Clarkia stellata Mosquin (Onagraceae) is an uncommon annual herb endemic to Plumas and Yuba Counties in northeastern California that has threatened populations due to noxious weeds, recreational and forest management activities, and development. The purpose of this study is clarify the species identification of populations of Clarkia stellata for management purposes, specifically, populations of C. stellata and C. rhomboidea that are difficult to differentiate in the field. A total of 11 populations of C. stellata and related species were sampled for morphometric analyses and nine populations were sampled for genetic analysis using amplified fragment length polymorphisms (AFLPs). Clarkia stellata can be separated from C. rhomboidea based on all floral characteristics except claw width, claw length, and isthmus width. These species also can be differentiated based on the following vegetative characteristics: petiole length, leaf length, leaf width, and plant height. The sympatric Clarkia mildrediae is easily differentiated from C. stellata by every character except petiole length, leaf length, and plant height; C. mildrediae differs from C. rhomboidea for all characters except petal speckling, pollen color, leaf width, and leaf length. Populations that were initially difficult to categorize as either C. stellata or C. rhomboidea were most similar to C. stellata; however, we were not able to identify a suite of characters that would distinguish these populations as either C. stellata or C. rhomboidea. An analysis of molecular variance (AMOVA) shows that although there was genetic variation among all populations (13.19%), the majority of variation is found within populations (86.81%). Genetic differentiation among all populations was low as calculated by Genetic Data Analysis (Φ_st = 0.132) and Hickory (θ^B = 0.0137); variance within populations was high (sigma-G = 32.645) and between populations was low (sigma-P = 4.96).

This work is consistent with a number of studies within Clarkia section Myxocarpa that have identified taxonomic difficulties due to recent speciation, local adaptation, rapid chromosomal evolution, sympatry, and hybridization.