Origin and Early Radiation of Angiosperms

The seemingly sudden appearance and dramatic rise of the angiosperms, Darwin's "abominable mystery", has been a continuous challenge for botanists. The scenarios which are currently envisaged for their origin try to combine the various fields of evidence into a coherent picture. Stebbins (1974) and Doyle (1978) have evaluated much of the evidence and the hypotheses that were based on it, and paleobotanical studies have substantially added to these views (Friis et al. 1987).

It has often been argued that the evolutionary success of angiosperms over gymnosperms was due to their reproductive ecology, intimately interconnected with their reproductive morphology. Relevant angiospermous advances include the protection of ovules, the possession of stigmas, the acceleration of gametophyte development, and double fertilization. Consequently, the rise and success of the angiosperms would have been particularly connected with insect pollination. At later stages, biotic interactions, not only with pollinators, but also with seed dispersers, symbionts, and predators, further shaped angiosperm diversification, mediated, no doubt, by the diversification of their secondary metabolism. The increased biosynthetic capacity of angiosperms in producing secondary metabolites, providing attractants, repellents, and other kinds of allelochemicals, has to be viewed in connection with their coevolution with arthropods, fungi, vertebrates, and other groups of organisms (Kubitzki and Gottlieb 1984a,b). Vegetative traits, such as the functional superiority of the conductive tissue of angiosperms in comparison with gymnospermous groups, or simply their higher growth rates, have also been considered as important factors for the evolutionary success of the angiosperms. On a more general level, the abbreviation of morphogenetic programs of angiosperms has led to an acceleration of their life cycle (Takhtajan 1976). The reduction of their gametophytes and the evolution of short-lived forms are two obvious manifestations of this trend. It is likely that the acceleration of reproductive maturity produced small, short-lived forms that were adapted to unstable or ephemeral environments in the lower Cretaceous. These forms may have represented one (the only?) transitional stage in early angiosperm evolution (Hickey and Doyle 1977; Doyle 1978). While there is little doubt that all the above features may have had some bearing upon the origin or radiation of angiosperms, the impact of each single factor can hardly be separately gauged.

In view of the enormous diversity of angiosperms, many authors have found it hard to envisage a mono-phyletic origin of this group. Several models ("floral theories", see Friis and Endress 1990 for a brief summary) have been elaborated, to support either a pleiophyletic or a monophyletic origin. Most of these hypotheses were based on extant forms rather than on fossils. Recently, however, the increasing knowledge of diverse fossil gymnosperm groups, such as the Pentoxylales, Caytoniales, and Corystospermales, has provided material for more meaningful comparisons. Meeuse's (summary: without year) anthocorm theory tried to bridge the gap among the latter gymnospermous groups and the angiosperms, and although he pleaded for a pleiophyletic origin of the latter, the evidence available now seems more than ever to favor monophylesis. In other words, the syndrome of angiospermy presumably originated in a single phylogenetic line in which the individual an-giospermous traits' were added to one another. This does not exclude the possibility that the diversifica tion of angiosperms proceeded in very distinct evolutionary lines from the very beginning.

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