XVI International Botanical Congess

The Devonian period clearly represents a highly dynamic phase in the evolutionary history of vascular plants. Recent attempts have been made at elucidating the historical patterns of change among major taxonomic groups, but we are still left with the complex task of answering critical questions regarding explanations of how particular patterns of vascular plant evolution are produced. Comparative method techniques designed for analyzing correlations among discrete cladistic characters were used to test the following hypotheses of 'causal' explanations of phylogenetic trends uniquely associated with the adaptive radiation of Devonian land plants: phylogenetic increase in plant height (independent variable) is a 'causal factor' for the evolution of 1) stelar dissection, 2) secondary growth and 3) heterospory (dependent variables). In addition to testing hypotheses of particular processes underlying vascular plant character evolution, large-scale phylogenetic patterns suggest a causal explanation for the iterative increase in morphological disparity among early vascular plant clades. Using phylogenetic comparative methodology, this study provides a statistical test of hypothesized, biological 'key innovations' (independent variables: heterospory, stelar dissection and leaves) underlying observed increases in vascular plant morphological disparity (dependent variable, measured using pairwise dissimilarity among taxa).