ARE MANY PLANT-SPECIES PARAPHYLETIC

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 allopatric 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, allopatric 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.