Typical occidentalis is a relatively uniform diploid (2n = 34) taxon of east-central and midwestern states, but variation needs further study. The species may be relatively close to mollis, but its ancestry remains somewhat obscure (Stepherns et al. 2015). A few colls. from Ky. appear to be hybridized with hirsutus (MEAD at KY), mollis (HARD at KY, R. Athey #4439 from MARS at APSC) and perhaps silphioides (R. Athey #1956 from CALL at WK and MEM, initially named "dowellianus"). At Fort Campbell in Tenn. at least (adjacent to TRIG), there appears to be extensive introgression from mollis in old fields reverting to native grassland. See also notes under var. dowellianus.