Genera in the Agaricales: Advances and retreats in the search for a natural system

The historical development of the genus concept in the Agaricales is reviewed. Evolutionary theory did not exert much influence on the factual content of the classification of the Agaricales; and even the justification of the classification did hardly change. The two mostly used classifications of the Agaricales are based on a mixture of both phylogenetic and phenetic considerations. Many genera of the Agaricales, which have been newly described in the last decades, are based on insufficient criteria for generic delimitation. This generification must be considered a retrograde step in the construction of a natural system of the Agaricales. Cladistic methods are sound principles to arrive at a natural classification. Application of cladistic methodology in the Agaricales is beset with many difficulties, e.g. the lack of a sufficient number of characters, uncertainty with regard to possible outgroups, lack of robustness, lack of truly synapomorphous characters, etc. For these reasons a direct translation of cladograms into a classification will yield an unstable system and hence diminish the utility of classifications for practical purposes. As long as cladistic methods will not result in stable classifications, the introduction of new genera should not be encouraged. A traditional eclectic classification seems therefore the best option for the time being. A consistent application of phylogenetic methods will ultimately result in a classification that is not of much use for mycofloristic practice, and will most likely result in giving up the Agaricales as a holophyletic group. The Agaricales, as now circumscribed, will turn out to be either paraphyletic or polyphyletic.