title: Shifts in diversification rates and clade ages explain species richness in higher-level sedge taxa (Cyperaceae).
abstract: OBJECTIVE
  Understanding heterogeneity in species richness across the tree of life is a challenge in evolutionary biology. The sedge family, Cyperaceae, is classified into tribes that exhibit a roughly 200-fold range in species richness. The Cyperaceae present an excellent case study in the determinants of species richness within higher-level taxa.
  
  
  METHODS
  We used secondary calibration based on prior studies and fossils from a rush (Juncaceae) and five sedges to calibrate two previously published Cyperaceae phylogenies, then compared our results to previous molecular clock analyses. We used an information-theoretic approach to identify shifts in lineage diversification rates and phylogenetic generalized least squares to fit alternative models of clade species richness.
  
  
  RESULTS
  Our results suggest a late Cretaceous origin for Cyperaceae (76-89 mya). The inferred 0.06 speciation events Ma(-1) is comparable to overall diversification rates in the order Poales but faster than angiosperm background rates. A threefold increase in diversification rate at the base of the species-rich SDC+FAEC clade is correlated with climatic changes during the Paleocene-Eocene boundary (ca. 55 mya). The greater driver of among-clade variance in species richness, however, is clade age (simple R(2) = 0.334, P = 0.0006).
  
  
  CONCLUSIONS
  Although shifts in diversification rates play a role in the generation of heterogeneous patterns of species richness, our study demonstrates that variance in clade age alone explains ca. 33% of among-clade variation in species diversity, which stands in contrast to the general pattern for angiosperms.
authors.affiliation: Department of Botany, The Field Museum of Natural History, 1400 S. Lake Shore Drive, Chicago, Illinois 60605 USA.
authors.forename: Marcial
authors.initials: M
authors.lastname: Escudero
authors.forename: Andrew
authors.initials: A
authors.lastname: Hipp
mesh_terms: D044822:Biodiversity; D005075:Biological Evolution; D029785:Cyperaceae / Q000235:genetics*; D005580:Fossils; D049810:Genetic Speciation; D019684:Magnoliopsida; D008957:Models, Genetic; D010802:Phylogeny; D013045:Species Specificity
keywords: Bounded diversification; Cretaceous; extinction; radiation; speciation; times of diversification