Bringing Together the Living and Dead: Integrating Extant and Fossil Biodiversity in Evolutionary Studies
Magallón, Susana .
Alternative uses of fossil data in relaxed molecular clock age estimation: a case study on the age of angiosperms.
RELAXED molecular clock (RMC) methods provide substantially different perspectives about the timing of angiosperm crown group origin and diversification than fossil earliest occurrences. Nevertheless, RMC and the fossil record agree in identifying a burst of phylogenetic branching soon after the origin of the clade giving rise to its major lineages, including Nymphaeales, Magnoliales, Chloranthaceae, monocots and eudicots. Molecular phylogeny estimates suggest that angiosperms are the sister to all living gymnosperms, however, this hypothesis does not preclude the possibility that one or several extinct gymnosperm lineages are more closely related to angiosperms. A partial explanation to the observed discrepancy between RMC and fossils regarding the timing of angiosperm origin and diversification may be that RMC methods are misguided by the long angiosperm stem lineage, in which landmarks that may limit their age are missing. To evaluate this idea, possible angiosperm stem relatives were identified on the basis of a combined phylogenetic analysis of morphological and molecular data for living and fossil seed plants. The resulting information was implemented in two different ways in strict molecular clock (SMC) and Penalized Likelihood (PL) age estimation: First, the age of the extinct sister group of angiosperms, Caytonia, was introduced as a maximum constraint of angiosperm age. Second, branch lengths of angiosperm stem relatives were optimized with maximum likelihood, and inserted in a phylogram for living taxa, which was subsequently used for age estimation. Imposing a maximum age constraint on the age of angiosperms resulted in ages that are closer to fossil ages, however, the use of optimized branch lengths for angiosperm stem relatives resulted in ages that are even older than those obtained with conventional SMC and PL estimation. Although entirely experimental in its approach, this study examines alternative implementations of fossil information to aid estimation of divergence times based on molecular data.
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1 - Universidad Nacional Autónoma de México, Department of Botany, 3er Circuito de Ciudad Universitaria, Del. Coyoacán, Mexico City, México D.F., 04510, Mexico
maximum age constraint
branch length optimization
seed plant phylogeny.
Presentation Type: Symposium or Colloquium Presentation
Location: 134/Performing Arts Center
Date: Tuesday, August 1st, 2006
Time: 4:30 PM