North American Dryopteris hybrid complex
Hybridization and polyploidy are common phenomena in ferns, and the genus "Dryopteris" is known to be one of the most freely-hybridizing fern genera. North American botanists recognized early that there were close relationships between many of the species of "Dryopteris" on the continent, and that these relationships reflected hybrid ancestry. The complex includes six sexual diploid parents (one of which, ""D. semicristata"", is hypothesized to be extinct), six sexual allopolyploids, and numerous sterile hybrids at various ploidal levels.