Species boundaries in the Astragalus cusickii complex delimited using molecular phylogenetic techniques

Understanding the source of phenotypic variability is a challenge in the biological sciences. Variation in phenotypes is the result of variation in the genetics and environment the organism experiences, but elucidating the relative contribution of these two parameters can pose problems, especially in the field of systematics. Systematists are challenged to classify biological diversity into groups that share common ancestry. Phenotypic variation can be useful to demonstrate common ancestry, but only when the primary contributor to the variation is under strong genetic control, and thus heritable. Cusick's milkvetch (Astragalus cusickii) is a perennial forb endemic to the northwestern intermountain region of the United States. The species currently comprises four varieties based on subtle morphological dissimilarities, such as leaf size and density, and the size and shape of the seed pods. The taxonomic organization of the varieties of A. cusickii and related species of Astragalus were reexamined through phylogenetic analysis of low copy nuclear, nuclear-ribosomal, and chloroplast gene regions. Maximum parsimony, maximum likelihood, Bayesian inference, the genealogical sorting index, and an approximately unbiased test were used to determine appropriate species boundaries under the phylogenetic species concept. The results support reclassification of A. cusickii var. packardiae and A. cusickii var. sterilis as separate species. Additionally, evidence suggests a chloroplast capture event may have occurred in one population of A. cusickii var. packardiae.