Pergamon Zoologica Scripta, Vol. 25, No. 2, pp. 183-185, 1996

Elsevier Science Ltd 0 1996 The Norwegian Academy of Science and Letters

Printed in Great Britain. All rights reserved 030&3256/96 $1 5.00 + 0.00 PII: SO300-3256(96)00015-3

Point of View Do w e need “phylogenetic taxonomy”?

MAGNUS LIDEN and BENGT OXELMAN

Systematisk Botanik, Goteborgs Universitet, Carl Skottsbergs gata 228. 941319 Goteborg, Sweden. E-mail: magnus.liden@systbot.gu.se and bengt.oxelman@systbot.gu.se

de Queiroz & Gauthier (1990, 1992, 1994) and de Queiroz (1992) suggest in a series of articles that we should base our formal nomenclatural definitions on the tenet of evolution, and they also give explicit examples of how this could be achieved. One of their main concerns is the legal possibility in the current code to lump or split, which (they argue) has undesirable consequences for nomenclatural stability. Their alternative system “phylogenetic taxonomy” has been positively reviewed in this journal by Sundberg & Pleijel (1994), and thoroughly discussed by Schander & Thollesson (1999, whereas wholly negative reactions are so far wanting. Schander and Thollesson very clearly analyse most of the problems with “phylogenetic taxon- omy” that we recognise below. A certain repetition of their arguments cannot be avoided, but we try to develop some points further, and we reach rather different conclusions.

Let us state from the outset that we agree with other proponents of phylogenetic systematics in considering taxa equivalent to named clades, and we consider the central role of formal taxonomy to be communication of phylo- geny. Whether it is wise or not to make reference to monophyly a formal requirement for taxon recognition is Hot our main objective to discuss here. What we question are the claims made by the above authors that the current nomenclatural system fails to accomplish the goals of phylogenetic systematics. Rather, we feel that it works admirably well. It was the recognition in “classical taxonomy” of a natural hierarchy that inspired develop- ment of a theory of evolution in the first place. de Queiroz & Gauthier (Q and G) are dissatisfied over the fact that no reference to phylogeny is obligatory in the nomenclatural codes, but the specific alternatives suggested by Q and G are certainly, as we show below, in conflict with conve- nience, stability, information content, and communication efficiency. In the current system we have the possibility to select well established categories and names for well supported clades, an advantage we shouldn’t dispose of without reflection. That the current nomenclatural system works well also for non-phylogenetic taxonomy is not enough reason to abandon it, and might even be considered a strength.

Let us first clarify the type concept, which seems to be enveloped in mystery. The zoological Code states a “.....name-bearing type provides the objective standard of

reference by which the application of the name it bears is determined, no matter how the boundaries of the taxon may change”. In the draft to the new code the intention of this paragraph is even more carefully spelled out. The Botanical code has a different wording (“A nomenclatural type is that element to which the name of a taxon is permanently attached.. .[it] is not necessarily the most typical or representative element of a taxon”), but the meaning is clearly the same. The type can thus be used to ‘point’ to a clade of which the type is (or rather, represents) a component. Nomenclatural types allow us to free systematics from typology by divorcing naming and diagnosis, and they make ostensive definitions of names possible. Neither formally (in the nomenclatural codes) nor, according to our experience, in the mind of the average taxonomist, is there any support for the claim that the characters of the type are in any sense defining properties of taxa, and we fail to understand how “..fixed classes can be unambiguously defined by the type concept” (Sundberg & Pleijel, 1994). Linnaeus’ classical formulation about characters not making the taxon (“Scias charac- terem non constituere genus, sed genus characterem”, Linnaeus, 1751), is echoed in the modern individuality thesis. Misunderstanding of the type method is probably partly behind the enthusiasm for ‘phylogenetic taxonomy’ as reflected in recent discussions on the world wide web.

It is true, however, that as a semaphoront can represent more than one (historical) individual (including ‘life’), types in the current system are not very rigid designators, and auxiliary criteria are necessary to show the intended clade (Schander & Thollesson, 1995). In the current system these criteria are informal, and the inclusiveness of for example Mammalia or Anemone is subjective. Even if we only allow monophyletic entities to be named, the same name can still be used by different authors for clades of different inclusiveness (or incorporativeness; Ghiselin, 1995). To prevent this, Q and G suggest (implicitly) that names be typified by two or more types, i. e. the application of a name is restricted to a specific clade by pointing to an ancestor (by the way, this is not - as stated by Q and G - a definition stated in terms of necessary properties, but a baptismal act; Ghiselin, 1995). In practice, however, the more-than-one-type system will be disastrous, as it may force renaming of well supported and familiar clades if our

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184 M . Lid& and B. Oxelman

ideas of intra-taxon relationships change. In practice, the flexibility (subjectivity) of the current system will favour stability.

Let us consider a real case, and please forgive us for taking a botanical example. The following discussion presupposes node-based typification. Alternatives to node-based typification (stem-based, and apomorphy- based) have similar and/or other drawbacks (Schander & Thollesson, 1995):

Angiosperms are characterized by a large number of unique synapomorphies and the monophyly is rarely challenged, but the internal relationships are controversial. Suppose that someone typified Angiospermae using the rbcL tree of Chase et al. (1993). Ceratophyllum, which is sister group to the rest of angiosperms in that tree, and some ‘higher dicotyledon’ would perhaps be selected. If evidence is later presented that Ceratophyllum is actually nested within Nymphaeales, and that some Magnoliid taxon has a basalmost position, the latter will not be a part of Angiospermae. Desirable? A zoological example is presented by Schander & Thollesson (1995). Their answer to this problem is not to use “...taxa with uncertain relationships in the formal definitions”, a solution we regard as overly optimistic. How do we prevent someone from using his/her private ‘phylogeny’ to designate types? Voting? Then, what have we achieved?

Another problem arises if a newly discovered species, sharing most of the synapomorphies of a well supported clade, falls ‘outside’ of the designated types. Agamidae is defined by Q and G as the clade stemming from the most recent common ancestor of Agama and Leiolepis. With this node-based ‘definition’ nothing outside this clade can ever be part of Agamidae. If a sistergroup is discovered which shares most of the synapomorphies of Agamidae it cannot be included, but a new more inclusive taxon will have to be described in order to accommodate it. A designation of numerous types cannot buffer the system in this case. Clearly, if we aim at formal recognition of the best supported clades (that also tend to be the most easily recognized), ‘phylogenetic taxonomy’ sensu Q and G will necessitate erection of numerous new names as knowledge accumulates.

A necessary corollary of ‘phylogenetic taxonomy’ is the abandonment of ranks and binomial nomenclature (Schander & Thollesson, 1999, thereby depriving names of much of their information, comfort, and mnemonic handles. Of course, ranks are redundant in an ‘indented list’, but taxon names do occur in other circumstances, and should carry some information even decoupled from a cladogram. In our opinion, a hierarchical pattern is best reflected by a hierarchic nomenclature in which subordi- nate levels have some sort of organizing tags. In the current system this is achieved by suffixes, and ultimately by binomials. In ‘phylogenetic taxonomy’ there is no neces- sary correlation between form and subordination.

The main advantages of the current system are: (A) its ability to reflect a phylogenetic (or other)

hierarchy with a series of nested ranks. In the Q and G system there is no correlation between the form of a name and its subordination in the hierarchy. Because the taxonomic hierarchy runs in opposite direction to phylo- geny (Lravtrup, 1973), the mandatory ranks can cause

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‘redundancy’, i.e. taxa at different formal ranks can have the same content (monotypic taxa). This is often consid- ered disturbing, but we cannot see why.

(B) its flexibility as to inclusiveness of a named entity at a specific rank, i.e. inclusiveness can be adjusted in accor- dance with tradition and convenience, should ideas on relationship change. The mandatory ranks (including species) can be selected for the best supported clades. In the newly proposed system we depend on a correct and complete picture of relationships for sound typification if stability, in particular of the names applied to the best supported clades, is desired. Otherwise numerous new names will have to be erected with new knowledge, and well known names will often refer to poorly supported or phenetically obscure clades. Concerning the least inclusive taxa (like ‘species’) the problems with new or reinterpreted information becomes enormous (see C).

(C) its strict and straightforward rules for typification. ‘Phylogenetic taxonomy’ makes typification at all ranks extremely complicated. Agamidae must be typified by reference to at least two clades, each of which must be typified by reference to at least two subordinate clades, and ultimately (Q and G avoid this point by regarding species as “population level taxa”) by sets of type specimens, attempting to represent the most distantly related popula- tions included in each species or subspecies.

Conclusion

Those who still regard the formal possibilities for lumping and splitting a shame, should mind for a second whether this is not a very modest price to pay for not having to deal with a nomenclatural system with multiple types and no ranks. If we started from scratch today in the construction of a taxonomic system aimed at reflecting and commu- nicating phylogeny in an efficient way, there are strong arguments for something very similar to the present.

Even disregarding the conclusions above, one might still ask if it is advisable to reformulate nomenclatural principles in accordance with evolutionary theory. We (or most of us) explain hierarchy by reference to phylogeny, but we do not need descent to describe hierarchy. The current system is perfectly compatible with phylogenetic systematics (sensu Hennig, 1966), yet completely indepen- dent of it. We consider this a strength. To paraphrase Voltaire, we may disagree with the world view of a colleague, but we will still defend his possibility to express it.

Acknowledgements

We are grateful to the editors and Mikael Thollesson for valuable comments.

References

Chase, M. W., Soltis, D. E., Olmstead, R. G., Morgan, D., Les, D. H., Mishler, B. D., Duvall, M. R., Price, R. A,, Hills, H. G., Qiu, Y.-L., Kron, K. A., Rettig, J. H., Contie, E., Palmer, J. D., Manhart, J. R., Sytsma, K. J., Michaels, H. J., Kress, W. J., Karol, K. G., Clark, W.

Do we need ‘3hylogenetic taxonomy”? 185

D., Hedren, M., Gaut, B. S., Jansen, R. K., Kim, K.-J., Wimpee, C. F., Smith, J. F., Furnier, G. R., Straws, S. H., Xiang, Q.-Y., Plunkett, G. N., Soltis, P. S., Swensen, S. M., Williams, S. E., Gadek, P. A., Quinn, C. J., Eguiarte, L. E., Golenberg, E., Learn, G. H. Jr, Graham, S. W., Barrett, S. C. H., Dayanandan, S. &Albert, V. A. 1993. Phylogenetics of seed plants.-Ann. Missouri Bot. Gard. 80: 528-580.

de Queiroz, K. 1992. Phylogenetic definitions and taxonomic philoso- phy.-Biol. Phil. 7: 295-3 13.

de Queiroz, K. & Gauthier, J. 1990. Phylogeny as a central principle in taxonomy; phylogenetic definitions of taxon names.-Syst. Zool. 39:

de Queiroz, K. & Gauthier, J. 1992. Phylogenetic taxonomy.-Ann. Rev. 307-322.

Ecol. Syst. 23: 449480.

de Queiroz, K. & Gauthier, J. 1994. Towards a phylogenetic system of

Ghiselin, M. T. 1995. Ostensive definitions of the names of species and

Hennig, W. 1966. Phylogenetic Systematics. University of Illinois Press,

Linnaeus, C. 1751. Philosophia botanica. Stockholm. Lsvtrup, S. 1973. Classification, convention, and logic.-Zool. Scr. 2:

Schander, C. & Thollesson, M. 1995. Phylogenetic taxonomy-some

Sundberg, P. & Pleijel, F. 1994. Phylogenetic classification and the

biological nomenclature.-Trend. Ecol. Evol. 9: 27-3 1.

clades.-Biol. Phil. 10: 219-222.

Urbana.

49-61.

comments.-Zool. Scr. 24: 263-268.

definition of taxon names.-Zool. Scr. 23: 19-25.

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