The phylogenetic status of plant species has become a critical issue in systematic and evolutionary botany, due in part to the influence of Hennigian principles on organismal classification. This paper reviews various modes of speciation and their frequency in plants, and discusses predicted phylogenetic consequences of different modes of speciation. The classic model of a1lopatric speciation by subdivision will typically generate monophyletic daughter species, whereas most geographically local models of speciation (e.g., the founder effect model), will produce a paraphyletic progenitor and monophyletic derivative species. Due to the theoretical difficulty of transforming widespread population systems through gene flow or selection, a1lopatric speciation by subdivision is likely to be less frequent than geographically local models of speciation. Low levels of gene flow will also increase the time required for the progenitor species to achieve monophyly. Thus, many plant species are likely to be paraphyletic, and predictably a species classification based on the criterion of monophyly is unlikely to be an effective tool for describing and ordering biological diversity.

The Begoniaceae consist of two genera, Begonia, with approximately 1400 species that are widely distributed in the tropics, and Hillebrandia, with one species that is endemic to the Hawaiian Islands and the only member of the family native to those islands. To help explain the history of Hillebrandia on the Hawaiian Archipelago, phylogenetic relationships of the Begoniaceae and the Cucurbitales were inferred using sequence data from 18S, rbcL, and ITS, and the minimal age of both Begonia and the Begoniaceae were indirectly estimated. The analyses strongly support the placement of Hillebrandia as the sister group to the rest of the Begoniaceae and indicate that the Hillebrandia lineage is at least 51-65 million years old, an age that predates the current Hawaiian Islands by about 20 million years. Evidence that Hillebrandia sandwicensis has survived on the Hawaiian Archipelago by island hopping from older, now denuded islands to younger, more mountainous islands is presented. Various scenarios for the origin of ancestor to Hillebrandia are considered. The geographic origin of source populations unfortunately remains obscure; however, we suggest a boreotropic or a Malesian-Pacific origin is most likely. Hillebrandia represents the first example in the well-studied Hawaiian flora of a relict genus.

Salix L. is the largest genus in the family Salicaceae (450 species). Several classifications have been published, but taxonomic subdivision has been under continuous revision. Our goal is to establish the phylogenetic structure of the genus using molecular data on all American willows, using three DNA markers. This complete phylogeny of American willows allows us to propose a biogeographic framework for the evolution of the genus. Material was obtained for the 122 native and introduced willow species of America. Sequences were obtained from the ITS (ribosomal nuclear DNA) and two plastid regions, matK and rbcL. Phylogenetic analyses (parsimony, maximum likelihood, Bayesian inference) were performed on the data. Geographic distribution was mapped onto the tree. The species tree provides strong support for a division of the genus into two subgenera, Salix and Vetrix. Subgenus Salix comprises temperate species from the Americas and Asia, and their disjunction may result from Tertiary events. Subgenus Vetrix is composed of boreo-arctic species of the Northern Hemisphere and their radiation may coincide with the Quaternary glaciations. Sixteen species have ambiguous positions; genetic diversity is lower in subg. Vetrix. A molecular phylogeny of all species of American willows has been inferred. It needs to be tested and further resolved using other molecular data. Nonetheless, the genus clearly has two clades that have distinct biogeographic patterns.

The hippocastanaceous Sapindaceae (family Hippocastanaceae) consists of two genera: Aesculus and Billia. The genus Aesculus (buckeyes) is distributed throughout the northern hemisphere. Twelve of the thirteen species of Aesculus are found in eastern Asia and North America, and one is native to Europe. The two species of the genus Billia are found in South and Central America. Aesculus is traditionally divided into five sections. This analysis, based on morphological characters, suggests that the monotypic section Parryanae (Aesculus parryi Gray) is sister to the remainder of the genus, and the other species are divided into two clades. One clade comprises all species from southeastern North America with the monotypic section Macrothyrsus (Aesculus parviflora Walt.) as sister to section Pavia. The other clade consists of all species found in Eurasia included in sections Aesculus and Calothyrsus, with the only North American member of this clade (Aesculus californica (Spach) Nutall) as sister to the rest of the group. According to this analysis, section Calothyrsus is rendered paraphyletic by the inclusion of section Aesculus. In contrast to previous studies that suggest an Asian origin for the genus, our study suggests an American origin for Aesculus from an ancestor similar to Billia with a single migration to Eurasia via the Bering land bridge.Résumé : Les Sapindaceae hippocastanoïdes (la famille des Hippocastanaceae) sont formées de deux genres, Aesculus et Billia. Le genre Aesculus (les marronniers) est répartie dans l'ensemble de l'hémisphère nord. La majorité des 13 espèces du genre se retrouvent en Asie de l'Est et en Amérique du Nord. Une seule espèce est indigène en Europe. Les deux espèces du genre Billia sont distribuées en Amérique Centrale et du Sud. Le genre Aesculus est traditionnellement divisé en cinq sections. Cette analyse phylogénétique fondée sur des caractères morphologiques place la section Parryanae (Aesculus parryi Gray) comme groupe frère du reste du genre, ce dernier étant subdivisé en deux clades. Le premier clade inclut toutes les espèces du sud-est de l'Amérique du Nord, c'est-à-dire les sections Macrothyrsus et Pavia. La section Macrothyrsus est monotypique (Aesculus parviflora Walt.) et groupe-frère de la section Pavia. Le second clade regroupe toutes les espèces retrouvées en Eurasie (sections Aesculus et Calothyrsus). Aesculus californica (Spach) Nutall, la seule espèce américaine de ce clade, se retrouve à la base. La section Calothyrsus est rendue paraphylétique par l'inclusion de la section Aesculus. Contrairement à certaines études antérieures qui suggèrent une origine asiatique pour le genre Aesculus, notre étude propose plutôt une origine américaine à partir d'un ancêtre similaire à Billia, suivi d'une seule migration vers l'Eurasie via le détroit de Béring.

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