Colloquium: Assembling the Tree of Life Colloquium
Nagalingum, Nathalie , Rai, Hardeep , Mathews, Sarah .
Patterns of species diversity in the evolutionary history of cycads.
The cycads have the oldest fossil record of all living seed plants, with specimens known from the Paleozoic. In the Mesozoic, cycad diversity was high; however, fossils of modern genera appeared relatively recently, during the Cenozoic. We have obtained a fully resolved estimate of relationships among cycad genera using nucleotide data from phytochrome loci. Consistent with observations in other molecular phylogenies, we find that branches within and in some cases among genera are remarkably short given the age of the group. We used molecular divergence time estimation to address whether these short branches are indicative of a recent radiation in modern cycads, or whether rates of nucleotide evolution in cycads have decelerated. In sharp contrast to their ancient origins, molecular divergence time estimates indicate that the species radiation of cycads appears to have occurred rather recently. The modern genera show unequal species diversity with some comprising nearly 100 species while others are monotypic. To better understand these patterns of extant species diversity we will compare and discuss diversification rates across cycads. We also will assess whether species radiations within cycads are coincident with abiotic events (e.g., climatic and geological change) and biotic factors (e.g., cospeciation with pollinators).
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1 - Harvard University, Arnold Arboretum, Harvard University Herbaria, 22 Divinity Avenue, Cambridge, MA, 02138, USA
divergence time estimates
Presentation Type: Symposium or Colloquium Presentation
Location: Cottonwood C/Snowbird Center
Date: Tuesday, July 28th, 2009
Time: 1:30 PM