Graduate Thesis Or Dissertation

A fistful of Astragalus : phenotypic and genotypic basis of the most taxon rich species in the North American flora

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  • The study of the infra-specific ranks (i.e., subspecies and variety) can be considered the study of the process of speciation. Astragalus lentiginosus Douglas ex Hooker (Fabaceae) is the most taxon rich species in the U.S. flora currently including 40 taxonomic varieties. These varieties were described using traditional taxonomic methods primarily through the work of M.E. Jones, P.A. Rydberg, and R.C. Barneby. Presently, three methodologies are employed to test the taxonomic hypotheses presented by these monographers as well as the more simplistic hypothesis of isolation by distance. These datasets include morphometric measurements made from herbarium samples, chloroplast simple sequence repeats (CpSSR), and amplified fragment length polymorphisms (AFLP). Morphometric analysis is an explicit test of the morphological characters employed to circumscribe taxa within the group. Analyses indicated cohesiveness to morphological varieties but an absence of significant discontinuity to define these taxa. This indicates that the varieties may make cohesive groups, but their distinction is arbitrary. Analysis of CpSSRs indicates that the greatest amount of variance is described by among population and within population components of variance as opposed to varietal or Rydbergian sectional components. Morphological and CpSSR data present an incongruence between phenotypic and molecular data, a pattern which has been interpreted as the possible signal of selection. The AFLP dataset demonstrates a greater concordance of molecular data to morphological taxonomy, but also presents some interesting differences. An east-west transect at approximately 36.5° latitude demonstrates a pattern where A. l. var. variabilis occurs in the basins while A. l. var. fremontii occurs in the ranges. Genetic differentiation along this transect appears to support taxonomy despite a confounding geographic pattern, suggesting differential gene-flow among morphological varieties or that the AFLP dataset has captured the signal of selectively constrained loci (or regions linked to these loci). Multiple datasets provide multiple perspectives on evolution within A. lentiginosus. A significant pattern of morphological diversity accompanied by two molecular datasets which do not support this phenotypic diversity are interpreted as a potential instance of selective divergence among the varieties of A. lentiginosus.
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