A biosystematic study of the section Pityopsis of the genus Heterotheca was undertaken. Cytological, morphological, chromatographic, and hybridization studies were utilized to provide a basis for taxonomic treatment of the taxa. The species were divided into two broad groups; the Heterotheca graminifolia group (H. adenolepis, H. aspera, H. graminifolia, H. microcephala var. microcephala and var. aequilifolia, and H. oligantha) and the H. pinifolia group (H. falcata, H. flexuosa, H. pinifolia, and H. ruthii) Heterotheca ruthii appears to be intermediate between the two groups and is closely related to H. oligantha.
Cytology indicates the diploid species (N=9) are prominent with only H. adenolepis (both diploids and tetraploid), H. graminifolia, and H. oligantha being tetraploids (N=8). Pollen measurements were obtained showing tetraploids being significantly larger (except for H. adenolepsis).
Paper chromatographs were somewhat inconclusive but generally followed the other taxonomic evidence for the section.
It is thought that H. graminifolia was derived via autopolyploidy from H. microcephala. Heterotheca adenolepsis is thought to have been derived from an allopolyploid hybrid between H. aspera and H. microcephala. Heterotheca oligantha has a limited range in Florida and Alabama but appears closely related to H. graminifolia and H. ruthii. Heterotheca pinifolia, H. falcata, H. flexuosa, and H. ruthii apparently were derived from one or more primitive ancestors which migrated from an Appalachian center of origin to the Coastal Plain, became geographically separated and evolved into the four entities listed above. Heterotheca ruthii appears to be the most primitive of the four species. All are limited in rang with H. ruthii found only in the Hiwassee Gorge of East Tennessee; H. flexuosa in a few counties in Florida: H. pinifolia in a few counties in the Sand Hills of Georgia and North and South Carolina, and H. falcata in sand areas in five northeastern states. These four species have relatively few genetic barriers but are widely separated geographically
