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Sven Buerki

Jodrell Laboratory, Royal Botanic Gardens, Kew
Richmond, Surrey
United Kingdom

From Dec. 1st, 2010-present:
Postdoc fellow at the Royal Botanic Gardens, KEW (UK), funded by the Marie Curie Action programme.

Research Associate, Missouri Botanical Garden, St. Louis, USA.

Dec 2009- Nov. 2010:
Postdoc fellow at the Real Jardin Botanico (Madrid, Spain), funded by the Swiss National Science Foundation.

Oct. 2005 - 12th June 2009:
PhD thesis at the laboratory of Evolutionary Botany (University of Neuchâtel): “Worldwide biogeography and systematics of Sapindaceae: a molecular and taxonomic survey combining large data sets and novel methodological approaches”: advisor: Prof Philippe Küpfer and co-advisor: Dr Nadir Alvarez.

Master of Science in Behaviour, Ecology and Evolution from the University of Neuchâtel, with high distinction: “Biogeography and systematics of the genus Gouania (Rhamnaceae) in the western Indian Ocean”; advisor: Prof Philippe Küpfer and co-advisors: Pete B. Phillipson and Dr Martin W. Callmander (Missouri Botanical Garden, USA).


Biogeography and diversification of angiosperms

Coevolutionary processes: interactions between plants and insects

Taxonomy of Sapindaceae and contributions to the knowledge of the Malagasy and New Caledonian flora

Journal articles

in press
M Borer*, T van Noort*, N Arrigo, S Buerki, N Alvarez (2011)  Does a shift in host plants trigger speciation in the Alpine leaf beetle Oreina speciosissima (Coleoptera, Chrysomelidae)?   BMC Evolutionary Biology 11: 310 October  
Abstract: Background Within the Coleoptera, the largest order in the animal kingdom, the exclusively herbivorous Chrysomelidae are recognized as one of the most species rich beetle families. The evolutionary processes that have fueled radiation into the more than thirty-five thousand currently recognized leaf beetle species remain partly unresolved. The prominent role of leaf beetles in the insect world, their omnipresence across all terrestrial biomes and their economic importance as common agricultural pest organisms make this family particularly interesting for studying the mechanisms that drive diversification. Here we specifically focus on two ecotypes of the alpine leaf beetle Oreina speciosissima (Scop.), which have been shown to exhibit orphological differences in male genitalia roughly corresponding to the subspecies Oreina speciosissima sensu stricto and Oreina speciosissima troglodytes. In general the two ecotypes segregate along an elevation gradient and by host plants: Oreina speciosissima sensu stricto colonizes high forb vegetation at low altitude and Oreina speciosissima troglodytes is found in stone run vegetation at higher elevations. Both host plants and leaf beetles have a patchy geographical distribution. Through use of gene sequencing and genome fingerprinting (AFLP) we analyzed the genetic structure and habitat use of Oreina speciosissima populations from the Swiss Alps to examine whether the two ecotypes have a genetic basis. By investigating a wide range of altitudes and focusing on the structuring effect of habitat types, we aim to provide answers regarding the factors that drive adaptive radiation in this phytophagous leaf beetle. Results While little phylogenetic resolution was observed based on the sequencing of four DNA regions, the topology and clustering resulting from AFLP genotyping grouped specimens according to their habitat, mostly defined by plant associations. A few specimens with intermediate morphologies clustered with one of the two ecotypes or formed separate clusters consistent with habitat differences. These results were discussed in an ecological speciation framework. Conclusions The question of whether this case of ecological differentiation occurred in sympatry or allopatry remains open. Still, the observed pattern points towards ongoing divergence between the two ecotypes which is likely driven by a recent shift in host plant use.
Notes: * These authors contributed equally to this work and are considered co-first authors.
S Buerki, PP Lowry II, S Andriambololonera, P B Phillipson, L Vary, M W Callmander (2011)  How to kill two genera with one tree: clarifying generic circumscriptions within an endemic Malagasy clade of Sapindaceae.   Botanical Journal of the Linnean Society 165: 223-234 March  
Abstract: Phylogenetic relationships in a Malagasy clade of Sapindaceae, encompassing Molinaea (with members also in the Mascarene Islands), Neotina, Tina and Tinopsis, were inferred by expanding a previous nuclear and plastid DNA data set for the family. The circumscription of these morphologically similar genera has remained prob- lematic since the first family-wide treatment. To investigate this situation, representative taxa were analysed to: (1) test the monophyly of the genera; (2) investigate their phylogenetic relationships; and (3) explore alternative circumscriptions that reflect phylogeny and yield genera that are morphologically coherent and easily charac- terized. Phylogenetic inferences supported the monophyly of the group and its subdivision into three clades. All species of Molinaea sampled belong to a clade (Clade I) that is sister to a clade comprising Neotina, Tina and Tinopsis, within which one clade (Clade II) encompasses Tinopsis and Neotina (with the latter nested within the former) and another (Clade III) comprises all taxa of Tina. These three genera can be easily distinguished from Molinaea by having two rather than three carpels, which represents an unambiguous synapomorphy. Given the paraphyly of Tinopsis with regard to Neotina and the strong support for the monophyly of Tina, two potentially viable options are available for the generic delimitation of the taxa in this clade: (1) to recognize two genera corresponding, respectively, to Clades II and III; or (2) to place all of the taxa in a single genus encompassing both clades. Based on a review of morphological evidence the second option is favoured and consequently a broad generic concept is applied.
S Buerki, P B Phillipson, M W Callmander (2011)  A Taxonomic Revision of Gouania (Rhamnaceae) in Madagascar and the Other Islands of the Western Indian Ocean (The Comoro and Mascarene Islands, and the Seychelles).   Annals of the Missouri Botanical Garden 98: 157-195 July  
Abstract: A taxonomic revision of the genus Gouania Jacq. (Rhamnaceae) is presented for Madagascar and the other western Indian Ocean islands. Seventeen species are recognized, of which nine are described and published as new (all endemic to Madagascar): G. ambrensis Buerki, Phillipson & Callm., G. callmanderi Buerki, G. cupreifolia Buerki, Phillipson & Callm., G. cupuliflora Buerki, Phillipson & Callm., G. gautieri Buerki, Phillipson & Callm., G. perrieri Buerki, Phillipson & Callm., G. phillipsonii Buerki, G. taolagnarensis Buerki, Phillipson & Callm., and G. zebrifolia Buerki, Phillipson & Callm. Sixteen species occur in Madagascar, of which 13 are endemic and three are common to Madagascar and one or more of the smaller Indian Ocean islands. The latter include G. laxiflora Tul., a species which is also present on mainland Africa. One species, G. mauritiana Lam., is endemic to Réunion Island. We recognize two subspecies within G. scandens (Gaertn.) R. B. Drumm.: G. scandens subsp. scandens and G. scandens subsp. glandulosa (Boivin ex Tul.) Buerki, Phillipson & Callm., the latter transferred from G. glandulosa Boivin ex Tul. Past confusion about the identity of this species is discussed. Five names are lectotypified: G. aphrodes Tul., G. glandulosa [= G. scandens subsp. glandulosa], G. laxiflora, G. lineata Tul., and G. tiliifolia Lam. Both lectotype and epitype are designated for G. mauritiana. Conservation assessments are provided for all species within their primary areas of occurrence.
S Buerki, F Forest, N Salamin, N Alvarez (2011)  Comparative performance of supertree algorithms in large data sets using the soapberry family (Sapindaceae) as a case study.   Systematic Biology 60: 32-44  
Abstract: For the last 2 decades, supertree reconstruction has been an active field of research and has seen the develop- ment of a large number of major algorithms. Because of the growing popularity of the supertree methods, it has become necessary to evaluate the performance of these algorithms to determine which are the best options (especially with regard to the supermatrix approach that is widely used). In this study, seven of the most commonly used supertree methods are investigated by using a large empirical data set (in terms of number of taxa and molecular markers) from the worldwide flowering plant family Sapindaceae. Supertree methods were evaluated using several criteria: similarity of the supertrees with the input trees, similarity between the supertrees and the total evidence tree, level of resolution of the supertree and computational time required by the algorithm. Additional analyses were also conducted on a reduced data set to test if the performance levels were affected by the heuristic searches rather than the algorithms themselves. Based on our results, two main groups of supertree methods were identified: on one hand, the matrix representation with parsimony (MRP), MinFlip, and MinCut methods performed well according to our criteria, whereas the average consensus, split fit, and most similar supertree methods showed a poorer performance or at least did not behave the same way as the total evidence tree. Results for the super distance matrix, that is, the most recent approach tested here, were promising with at least one derived method performing as well as MRP, MinFlip, and MinCut. The output of each method was only slightly improved when applied to the reduced data set, suggesting a correct behavior of the heuristic searches and a relatively low sensitivity of the algorithms to data set sizes and missing data. Results also showed that the MRP analyses could reach a high level of quality even when using a simple heuristic search strategy, with the exception of MRP with Purvis coding scheme and reversible parsimony. The future of supertrees lies in the implementation of a standardized heuristic search for all methods and the increase in computing power to handle large data sets. The latter would prove to be particularly useful for promising ap- proaches such as the maximum quartet fit method that yet requires substantial computing power.
S Buerki, F Forest, N Alvarez, J Nylander, N Arrigo, I Sanmartín (2011)  An evaluation of new parsimony-based versus parametric inference methods in biogeography: a case study using the globally distributed plant family Sapindaceae.   Journal of Biogeography 38: 531-550  
Abstract: <b>Aim</b> Recently developed parametric methods in historical biogeography allow researchers to integrate temporal and palaeogeographical information into the reconstruction of biogeographical scenarios, thus overcoming a known bias of parsimony-based approaches. Here, we compare a parametric method, dispersalâ extinctionâcladogenesis (DEC), against a parsimony-based method, dispersalâ vicariance analysis (DIVA), which does not incorporate branch lengths but accounts for phylogenetic uncertainty through a Bayesian empirical approach (Bayes-DIVA). We analyse the benefits and limitations of each method using the cosmopolitan plant family Sapindaceae as a case study. Location World-wide. <b>Methods</b> Phylogenetic relationships were estimated by Bayesian inference on a large dataset representing generic diversity within Sapindaceae. Lineage divergence times were estimated by penalized likelihood over a sample of trees from the posterior distribution of the phylogeny to account for dating uncertainty in biogeographical reconstructions. Wecompared biogeographical scenarios between Bayes-DIVA and two different DEC models: one with no geological constraints and another that employed a stratified palaeogeographical model in which dispersal rates were scaled according to area connectivity across four time slices, reflecting the changing continental configuration over the last 110 million years. <b>Results</b> Despite differences in the underlying biogeographical model, Bayes- DIVA and DEC inferred similar biogeographical scenarios. The main differences were: (1) in the timing of dispersal events â which in Bayes-DIVA sometimes conflicts with palaeogeographical information, and (2) in the lower frequency of terminal dispersal events inferred by DEC. Uncertainty in divergence time estimations influenced both the inference of ancestral ranges and the decisiveness with which an area can be assigned to a node. <b>Main conclusions</b> By considering lineage divergence times, the DEC method gives more accurate reconstructions that are in agreement with palaeogeographical evidence. In contrast, Bayes-DIVA showed the highest decisiveness in unequivocally reconstructing ancestral ranges, probably reflecting its ability to integrate phylogenetic uncertainty. Care should be taken in defining the palaeogeographical model in DEC because of the possibility of overestimating the frequency of extinction events, or of inferring ancestral ranges that are outside the extant species ranges, owing to dispersal constraints enforced by the model. The wide-spanning spatial and temporal model proposed here could prove useful for testing large-scale biogeographical patterns in plants.
N Arrigo, S Buerki, A Sarr, R Guadagnuolo, G Kozlowski (2011)  Phylogenetics and phylogeography of the monocot genus Baldellia (Alismataceae): Mediterranean refugia, suture zones and implications for conservation.   Molecular Phylogenetics and Evolution 58: 1. 33-42 Jan  
Abstract: Aquatic plants, and especially the emblematic genus Baldellia (Alismataceae), are among the most threatened organisms, due to unprecedented human-driven habitat destructions. Therefore protection plans are crucially needed and call for thoroughly documenting the genetic diversity and clarifying the taxonomy of this endangered genus. Our sampling included 282 individuals from 42 natural populations and covered the whole geographical range of the genus, across Europe and the Mediterranean. We combined sequencing of nuclear internal transcribed spacer (ITS) and chloroplastic trnL-ndhF regions with amplified fragment length polymorphism (AFLP) genotyping to investigate the Alismataceae phylogeny, and produce a phylogeography of Baldellia. Our phylogeny strongly supported the monophyly of Baldellia and placed it as the sister clade to Luronium and Alisma, therefore excluding, as previously supposed, a close genetic relatedness to the predominantly neotropical genus Echinodorus. The phylogeography of Baldellia outlined patterns consistent with a hypothesis considering glacial refugia located in the Iberian Peninsula and the Italy/Balkan region from which two distinct genetic lineages re-colonized Europe. These two lineages corresponded respectively to Baldellia ranunculoides (Italy/Balkan derived populations) and Baldellia repens (populations recovered from the Iberian Peninsula refuge), therefore supporting differences outlined between the two taxa in previous ecological and morphological studies. These results allowed clarifying taxonomic uncertainties by confirming the genetic distinctness of B. repens according to B. ranunculoides. A third lineage, Baldellia alpestris, originated and remained endemic to the mountainous regions of the Iberian Peninsula. Unexpectedly, B. repens populations collected in northern Africa, appeared to be genetically distinct from their European counterparts, this calls for further investigation to fully address their genetic and conservation status. Finally, we detected a large hybridization zone in northwestern Europe between B. repens and B. ranunculoides. These results were discussed in light of conservation approaches for Baldellia populations.
A Espíndola, S Buerki, M Bedalov, P Küpfer, N Alvarez (2010)  New insights into the phylogenetics and biogeography of Arum (Araceae): unravelling its evolutionary history.   Botanical Journal of the Linnean Society 163: 14-32  
Abstract: The heat- and odour-producing genus Arum (Araceae) has interested scientists for centuries. This long-term interest has allowed a deep knowledge of some complex processes, such as the physiology and dynamics of its characteristic lure-and-trap pollination system, to be built up. However, mainly because of its large distributional range and high degree of morphological variation, species' limits and relationships are still under discussion. Today, the genus comprises 28 species subdivided into two subgenera, two sections and six subsections. In this study, the phylogeny of the genus is inferred on the basis of four plastid regions, and the evolution of several morphological characters is investigated. Our phylogenetic hypothesis is not in agreement with the current infrageneric classification of the genus and challenges the monophyly of several species. This demonstrates the need for a new infrageneric classification based on characters reflecting the evolution of this enigmatic genus. To investigate the biogeography of Arum deeply, further spatiotemporal analyses were performed, addressing the importance of the Mediterranean basin in the diversification of Arum. Our results suggest that its centre of origin was the EuropeanâAegean region, and that major diversification happened during the last 10 Myr.
S Buerki, PP Lowry II, N Alvarez, S G Razafimandimbison, P Küpfer, M W Callmander (2010)  Phylogeny and circumscription of Sapindaceae revisited: molecular sequence data, morphology and biogeography support recognition of a new family, Xanthoceraceae.   Plant Ecology and Evolution 143: 2. 1-20  
Abstract: Background and aims - Recent studies have adopted a broad definition of Sapindaceae that includes taxa traditionally placed in Aceraceae and Hippocastanaceae, achieving monophyly but yielding a family difficult to characterize and for which no obvious morphological synapomorphy exists. This expanded circumscription was necessitated by the finding that the monotypic, temperate Asian genus Xanthoceras, historically placed in Sapindaceae tribe Harpullieae, is basal within the group. Here we seek to clarify the relationships of Xanthoceras based on phylogenetic analyses using a dataset encompassing nearly ¾ of sapindaceous genera, comparing the results with information from morphology and biogeography, in particular with respect to the other taxa placed in Harpullieae. We then re-examine the appropriateness of maintaining the current broad, morphologically heterogeneous definition of Sapindaceae and explore the advantages of an alternative family circumscription. Methods - Using 243 samples representing 104 of the 142 currently recognized genera of Sapindaceae s. lat. (including all in Harpullieae), sequence data were analyzed for nuclear (ITS) and plastid (matK, rpoB, trnD-trnT, trnK-matK, trnL-trnF and trnS-trnG) markers, adopting the methodology of a recent family-wide study, performing single-gene and total evidence analyses based on maximum likelihood (ML) and maximum parsimony (MP) criteria, and applying heuristic searches developed for large datasets, viz. a new strategy implemented in RAxML (for ML) and the parsimony ratchet (for MP). Bootstrap analyses were performed for each method to test for congruence between markers. Key results - Our findings support earlier suggestions that Harpullieae are polyphyletic: Xanthoceras is confirmed as sister to all other sampled taxa of Sapindaceae s. lat.; the remaining members belong to three other clades within Sapindaceae s. lat., two of which correspond respectively to the groups traditionally treated as Aceraceae and Hippocastanaceae, together forming a clade sister to the largely tropical Sapindaceae s. str., which is monophyletic and morphologically coherent provided Xanthoceras is excluded. Conclusion - To overcome the difficulties of a broadly circumscribed Sapindaceae, we resurrect the historically recognized temperate families Aceraceae and Hippocastanaceae, and describe a new family, Xanthoceraceae, thus adopting a monophyletic and easily characterized circumscription of Sapindaceae nearly identical to that used for over a century.
S Buerki, P B Phillipson, P P Lowry II, M W Callmander (2010)  Molecular phylogenetics and morphological evidence support recognition of Gereaua, a new endemic genus of Sapindaceae from Madagascar.   Systematic Botany 35: 172-180  
Abstract: A recent worldwide phylogeny of Sapindaceae inferred from nuclear and plastid DNA regions segregated the Malagasy <i>Haplocoelum perrieri</i> Capuron from the African <i>Haplocoelum foliosum</i> (Hiern) Bullock. Additional phylogenetic analyses conducted here (including material of <i>Haplocoelum inopleum</i> Radlk., the generic type) supported the result from the previous analysis and showed that maintaining a broad circumscription of <i>Haplocoelum</i> to include the Malagasy species would render the genus polyphyletic. To maintain monophyly, it is necessary to exclude <i>H. perrieri</i> , which we transfer to a new, monotypic genus, described here as <i>Gereaua</i> . This taxon is easily distinguished from the species retained in <i>Haplocoelum</i> by the following morphological characters: (1) sexually dimorphic inflorescences in racemules (vs. monomorphic inflorescences in fascicule of cymes); (2) 2-locular ovary (vs. 3-locular ovary); (3) rudimentary pistillode in staminate flowers (vs. no pistil- lode in staminate flowers); (4) corolla with 4 or 5 petals (vs. apetalous); (5) pubescent fruit (vs. glabrous fruit). Relationships between the new genus and its most closely related genera, included in the Macphersonia group, are discussed in light of molecular, morphological and biogeographic evidence. A preliminary threat assessment of <i>Gereaua perrieri</i> using the IUCN Red List criteria indicates a status of Least Concern.
N Arrigo, F Felber, C Parisod, S Buerki, N Alvarez, J David, R Guadagnuolo (2010)  Origin and expansion of the allotetraploid Aegilops geniculata, a wild relative of wheat.   New Phytologist 187: 4. 1170-1180 Sep  
Abstract: *This study reconstructs the phylogeography of Aegilops geniculata, an allotetraploid relative of wheat, to discuss the impact of past climate changes and recent human activities (e.g. the early expansion of agriculture) on the genetic diversity of ruderal plant species. *We combined chloroplast DNA (cpDNA) sequencing, analysed using statistical parsimony network, with nonhierarchical K-means clustering of amplified fragment length polymorphism (AFLP) genotyping, to unravel patterns of genetic structure across the native range of Ae. geniculata. The AFLP dataset was further explored by measurement of the regional genetic diversity and the detection of isolation by distance patterns. *Both cpDNA and AFLP suggest an eastern Mediterranean origin of Ae. geniculata. Two lineages have spread independently over northern and southern Mediterranean areas. Northern populations show low genetic diversity but strong phylogeographical structure among the main peninsulas, indicating a major influence of glacial cycles. By contrast, low genetic structuring and a high genetic diversity are detected in southern Mediterranean populations. Finally, we highlight human-mediated dispersal resulting in substantial introgression between resident and migrant populations. *We have shown that the evolutionary trajectories of ruderal plants can be similar to those of wild species, but are interfered by human activities, promoting range expansions through increased long-distance dispersal and the creation of suitable habitats.
J Burnier*, S Buerki*, N Arrigo, P Küpfer, N Alvarez (2009)  Genetic structure and evolution of Alpine polyploid complexes: Ranunculus kuepferi (Ranunculaceae) as a case study.   Molecular Ecology 18: 3730-3744  
Abstract: The alpine white-flowered buttercup, Ranunculus kuepferi Greuter & Burdet, is a polyploid complex with diploids endemic to the southwestern Alps and polyploids - which have been previously described as apomictic - widespread throughout European mountains. Due to the polymorphic status of both its ploidy level and its reproductive mode, R. kuepferi represents a key species for understanding the evolution of polyploid lineages in alpine habitats. To disentangle the phylogeography of this polyploid taxon, we used cpDNA sequences and AFLP (amplified fragment length polymorphism) markers in 33 populations of R. kuepferi representative of its ploidy level and distribution area. Polyploid individuals were shown to be the result of at least two polyploidization events that may have taken place in the southwestern Alps. From this region, one single main migration of tetraploids colonized the entire Alpine range, the Apennines and Corsica. Genetic recombination among tetraploids was also observed, revealing the facultative nature of the apomictic reproductive mode in R. kuepferi polyploids. Our study shows the contrasting role played by diploid lineages mostly restricted to persistent refugia and by tetraploids, whose dispersal abilities have permitted their range extension all over the previously glaciated Alpine area and throughout neighbouring mountain massifs.
Notes: * These authors contributed equally to this work and are considered co-first authors.
S Buerki, F Forest, P Acevedo-Rodríguez, M W Callmander, J A A Nylander, M Harrington, I Sanmartín, P Küpfer, N Alvarez (2009)  Plastid and nuclear DNA markers reveal intricate relationships at subfamilial and tribal levels in the soapberry family (Sapindaceae).   Molecular Phylogenetics and Evolution 51: 238-258  
Abstract: The economically important soapberry family (Sapindaceae) comprises about 1900 species mainly found in the tropical regions of the world, with only a few genera being restricted to temperate areas. The infrafamilial classification of the Sapindaceae and its relationships to the closely related Aceraceae and Hippocastanaceae â which have now been included in an expanded definition of Sapindaceae (i.e., subfamily Hippocastanoideae) â have been debated for decades. Here we present a phylogenetic analysis of Sapindaceae based on eight DNA sequence regions from the plastid and nuclear genomes and including 85 of the 141 genera defined within the family. Our study comprises 997 new sequences of Sapindaceae from 152 specimens. Despite presenting 18.6% of missing data our complete data set produced a topology fully congruent with the one obtained from a subset without missing data, but including fewer markers. The use of additional information therefore led to a consistent result in the relative position of clades and allowed the definition of a new phylogenetic hypothesis. Our results confirm a high level of paraphyly and polyphyly at the subfamilial and tribal levels and even contest the monophyletic status of several genera. Our study confirms that the Chinese monotypic genus Xanthoceras is sister to the rest of the family, in which subfamily Hippocastanoideae is sister to a clade comprising subfamilies Dodonaeoideae and Sapindoideae. On the basis of the strong support demonstrated in Sapindoideae, Dodonaeoideae and Hippocastanoideae as well as in 14 subclades, we propose and discuss informal groupings as basis for a new classification of Sapindaceae.
S Buerki, M W Callmander, F Schüpfer, M Ravokatra, N Alvarez (2009)  Malagasy Dracaena Vand. ex L. (Ruscaceae): an investigation of discrepancies between morphological features and spatial genetic structure at a small evolutionary scale.   Plant Systematics and Evolution 280: 15-28  
Abstract: Malagasy Dracaena (Ruscaceae) are divided into four species and 14 varieties, all of them showing a high level of morphological diversity and a putatively artefactual circumscription. In order to reveal relationships between those entangled entities, a span of Malagasy Dracaena were sampled and analyzed using cpDNA sequences and AFLP. The cpDNA analyses resolved three biogeographic clades that are mostly inconsistent with morphology, since similar phenotypes are found across the three clades. Bayesian inference clustering analyses based on the AFLP were not in accordance with the cpDNA analysis. This result might be explained by (1) a recent origin of the Malagasy species of Dracaena with an incomplete sorting of chloroplast lineages; (2) a high amount of hybridizations; (3) a complex migration pattern. Interestingly, when the AFLP are analyzed using the parsimony criterion, a trend towards a directional evolution of inflorescence types and ecological features was observed. This might be considered either as phenotypic plasticity and/or as the result of fast evolution in flower characters according to habitat preferences. Overall, our results point to the difficulty of defining evolutionarily significant units in Malagasy Dracaena, emphasizing the complex speciation processes taking place in tropical regions.
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