils and explicitly rejected the use of fossils in phylogenetics, it is seen here that, while Hennig only reluctantly endorsed their use, he did indeed do so, and with similar restrictions as those later suggested by Farris (1976).


pp. 192-193 - Here he points out something interesting--as information from paleontology increases in scope and depth, the increase in information content aids phylogenetic classification while hindering typological approaches (by making the definition of "types" increasingly problematic).


pp. 199ff. - Hennig steps onto some very shaky ground here when he delves into utilizing phylogenies to make statements about evolutionary processes. This has been more recently suggested by proponents of maximum likelihood estimation of phylogenies who talk somewhat cavalierly about evolutionary rates and by others who attempt to derive functional explanations for the presence or absence of features in certain taxa. Even if one accepts Hennig's contention that evolution proceeds in a fashion that "conforms to law," there is no rational justification for using homoplasy as a grouping criterion or explanation. Kluge (ms. in review) discusses this idea in considerably greater detail, but I will leave it at this.


pp. 209-211 - Here Hennig discusses the idea that cladistics demands that species split in a dichotomous fashion. Of course, cladistics makes no such evolutionary assumptions. It was refreshing to see this misconception dispelled with Hennig's own words.


pp. 214-215 - Hennig's critique of Simpson is incisive. Simpson argued for the nonmonophyly of the Mammalia by pointing out that some fossil mammals share a character state with all Recent mammals. Hennig points out that all Simpson has demonstrated is the homoplastic evolution of that character; he hadn't examined the relationships among the taxa at all.


p. 219 - I'm not sure I agree with this notion of lost evolutionary potential though time, although this is a topic on which I should do some further reading. It seems to me that Hox gene research shows that much evolutionary potential remains untapped within the genome, just waiting (so to speak) for the occasion that may call for it.


p. 220 - Hennig makes an interesting and critical point here: "the action of a law is not restricted to a particular case, whereas phylogeny as a whole is a nonrepetitive process." In other words, the products of phylogeny reconstruction are descriptions of singular historical events, to which lawlike generalities cannot be ascribed (at least, to our understanding of these concepts).


p. 222 - On phenetics: "there is no definite correlation between magnitude of morphological difference and degree of phylogenetic kinship."


p. 232 - Hennig's "progression rule" is described here. The idea is that if an orthogenetic series of character states is shown in a group of taxa and correlated with the distributions of those taxa, it may be postulated that the evolution of the character within that group is connected to its geographical distribution.


That's all, folks! Feel free to e-mail me, if you like, regarding these points.



Works cited

de Queiroz, K. and M.J. Donoghue. 1988. "Phylogenetic Systematics and the Species Problem." Cladistics 4: 317-338.

Farris, J.S. 1976. "Phylogenetic Classification of Fossils with Recent Species." Systematic Zoology 25: 271-282.

Forey, P.L. 1982. "Neontological Analysis Versus Palaeontological Stories," pp. 119-157 in Joysey, K. A. and A. E. Friday, eds. Problems of Phylogeny Reconstruction. Academic Press, New York.

Hennig, W. 1953. Kritische Bemerkungen zum phylogenetischen System der Insekten. Beitr. Ent. 3, Sonderheft, pp. 1-85.

Hennig, W. 1966. Phylogenetic Systematics. Davis, D.D. and R. Zangerl, transl. University of Illinois Press, Urbana. [1999 reissue]

Kluge, A.G. 1993. "Three-Taxon Transformation in Phylogenetic Inference: Ambiguity and Distortion as Regards Explanatory Power." Cladistics 9: 246-259.

Mayr, E. 1969. Principles of systematic zoology. McGraw-Hill Book Co., New York. [cited in Mayr, E. 1982. The Growth of Biological Thought. Belknap Press, Harvard.]

Nelson, G.J. 1994. "Homology and Systematics," pp. 101-149 in Hall, B. K., ed. Homology: The Hierarchical Basis of Comparative Biology. Academic Press, New York.

Norell, M.A. 1992. "Taxic Origin and Temporal Diversity: The Effect of Phylogeny," pp. 89-119 in Novacek, M. J., and Q.D. Wheeler, ed. Extinction and Phylogeny. Columbia University Press, New York.

Patterson, C. 1982. "Morphological Characters and Homology," pp. 21-74 in Joysey, K. A. and A. E. Friday, eds. Problems of Phylogenetic Reconstruction. Academic Press, New York.

Wiley, E.O., D. Siegel-Causey, D.R. Brooks, and V.A. Funk. 1991. The Compleat Cladist. University of Kansas Museum of Natural History, Lawrence.


Last updated 2-18-00
© 1999-2000 Brian R. Warren