Pellaea gastonyi is an apogamous tetraploid that has originated through repeated hybridization between P . atropurpurea and P . glabella . Isozyme studies (G. J. Gastony 1988) indicate that P . glabella subsp. missouriensis was the diploid parent of plants found in Missouri, whereas diploid P . glabella subsp. occidentalis was involved in the origin of P . gastonyi populations occurring in western North America. Pellaea gastonyi is most often confused with P . atropurpurea , from which it differs in having sparsely villous rachises, smaller ultimate segments, and spores averaging more than 62 µm in diameter.