About 25 years ago, a revolution began in evolutionary studies of seed-free vascular plants. Whereas common wisdom and laboratory-based observations had averred that minute spores and inbreeding of individual bisexual gametophytes diminished barriers to long distance migration, genetic analyses of sporophyte populations demonstrated outcrossing breeding systems that required two spores for each successful migration event. After those population-based discoveries, the processes controlling biogeographic patterns of ferns appeared to resemble those of seed plants, and vicariance took on renewed significance. More recently, data from DNA sequencing predicted that some of the most diverse extant fern families originated after the isolation of major land masses, and these new hypotheses also demanded fresh consideration of biogeographic assumptions. The Polypodiaceae yield phylogenetic insights through integration of DNA sequence analysis and biogeography. New evidence shows separate yet simultaneous radiations in the New and Old Worlds. By combining sequence data, vicariance, and a reassessment of morphological features, new family and generic boundaries are obtained. Contributors to this symposium discovered similar patterns in the systematics and biogeography of the seed-free vascular plants they studied. Although long distance migration remains an important factor in explaining fern distributions, local and recent radiations that result in species complexes are also significant in explaining fern biogeography.
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Vol. 59 • No. 2