Casearia is a pantropical genus of circa 240 species of the Salicaceae family, and the most species rich genus of the tribe Samydeae. The Neotropics harbours half of all Casearia species, which are present in every biome of the New World. Despite its abundance in tropical biomes, Casearia has been poorly studied, mostly because of the complexity of identification at the species level. This thesis presents the first well-resolved phylogenetic study of the genus Casearia. The objectives were to test its monophyly and to elucidate its relationships to other taxa of the Samydeae tribe, as well as to study the biogeography of the genus. Furthermore, species boundaries of some Neotropical taxa were investigated, using an integrative taxonomy approach. Finally, the last objective was to elaborate a reliable World checklist of every currently accepted Casearia species, a tool most needed for taxonomy, conservation and biodiversity management. A molecular dataset of 103 accessions from four plastid regions (petD, rpl16, rps4/trnLF and trnK/matK) and on nuclear region (ITS) was generated, corresponding to 54 species, of which 42 belong to Casearia. The study focuses on Neotropical species and the dataset included five Samydeae genera: Euceraea, Lunania, Neoptychocarpus, Ryania and Tetrathylacium, to understand the relationship between Casearia and the Samydeae tribe. Phylogenetic relationships were inferred using Bayesian inference, maximum likelihood and parsimony analysis. The origin of Casearia in the Caribbean and its biogeographical relationships to species from the South American mainland was investigated. Therefore, divergence time and ancestral area were estimated. Seven key morphological characters were studied, in order to retrieve synapomorphies for the genus, using BayesTraits. The results of the phylogenetic reconstruction showed a well-supported Samydeae clade and a well-supported Casearia clade, as sister to the other Samydeae. The genus Casearia was identified as not monophyletic, because the included species of the two Neotropical genera Euceraea and Neoptychocarpus were found nested with good support within the Casearia clade. Several major clades were retrieved, mostly constituted of entirely Neotropical species, one of which being consisted of only species from the Caribbean. A clade of all Paleotropical species was found nested in the clade with the New World Casearia taxa. Two morphological characters that work well to delimitate a monophyletic genus concept were identified. First, a branched inflorescence (fasciculate, glomerulous or cymous) and second a single row of stamen. In addition, some traits previously used as diagnostic for the genus, have been identified as homoplastic.For example, the presence of pellucid dots or lines on the leaves, that appeared earlier within Samydeae and were lost in Ryania and Tetrathylacium, a dioeceous reproduction system, used to diagnose Neoptychocarpius, or a higher number of stamen, used to diagnose Laetia and Zuelania. The time estimation analysis of the present study showed that the Samydeae clade originated in South America during the Cretaceous, 102 Ma, which corresponds to the rapid radiation of the Malpighiales. The Casearia clade is of South American origin and begins to diversify in the Eocene, 90 Ma. The Paleotropical clade of Casearia was found to have originated later than the Neotropical one, therefore from New World ancestors, in the Oligocene/Miocene boundary, 40 Ma and that it was divided into two clades, an African and an Asian one. More sampling from Old World taxa would be needed, in order to identify the origin of this Paleotropical clade. The results show that Casearia migrated multiple times to the Caribbean, and gave rise to a clade of Caribbean endemics in the late Miocene, 9 Ma, most likely of South American origin. In addition, the species limits of five Neotropical taxa were investigated. Those taxa were retrieved as nested singletons, which are taxa with a single or few accessions of an accepted species, nested within a clade of another accepted species. Here, an integrative taxonomy approach was applied as a successful way to study the taxonomical status of those taxa, to evaluate whether the singletons may be considered as a synonym, or if they constitute an emerging species, using both morphological and ecological analysis. To that aim, morphological and ecological differentiations were calculated, or on the contrary the absence of differentiation, between those nested singleton and the taxa in which it it nested. Phenotypic differentiation was analysed using morphometry based on the characters that are used as diagnostic characters of the taxa. For the ecological analysis, the niche space differentiation was evaluated and ecological niche equivalency and similarity tests were performed. Furthermore, the distribution range of those species was mapped. One species, C. grandiflora, is recovered nested within a C. arborea clade. However, the analyses showed that it presents some degree of differentiation, both morphological and ecological. Therefore, the results suggest that they are indeed two species and C. grandiflora is most likely a recent emerging lineage. Two other cases of nested singleton were found, C. selloana and C. zizyphoides, retrieved within the C. sylvestris clade. The results show that they present little to no morphological and ecological differentiation and a narrower distribution range as the widely distributed C. sylvestris. Therefore, it suggests that they most likely represent a subspecies of C. sylvestris. Another nested singleton of C. mariquitensis is retrieved within a C. mollis clade and they are most likely synonyms, given the absence of morphological or ecological differentiation. Finally, an up-to-date checklist of all currently accepted Casearia species is presented, which includes both Neotropical and Paleotropical taxa. The database was created using information from floristic treatments, monographs, regional checklists and taxonomic treatments at species level. For each taxon, the complete information on protologue, types and synonyms has been provided. Our aim was to give the genus a taxonomic backbone that can serve as a basis for further research in taxonomy, ecology and species conservation. The EDIT platform for Cybertaxonomy was used; it is an open-source platform that presents all the tools necessary to establish a taxonomic checklist. It provides the user with a taxonomic editor where every relevant taxonomic information can be entered and an online data portal Here, the most complete checklist of Casearia species to date at a global scale is presented. 708 names, 222 accepted species, 464 synonyms and 22 unresolved names were included, combining information from taxonomical revision, floristic treatments and regional checklists.
