Contrary to D. B. Lellinger's (1985) hypothesis, isozyme data indicate that neither Pellaea glabella
nor P . ternifolia was involved in the origin of this apogamous triploid. Instead, it appears that P . atropurpurea is an autopolyploid derivative of a single diploid taxon that has not yet been located. A thorough survey of spore number per sporangium in this species should be undertaken to determine whether the diploid progenitor is still extant. Collections from western Canada identified as P . atropurpurea actually represent P . gastonyi , an apogamous tetraploid produced by hybridization between P . atropurpurea and diploid populations of P . glabella . Pellaea atropurpurea
has also hybridized with P . wrightiana ; the hybrid is a rare apogamous pentaploid known only from western Oklahoma. Pellaea lyngholmii
is the apogamous tetraploid hybrid between P . atropurpurea and P . truncata . Pellaea atropurpurea
is distinguished from all these hybrids by having rachises that are densely pubescent adaxially, larger ultimate segments, and spores averaging less than 62 µm in diameter.