Bradleya 2010
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Bradleya 28/2010 pages 125 – 144 An up-to-date familial and suprafamilial classification of succulent plants Reto Nyffeler 1 and Urs Eggli 2 1 Institut für Systematische Botanik, Universität Zürich, Zollikerstrasse 107, CH-8008 Zürich, Switzerland (email: [email protected]). 2 Sukkulenten-Sammlung Zürich, Grün Stadt Zürich, Mythenquai 88, CH-8002 Zürich, Switzerland (email: [email protected] ; author for correspondence). Summary : We provide a short discussion of how Anzahl Arten und Gattungen) und die vorkom - the use of molecular data and sophisticated menden Sukkulenzformen. Schliesslich disku - analytical methods has expanded our knowledge tieren wir kurz die wichtigsten neueren about the phylogenetic relationships among flow - Ergänzungen der Klassifikation der Blüten - ering plants and how this affects the familial and pflanzen und beleuchten die Argumente für die suprafamilial classification of succulents. A tree vorgeschlagenen Veränderungen soweit Sukku - diagram illustrates the current hypothesis on lenten betroffen sind. Insbesondere gehen wir their interrelationships and a table lists all 83 auf die kontrovers diskutierte Familien - families that include succulent representatives klassifikation der einkeimblättrigen Ordnung (c.12,500 species from c.690 genera), together Asparagales ein und unterbreiten Argumente für with information on taxonomic diversity (i.e. eine überarbeitete Klassifikation, welche die number of estimated species and genera) and deutlichen Variationsmuster in diesem Clade architectural types of succulence. Furthermore, berücksichtigt. we briefly discuss some important recent modifi - cations to the family classification of flowering Introduction plants and provide arguments for the proposed The advent of molecular systematic methods in changes as far as succulents are concerned. In the 1980s and their subsequent widespread use particular, we focus on the controversially has resulted in a burst of novel insights about discussed family classification of the monocotyle - the phylogenetic relationships among flowering donous order Asparagales and provide argu - plants (e.g. Savolainen et al. , 2000; Soltis et al. , ments for a revised classification that considers 2007). These findings are now increasingly the distinct variation patterns in this clade. considered for updating current classification systems (APG, 1998, 2003, 2009). Furthermore, Zusammenfassung : In einer kurzen Diskussion recently published manuals and textbooks on zeigen wir auf, wie die Nutzung molekularer plant systematics introduce these modern phylo - Daten und komplexer analytischer Methoden genetic systems to a broader audience (e.g., unsere Kenntnisse bezüglich der phylogeneti - Simpson, 2006; Judd et al., 2007; Mabberley, schen Verwandtschaften innerhalb der 2008). Blütenpflanzen beeinflusst haben, und erklären, In the past, plant classification was mainly welche Auswirkungen das auf die Klassifikation based on morphological characters, supple - der Sukkulenten in Familien und übergeordnete mented by evidence from anatomy, cytology, or Einheiten hat. Ein Baumdiagramm illustriert phytochemistry (e.g. Cronquist, 1981; Takhtajan, die aktuelle Hypothese der Verwandtschaften. 1997). Now, the possibilities offered by molecular Eine Tabelle listet alle 83 Pflanzenfamilien mit phylogenetic methods to directly analyze and sukkulenten Vertretern auf (c.12,500 Arten aus compare parts of DNA sequences allow for the c.690 Gattungen), zusammen mit Angaben über first time thorough verification of “traditional” die taxonomische Diversität (Schätzung der classification systems on the basis of large Bradleya 28/2010 125 comparative datasets of completely independent classification and naming of organisms, which is and novel information gained directly from the a challenge to the goal of continuity and stability. individual genomes. The theory and methodology Our suggestions for an updated classification of of phylogenetic systematics focuses on recon - succulents considers this tension and aims at structing phylogenetic relationships and to offering a compromise between well supported present them as hypotheses in the form of tree facts and the stability of long established classifi - diagrams. Ranked hierarchical classification cation systems as well as well-known names for systems are then deduced from the branching rather small but more homogeneous families (e.g. pattern of the inferred tree diagram (e.g. Chase Kubitzki, 1998). It should be kept in mind, et al., 2000). Phylogenetic classification systems however, that the short-term disruption to classi - are strictly genealogical and ideally recognize ficatory stability is balanced by the possibilities it monophyletic taxa (conforming to clades of the offers towards a better understanding of the phylogeny) of extant species only. Hence, they do interrelationships among the recognized taxa for not primarily mirror the degree of divergence (i.e. an appreciation of their evolution and diversifi - the amount of evolutionary change, be it in the cation. The main question to ask before imple - form of differences in nucleotide sequences [but menting any changes is thus the question about see Chase et al. , 2000: 688 for a statement to the gains (or losses) of information connected to a contrary!], be it in the form of observable differ - newly proposed classification. Decisions about ences in morphology, anatomy, etc.) for assigning which clades in a phylogeny should receive a ranks at the different level in the Linnaean hier - formal name should primarily be based on “(1) archy – a strong contrast to tradition where the maximizing phylogenetic information and (2) degree of observable morphological divergence support for monophyly” (Chase et al. , 2000: 687). strongly influenced decisions. An example from Gains in phylogenetic information constitute the world of succulents should help to clarify this excellent reasons for change. If such an informa - point: the species of the former family tion gain is minimal or lacking altogether, Asclepiadaceae are clearly distinct because of changes to existing classifications should be their very elaborate floral architecture, though, exserted with great care, because changes are phylogenetically they only represent an evolu - always accompanied by some degree of disrup - tionary lineage derived from within the broader tion, especially when access to information in diversity of the species included in Apocynaceae. existing literature is considered. In order to reflect this insight and to ensure a During the past two decades, molecular consistent classification of these taxa, phylogenetic studies have resolved numerous Asclepiadaceae is nowadays included as part of problematical and controversial cases in plant Apocynaceae. Charles Darwin (1809–1882) classification. In many cases, these studies stated in his famous publication On The Origin of provided strong support for relationships that Species that all classification “must be strictly were previously based largely on intuition. But in genealogical” (Darwin, 1859: 420) in order to other cases they have shown that groups whose adequately represent the patterns of phyloge - circumscription or affiliation went by unques - netic interrelationships (e.g. Ghiselin, 1969, tioned for a long time as “natural” taxa turn out 2004; but see Mayr, 1985, for a different inter - to comprise lineages with distinctly different pretation and opinion). This challenge is now phylogenetic affinities. This results in new increasingly fulfilled with classification systems research questions and challenges to established derived from molecular phylogenetic analyses. classification systems. So far, the most profound At this point, we should clearly state that impact of recent studies of flowering plant classification systems of organisms have a three- phylogeny has been at the rank of families and fold purpose: they should, at the same time, be an above. This is testified by the recognition of objective and universal representation of inferred numerous new families (for succulents e.g. phylogenetic relationships, a stable framework Anacampserotaceae, Montiaceae, and Talinaceae for the naming of organisms, as well as a prac - segregated from Portulacaceae; Nyffeler & Eggli, tical tool consisting of easily identified and char - 2010) as well as the disappearance of old acterized entities (e.g. Thiele & Yeates, 2002; favourites (e.g. Aloaceae to be included in Stuessy, 2008). Newly gained knowledge from Asphodelaceae [Dahlgren et al. , 1985; Smith & phylogenetic studies often calls for changes in the Wyk, 1991]; Asclepiadaceae to be included in 126 Bradleya 28/2010 Apocynaceae [Endress et al. , 1997; Endress & current use, and it is our hope that in the near Bruyns, 2000; Endress & Stevens, 2001]; future the same will apply to the family Bombacaceae and Sterculiaceae included in classification. Malvaceae [Baum et al. , 1998, Alverson et al. , 1999; Bayer et al. , 1999]). All of these cases are Goals the result of improved knowledge of the genealog - Traditional succulent plant literature does not ical interrelationships (= “topology” of the clado - usually give much room to suprafamilial classifi - gram) between the taxa involved, and have cation. However, the phylogenetic classification helped to eliminate paraphyletic or polyphyletic of families into higher taxa provides the all- taxa. These changes are thus in strong accord important backbone towards an understanding of with both goals (i.e. maximizing phylogenetic