Loren H. Rieseberg, Stephen M. Beckstrom-Sternberg, Aaron Liston, Dulce M. Arias, Phylogenetic and Systematic Inferences from Chloroplast DNA and Isozyme Variation in Helianthus sect. Helianthus (Asteraceae), Systematic Botany, Vol. 16, No. 1 (Jan. - Mar., 1991), pp. 50-76
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Rieseberg et al. (1991) studied this question.
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