The Comparative - Phylogenetic Method of Reconstructing Evolutionary History
DiMichele, William A. , Bateman, Richard M .
Lycopsids versus the other vascular land-plants: contrasting solutions to similar problems.
IT is widely accepted that the lycopsids are a monophyletic group of vascular land-plants that diverged prior to any other extant group of pteridophytes. Morphological phylogenetic analysis of fossils and first appearances in the stratigraphic record agree that they last shared a common ancestor with the rest of the vascular land-plants in the Late Silurian or Early Devonian. The lycopsids can be distinguished from other vascular plants on the basis of the development, morphology and position of sporangia, embryology, phloem ontogeny, leaf development, xylem maturation, and root–shoot differentiation. The separation of the zosterophyll–lycopsid and trimerophyte lineages from the ancestral cooksonioid plexus occurred prior to the subsequent independent evolution of similar morphological features within the more derived portions of each clade, including bipolar growth, secondary xylem and periderm (lateral meristems), leaves, roots, strobilar sporangial aggregation, heterospory and seed-like organs, and tree-habit. Plesiomorphic features of note include axial organization, centrally positioned vascular tissues, lignin production, photosynthate translocation tissues, and determinate aerial shoot growth. The three major clades of extant lycopsids – lycopodioids sister to selaginelloids plus isoetoids – are themselves distinct organizationally, with the most radical body-plan innovations evolving within the isoetoids. The remarkable parallels between the lycopsids and the rest of the vascular plants, which evolved long after evolutionary separation, raise many questions about the factors that control and constrain morphological evolution. Underlying “central tendencies” in development may have predisposed lineages to the evolution of features such as appendicular organs and positive geotropism. Successive constraints imposed by the fundamentally irreversible, Markovian nature of the evolutionary process may be the underlying cause of parallelism in reproductive organization and in the ways in which the tree habit was attained. Ultimately, evolutionary understanding will derive from integrated studies of comparative morphology and development explored in an explicit phylogenetic context.
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1 - NMNH Smithsonian Institution, Department of Paleobiology, MRC-121, Washington, DC, 20560, USA
2 - Natural History Museum, Department of Botany, Cromwell Road, London, SW7 5BD, England
Presentation Type: Symposium or Colloquium Presentation
Location: 314/Bell Memorial Union
Date: Wednesday, August 2nd, 2006
Time: 1:30 PM