Significance of Orthogenesis in Angiosperm Taxonomy

Abstract

Higher categories of vertebrate animals (especially Reptilia. Aves, Mammalia, and orders within the Mammalia) are readily distinguished by structural features showing adaptation to environment. The more numerous higher categories among angiosperms, on the contrary, are distinguished by combinations of many structural features showing no apparent adaptation to environment. Darwinian evolution is prominent at all taxonomic levels among the higher vertebrate animals. Orthogenetic evolution is prominent at higher taxonomic levels among angiosperms, while Darwinian evolution apparently is operative chiefly at the rank of genus and below. The great difference in the ease with which higher animal categories can be recognized and handled taxonomically as compared with corresponding categories among the flowering plants is the consequence of a fundamental difference in the prevailing mode of evolution. The difficulty with which the higher categories of angiosperms are recognized stands in sharp contrast to the relative ease with which such categories are discerned in the mammals, or in vertebrates as a whole. The three classes of essentially land vertebrates, the Reptilia, Aves, and Mammalia, are pretty clear even to the unlearned. Such mammalian orders as the chiroptera (bats(, Primates, Rodentia, Lagomorpha (rabbits and hares), Carnivora, Pinnipedia (seals and walruses), Proboscidea (elephants), Perissodactyla (horses, etc.), Artiodactyla (cloven-hoofed animals), and Cetacea (whales and porpoises) are familiar to and readily recognized by a great many laymen, even though they may never have heard of the scientific ordinal names. Likewise such mammalian families as the Canidae, the Felidae, the Ursidae, the Equidae, the Cervidae, and the Sciuridae. The situation among the angiosperms is quite otherwise. Rare indeed is the layman who knows the difference between monocots and dicots, and rarer still one who recognizes any dicotyledonous orders beyond perhaps the legumes and the Cactales, and the Cactales would have to be defined to include a lot of spiny African euphorbiads. The monocots may fare just a little better: The orders Graminales, Liliales, and Palmales, and the more showy members of the family Orchidaceae might well be recognized by many laymen. IPart of an address given at the Third Annual Symposium on Systematics held at the Missouri Botanical Garden on October 27, 1956. This content downloaded from 207.46.13.169 on Sat, 01 Oct 2016 05:34:56 UTC All use subject to http://about.jstor.org/terms