Studies in the Capparidaceae. III. Evolution and Phylogeny of the Western North American Cleomoideae

Abstract

One of the best ways to visualize evolution is to arrange related forms in the order of their complexity. With fairly large and complete it is possible to show graphically the homology of unlike extremes by observing the intermediates. Use of the so-called series (Woodson, 1935) such as the modifications described by Bower (1923) and by Zohary (1948) is open to severe criticism, since they are composed of contemporary types not in direct line of descent (Lam, 1951). Nevertheless, they help to elucidate such problems as relationships between species and species groups, direction of evolution, and morphology of derived structures, particularly in cases where paleobotanical and cytogenetical evidence is lacking. The Capparidaceae, as well as the whole of Rhoeadales, provides a wealth of series illustrating many evolutionary problems (Camp, 1950). Four western North American genera of the subfamily Cleomoideae, namely Cleome sect. PERITOMA, Cleomella, Wislizenia, and Oxystylis, constitute one of the most clear-cut reduction series, and their taxonomy, morphology, geographical distribution, intraand inter-generic relationships, and intra-specific variations are discussed in this paper. Based on the above, an attempted evolutionary interpretation is correlated with the geologic history of North America and is discussed in the light of the probable great importance of desert evolution in this group and the angiosperms generally.