title: Insights from population genetics for range limits of a widely distributed native plant.
abstract: OBJECTIVE
  Species' range limits are determined by a combination of history, ecology, and genetics. While much of our understanding of range limits comes from ecological studies, molecular population genetic methods can provide insight into the contribution of historical range shifts and gene flow to current range limits.
  
  
  METHODS
  We sequenced nine nuclear loci in 68 individuals from four populations (two within the range interior, one at the western range edge, and one at northern range edge) of the native annual legume Chamaecrista fasciculata in the Upper Midwest of North America.
  
  
  RESULTS
  Molecular diversity was greatest in an interior population and significantly reduced at both the western and northern range edges. We found no molecular genetic evidence for historical demographic expansion or contraction in any of the populations. Coalescent simulations indicate that the absence of a genetic signal for expansion or contraction could be due to inadequate statistical power or to a bottleneck that was weak and old. Populations were significantly differentiated and clustered into three demes, with the fourth population containing individuals that were assigned to the two nearest populations. We found evidence that recent migration between the northern edge and the most proximate interior population has been slight, within the distribution of values that theoretical models predict would alleviate genetic load without impeding adaptation.
  
  
  CONCLUSIONS
  When coupled with results from transplant experiments, these results suggest that ecological-genetic factors other than gene flow currently limit range expansion of C. fasciculata. Further, they highlight challenges in using molecular data to make inferences about species' distributions that have complex demographic histories.
authors.affiliation: Department of Ecology, Evolution and Behavior, 100 Ecology Building, 1987 Upper Buford Circle, University of Minnesota, St. Paul, Minnesota 55108, USA. johnsg@uvm.edu
authors.forename: John
authors.initials: J
authors.lastname: Stanton-Geddes
authors.forename: Ruth G
authors.initials: RG
authors.lastname: Shaw
authors.forename: Peter
authors.initials: P
authors.lastname: Tiffin
mesh_terms: D007887:Fabaceae / Q000235:genetics*; D051456:Gene Flow; D014644:Genetic Variation; D006239:Haplotypes; D015146:Midwestern United States; D058974:Phylogeography