Willi Hennig (1913 - 1976)

The great entomologist Willi Hennig (1913—1976), founder of ‘Phylogenetic Systematics’, was born on this day. Had he lived, he’d be 98.

The Science of Systematics

In a recent post, by John Wilkins (Evolving Thoughts), there is a quote by Borgmeier (1957):

"As the science of order ("taxonomy"), Systematics is a pure science of relations, unconcerned with time, space, or cause. Unconcerned with time: systematics is non-historic and essentially static; it knows only a simple juxtaposition of different conditions of form. Unconcerned with space: geographical factors are not primary criteria in the definition of taxonomic units. Unconcerned with cause: systematics has no explanatory function as far as the origin of the system is concerned; it is merely comparing, determining, and classifying" (Borgmeier, 1957: 53).

On further reading we find this:

"Systematics is independent of the theory of descent. This is admitted today [1957] even by convinced evolutionists. The reasons are as follows. (1) Systematic methods provide definite results without reference to the idea of evolution; phylogenetics has no special methods, it is essentially the interpretation of systematic facts. (2) Systematics is a science; phylogeny is a hypothesis of a historical process containing a fundamentally unverifiable element (Thompson) and can therefore never be the foundation of a science. (3) Systematics is [an] investigation of facts; phylogenetics is often 'a dangerous play with mere possibilities' (Hennig); Kant called it 'a daring adventure of the mind'

Of course, any systematist is free to speculate on the probable phylogeny of certain species or genera, on the basis of systematic facts" (Borgmeier, 1957: 54-55; see also Williams & Ebach, 2009).

Notice the date – 1957. Cladism was gestating at the height of the Modern Synthesis. Mayr had already named his enemy – 'typology' – and created the essentialist myth. What was Borgmeier playing at? Was he an early cladist (Wilkins pers comm., 2009), or, was he someone, like Candolle (1813), who understood the importance of classification over inference?

The insistence that systematics and phylogeny should be treated separately would ally Borgmeier with Naef (1919) and Systematic morphology, rather than with the emerging numerical methods that would later predominate Hennigian cladistics. Borgmeier was not a cladist, but someone who understood the difference between phylogenetic inference and systematic classification, two fields that were as confused then as they are now. Borgmeier's message was aimed at readers of Systematic Zoology, namely evolutionary biologists and phylogenetists who insisted that their phylogenies (trees) were classification schemes (cladograms).

Borgmeier's points above may appear slightly heretical to modern day evolutionary biologists. After all 2009 is Darwin Year and a time to celebrate the achievements of evolutionary biology, rather than to dissect them. But if we do wield our scalpel at the underbelly of evolutionary biology, what do Borgmeier's three points mean for present day phylogenetics?

Decoding Borgmeier's Points for the 21 Century Phylogenetist

Point 1 "Systematic methods provide definite results without reference to the idea of evolution; phylogenetics has no special methods, it is essentially the interpretation of systematic facts." This quote consists of three parts. Take the relationship A(BC) for example. The result is definite in the sense that it states a relationship (i.e., homology, monophyly). This relationship is part of a classification, not a genealogical or phylogenetic lineage. That is, A does not necessarily have to be an ancestor of of either B or C. In fact A could be an extant mammal where as B and C could be trilobites (now extinct). This means there is no notion of time or transformation in cladograms (e.g., trilobites did not evolve from mammals). Cladograms are classifications, which depict systematic relationships that may include numerous hypothesized genealogical or phylogenetic lineages. If we skip to the third part of Borgmeier's point, namely '... it is essentially the interpretation of systematic facts', we see that phylogenetic inference comes from systematic relationships, not the other way around. In a modern context, we can hypothesize genealogical or phylogentic lineages once we discover cladograms and not the other way around (i.e., evolutionary taxonomists hypothesize lineages prior to finding classifications, hence the creation and acceptance of paraphyletic groups).

The second part to Borgmeier's point, '... phylogenetics has no special methods' is more relevant today than it was in 1957. Phylogenetic 'methods' are based only on inference. This means phylogenetic 'methods' are confused with phylogenetic models. Since models are immune to testing hypotheses, from a historical context, they fail. That is, models themselves are hypothetical and not based on actual observations. Phylogenies therefore remain hypothetical whether they fit the model or not. We will never know if A is actually ancestral to B or C through scientific methodology (i.e., testing). We can however only hypothesize which model is most 'likely', 'parsimonious' or 'similar'.

Point 2 "Systematics is a science; phylogeny is a hypothesis of a historical process containing a fundamentally unverifiable element (Thompson) and can therefore never be the foundation of a science." Point 2 proposes an interesting problem - Systematics as a science and phylogenetics as a hypothesis. Given this, the term Phylogenetic systematics appears to be an oxymoron. At one level taxa are treated in a systematic way (i.e., no concept of time or transformation), whereas on the other, characters and their states are treated as phylogenies (i.e., transformation, reversals, dating nodes etc.). Borgmeister may have seen the flaw in Hennig's system, however he didn't refer to it directly. Separating systematics as 'a science' from phylogenetics as 'a historical process' muddies the waters of cladistics. Hennigian cladistics happily confuses the two, whereas pattern cladists (sensu Brady, 1982) treats them separately (see Ebach et al. 2008). Moreover, Borgmeier's statement today would read differently; phylogenetics is now considered as a science and systematics its method. The confusion still continues.

Point 3 "Systematics is [an] investigation of facts; phylogenetics is often 'a dangerous play with mere possibilities' (Hennig)" I will not go into what 'facts' are, but for the purpose of this argument we may refer to systematics as an investigation of relationships. Point 3 is a more concise rephrasing of Point 1. Any systematic relationship may contain multiple hypothetical phylogenetic or genealogical lineages. The 'play with possibilities' becomes 'dangerous' once we use systematics to choose between them. Considering that systematics is silent about time, transformation and descent, it is impossible to use cladograms to choose one possible phylogeny over another. Other evidence is needed. After all, it is not the goal of systematics to find or propose lineages or find ancestors.

Borgmeier's three points are still relevant today. Whether phylogenetists will understand the dangers of confusing systematics with phylogenetics is another matter. The literature on this topic is readily available, but many do not realize that there is a problem. Understanding the nature of systematics and phylogenetics, their role in our research and their limitations, has more to offer than just another computer algorithm.

Malte C. Ebach & David M. Williams

References

Candolle de, A. P. (1813). Théorie élémentaire de la botanique ou exposition des principes de classification naturelle et de l'art de décrire les végétaux. Paris.
Borgmeier, T. (1957). Basic Questions of Systematics Systematic Zoology, 6, 53-69
Brady, R.H. (1982) Theoretical issues and 'pattern cladistics'. Systematic Zoology 31: 286–291.
Ebach, M.C., Morrone, J.J. & Williams, D.M (2008). A new cladistics of cladists. Biology & Philosophy 23: 153-156.
Naef, A. (1919). Idealistische Morphologie und Phylogenetik (zur Methodik der systematischen). Verlag von Gustav Fischer, Jena.
Williams, D.M & Ebach, M.C. (2009). What, Exactly, is Cladistics? Re-writing the History of Systematics and Biogeography. Acta Biotheoretica DOI:10.1007/s10441-008-9058-5.