Molecular Phylogenies and the Diversification of the Angiosperms

Abstract

The angiosperms, with approximately 440 families and nearly 250,000 species (Thorne, 1992b), represent the most significant radiation in the plant kingdom. As a result, understanding the manner and tempo of angiosperm evolution has preoccupied botanists for well over 100 years. Earlier chapters of this book have reviewed the various sources of data that have been brought to bear on this problem, which range from fossil evidence, biogeographical distributions, comparative morphology and anatomy, and, more recently, protein and nucleotide sequences. Phylogeny-based analysis of the origin and radiation of angiosperms is well underway and is utilizing both morphological and molecular data (Crane, 1985; Loconte and Stevenson, 1991; Chase et al., 1993; Qiu et al., 1993; Doyle and Donoghue, 1993; Albert et al., 1994; Donoghue, 1994; Meacham, 1994; Nixon et al., 1994; Sanderson and Donoghue, 1994; Nickrent and Soltis, 1995; reviewed in Sytsma and Hahn, 1994, and Crane et al., 1995). The questions eliciting the most interest include: Are the angiosperms a monophyletic group? What is/are the closest gymnosperm(s) to the angiosperms? What are the major angiosperm lineages? What are the basal angiosperm lineages? What did the earliest angiosperms look like? When did the angiosperms arise? Why are the angiosperms so diverse? Answering these questions depends on resolving the phylogeny of the early angiosperm radiation.