American Journal of Botany 96: 366-381 (2009)
© 2009 Botanical Society of America, Inc.
Department of Plant Biology, Cornell University, Ithaca, New York 14853 USA
ABSTRACT
The rapid diversification and ecological dominance of the flowering plants beg the question "Why are there so many angiosperm species and why are they so successful?" A number of equally plausible hypotheses have been advanced in response to this question, among which the most widely accepted highlights the mutually beneficial animal–plant relationships that are nowhere better developed nor more widespread than among angiosperm species and their biotic vectors for pollination and dispersal. Nevertheless, consensus acknowledges that there are many other attributes unique to or characteristic of the flowering plants. In addition, the remarkable coevolution of the angiosperms and pollination/dispersal animal agents could be an effect of the intrinsic adaptability of the flowering plants rather than a primary cause of their success, suggesting that the search for underlying causes should focus on an exploration of the genetic and epigenetic mechanisms that might facilitate adaptive evolution and speciation. Here,we explore angiosperm diversity promoting attributes in their general form and draw particular attention to those that, either individually or collectively, have been shown empirically to favor high speciation rates, low extinction rates, or broad ecological tolerances. Among these are the annual growth form, homeotic gene effects, asexual/sexual reproduction, a propensity for hybrid polyploidy, and apparent "resistance" to extinction. Our survey of the literature suggests that no single vegetative, reproductive, or ecological feature taken in isolation can account for the evolutionary success of the angiosperms. Rather, we believe that the answer to Darwin’s second "abominablemystery" lies in a confluence of features that collectively make the angiosperms unique among the land plants.
Key Words: agamospermy • angiosperm diversification • annual growth form • floral trait evolution • homeotic genes • polyploidy • species morphospace
Received for publication 3 April 2008. Accepted for publication 13 October 2008.
FOOTNOTES
1 The authors thank Drs. B. H. Tiffney (University of California, Santa Barbara) and K. Pigg (Arizona State University) for valuable suggestions to improve an early draft of this paper, which we dedicate to Verne Grant whose pioneering work in the field of angiosperm speciation and biology is as relevant today as it was in the last century.
2 Author for correspondence (e-mail: wlc1@cornell.edu)
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