Floral diagrams and molecular phylogeny

Molecular and morphological characters

Since the early 1990s molecular systematics has dramatically changed the approach to studying relations of plants and led to major changes in the classification of plant groups. Premolecular classifications such as those of Cronquist (1981), Thorne (1992), Takhtajan (1997) and to a lesser extent Dahlgren (1975, 1983), were mostly intuitive, with specifically selected characters considered to be more important than others. Especially for flowers, certain characters considered as important were shown to be mere convergences by the molecular phylogenies. For example, families sharing three carpels with parietal placentation grouped in an order Violales or Parietales sensu Engler were shown to belong to three different lineages. Other comparable earlier associations of families include the ‘Contortae’ (based on contorted petal aestivation), ‘Sympetalae’ (taxa with stamen-petal tubes) or ‘Rhoeadales’ (Papaveraceae and Brassicaceae) and were largely used in the book of Eichler (1875, 1878) and the Englerian systems.

Molecular phylogenies have proposed several shifts in relationships, some of them predicted by other characters (e.g. the link between Salicaceae and former Flacourtiaceae), others unexpected (e.g. the circumscription of Proteales) or apparently questionable because of superficial morphological similarities (e.g. the separation of Oxalidaceae and Geraniaceae), and a few controversial (e.g. Anisophyllaceae in Cucurbitales). Two major milestones, the Angiosperm Phylogeny Group or APG I (1998) and APG II (2003), have increasingly brought stability to the system with most major groupings firmly supported by a wide range of genes.