title: Physiological performance and mating system in Clarkia (Onagraceae): does phenotypic selection predict divergence between sister species?
abstract: OBJECTIVE
  The evolution of self-fertilization often occurs in association with other floral, life history, and fitness-related traits. A previous study found that field populations of Clarkia exilis (a predominantly autogamous selfer) and its sister species, Clarkia unguiculata (a facultative outcrosser) differ in mean photosynthetic rates and instantaneous water use efficiency (WUE(i)). Here, we investigate the strength and direction of selection on these traits in multiple populations of each taxon to determine whether natural selection may contribute to the phenotypic differences between them.
  
  
  METHODS
  In spring 2008, we measured instantaneous gas exchange rates in nine populations during vegetative growth (Early) and/or during flowering (Late). We conducted selection gradient analyses and estimated selection differentials within populations and across pooled conspecific populations to evaluate the strength, direction, and consistency of selection on each trait early and late in the season.
  
  
  RESULTS
  The direction and relative strength of selection on photosynthetic rates in these taxa corresponds to the phenotypic difference between them; C. exilis has higher photosynthetic rates than C. unguiculata, as well as stronger, more consistent selection favoring rapid photosynthesis throughout the growing season. Patterns of selection on transpiration, WUE(i), and the timing of flowering progression are less consistent with phenotypic differences (or lack thereof) between taxa.
  
  
  CONCLUSIONS
  We detected several examples where selection was consistent with the phenotypic divergence between sister taxa, but there were also numerous instances that were equivocal or in which selection did not predict the realized phenotypic difference between taxa.
authors.affiliation: Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106, USA. dudley@lifesci.ucsb.edu
authors.forename: Leah S
authors.initials: LS
authors.lastname: Dudley
authors.forename: Alisa A
authors.initials: AA
authors.lastname: Hove
authors.forename: Susan J
authors.initials: SJ
authors.lastname: Mazer
mesh_terms: D005075:Biological Evolution; D031663:Clarkia / Q000235:genetics* / Q000502:physiology*; D049810:Genetic Speciation; D014644:Genetic Variation; D010788:Photosynthesis / Q000502:physiology; D018526:Plant Transpiration; D054817:Pollination; D012098:Reproduction; D012641:Selection, Genetic; D014867:Water
chemical_list: D014867:Water