As a whole, genetic divergence was surprisingly similar among all three species comparisons (Table 1) despite the fact that the divergence of H. annuus with H. petiolaris and H. debilis clearly precedes the H.petiolaris-H. debilis divergence . Hybridization between recognized species is known to participate in shaping the evolutionary diversity of North American sunflowers and probably explains the patterns observed here. H. petiolaris and H. debilis are allopatric sister species, but both hybridize with the sympatric but evolutionary more distant H. annuus. H. annuus and H. debilis are known to have a long history of introgression in Texas, where these individuals were collected . Similarly, H. petiolaris forms numerous hybrid zones with H. annuus throughout their ranges . As such, these more distantly related species have exchanged genes across much of the genome while remaining morphologically and ecologically distinct , presumably accounting for lower than expected average FST values.