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Chordate phylogeny and the meaning of categorial ranks in modern evolutionary biology

2015; Royal Society; Volume: 282; Issue: 1807 Linguagem: Inglês

10.1098/rspb.2014.2327

1471-2954

Markus Lambertz, Steven F. Perry,

Plant Diversity and Evolution

You have accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Lambertz Markus and Perry Steven F. 2015Chordate phylogeny and the meaning of categorial ranks in modern evolutionary biologyProc. R. Soc. B.2822014232720142327http://doi.org/10.1098/rspb.2014.2327SectionYou have accessComments and invited repliesChordate phylogeny and the meaning of categorial ranks in modern evolutionary biology Markus Lambertz Markus Lambertz [email protected] Google Scholar Find this author on PubMed Search for more papers by this author and Steven F. Perry Steven F. Perry Google Scholar Find this author on PubMed Search for more papers by this author Markus Lambertz Markus Lambertz [email protected] Google Scholar Find this author on PubMed and Steven F. Perry Steven F. Perry Google Scholar Find this author on PubMed Published:22 May 2015https://doi.org/10.1098/rspb.2014.2327In a recent review, Satoh et al. [1] discuss the evolutionary relationships among chordates and several of the scenarios that have been proposed to elucidate their origin. While we believe that the article is interesting in principle, as is the topic itself, we see, however, a major conceptual problem in this study. Besides the general overview of chordate evolution and the various ideas of how it took place, the main aim of the study is to propose a new classificatory system for this taxon, which means in the view of the authors: a new categorial and rank-based scheme. Here, we will address why this approach is destined to fail.Willi Hennig (1913–1976) used his time as a prisoner of war to draft what would become one of the most important conceptual contributions to historical evolutionary biology of the twentieth century: his theory of phylogenetic systematics [2,3]. Although it took in fact several decades after the original publication in 1950, this approach eventually was adopted and further refined by subsequent authors (e.g. [4–7]), and doubtlessly became the current gold standard of how to assess the relationships of organisms. One of the fundamental conclusions from Hennig's work is that every taxon has only one sister taxon. As this article is not aimed at in-depth phylogenetic theory and its application, we will not discuss the intriguing problems that are associated with asexually reproducing organisms, hybridization and reticulate evolution or the like, within this context. However, the general principle of hierarchical and dichotomous lineage splitting, which is commonly accepted and for instance provides the basis for any computer-aided reconstructions of phylogeny, had a severe impact on the traditional Linnaean system of hierarchical categorial ranks: it proved it to be arbitrary and flawed. In fact, earlier authors had already recognized that the ranks are more or less arbitrarily chosen, although they maintained them because of the lack of any alternative (e.g. [8]).The major problem with the rank-based classificatory system is the lack of any objective criteria to assign them. If one just sticks to the rank 'Phylum', which is the main rank discussed by Satoh et al. [1], what could be a rationale to assign it to a given taxon? Is it the number of species that belong to it? Is it its relative 'deepness' of phylogenetic origin, meaning its age? One perfect example that illustrates how artificial this system is, is Trichoplax adhaerens Schulze, 1883 [9]. This species is one of only a handful of recognized extant lineages of the entire clade Placozoa, which usually is attributed the rank of a 'Phylum' [10]. On the other hand, the Placozoa indeed are one of the most [11], if not the most [12] basal taxon of metazoans. But how do we compare this 'Phylum' to something like Chordata or Arthropoda, which usually also are assigned the same rank? Both of the latter contain a tremendous diversity of species, but are simultaneously much more derived lineages (i.e. phylogenetically younger). The rank itself does not give us any clue about a systematic aspect, or in fact about anything else. For instance, if one does not know that the 'Family' Emydidae (pond turtles) is a subordinated taxon of the 'Order' Testudines, one will never be able to infer that they are turtles just from their rank.The only at least theoretically viable objective criterion would be to assign equal ranks to sister taxa only. However, this would require an almost infinite number of different qualifying prefixes (such as 'supra-' or 'infra-') and/or a similar quantity of completely new ranks to even be able to assign one to chordates, not even thinking of evolutionarily young lineages such as hominins. But even then there is a major problem: what if new fossils are found and they are nested somewhere down the stem line? What about fossils in general? In a consistent system, it would require taking into account all taxa, including extinct ones, and a re-ranking whenever something new is discovered. This goal is neither desirable nor attainable.In summary, the result of this lack of objectivity is that the rank-based system is arbitrarily chosen, lacks any comparability and furthermore does not convey any scientifically relevant information. What can we learn from these ranks? We can neither infer anything about the phylogenetic relations and position of a given taxon, nor anything about its biodiversity. It is consequently absolutely irrelevant whether chordates are ranked as a 'Phylum' or a 'Superphylum', because in fact they are neither. The only thing that (at least so far) seems unambiguous is that they represent a monophyletic taxon within deuterostomes. The important and interesting questions concern the search for the sister taxon and the phylogenetic relationships within chordates, which indeed is discussed by Satoh et al. [1] and others (e.g. [13,14]). The hierarchical rank, however, is at best superfluous.As a final conclusion, we would like to quote the late Peter Ax (1927–2013): 'Ein konsequent phylogenetisches System kann nur unter Ausmerzung aller Kategorien errichtet werden und ist in der Lehre entsprechend frei von Kategorien wiederzugeben'. ('A consistent phylogenetic system can only be constructed by the eradication of all ranks, and accordingly has to be taught without ranks', p. 19 [translated by the authors]) [11].Footnotes© 2015 The Author(s) Published by the Royal Society. All rights reserved.References1Satoh N, Rokhsar D& Nishikawa T. 2014Chordate evolution and the three-phylum system. Proc. R. Soc. B 281, 20141729. (doi:10.1098/rspb.2014.1729). 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