ARE MANY PLANT-SPECIES PARAPHYLETIC

The phylogenetic status of plant species has become a critical issue i n 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 discuss es predicted phylogenetic consequences of different modes of speciatio n. The classic model of allopatric speciation by subdivision will typi cally generate monophyletic daughter species, whereas most geographica lly 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 populati on systems through gene flow or selection, allopatric speciation by su bdivision is likely to be less frequent than geographically local mode ls of speciation. Low levels of gene flow will also increase the time required for the progenitor species to achieve monophyly. Thus, many p lant species are likely to be paraphyletic, and predictably a species classification based on the criterion of monophyly is unlikely to be a n effective tool for describing and ordering biological diversity.