Why are some traits and trait combinations exceptionally common across the tree of life, whereas others are vanishingly rare? The distribution of trait diversity across a clade at any time depends on the ancestral state of the clade, the rate at which new phenotypes evolve, the differences in speciation and extinction rates across lineages, and whether an equilibrium has been reached. Here we examine the role of transition rates, differential diversification (speciation minus extinction) and non-equilibrium dynamics on the evolutionary history of angiosperms, a clade well known for the abundance of some trait combinations and the rarity of others. Our analysis reveals that three character states (corolla present, bilateral symmetry, reduced stamen number) act synergistically as a key innovation, doubling diversification rates for lineages in which this combination occurs. However, this combination is currently less common than predicted at equilibrium because the individual characters evolve infrequently. Simulations suggest that angiosperms will remain far from the equilibrium frequencies of character states well into the future. Such non-equilibrium dynamics may be common when major innovations evolve rarely, allowing lineages with ancestral forms to persist, and even outnumber those with diversification-enhancing states, for tens of millions of years.
Parsimony analysis was used to develop phylogenetic hypotheses for Rosidae and other nonmagnoliid dicotyledons, especially Asteridae. Rosidae were placed among "lower" Hamamelidae as the sister group oi Platanus and Hamamelidaceae, "Higher" Hamamelidae (Fagales, Juglandales, and Casuarinaceae), Dilleniidae, and Asteridae nest within a paraphyletic Rosidae. With some expansion, the traditional Asteridae are monophyletic. For example, the problematic Columellia was placed among Asteridae as the sister group of Caprifoliaceae. Asteridae were placed as the sister group of Ericales among rosids circumscribed as Corniflorae in recent classifications of Dahlgren, Special attention was given to problematic groups that have been allied variously with Asteridae, Dilleniidae, and Rosidae. For example, Actinidiaceae and Fouquieriaceae were placed among Ericales, Loasaceae and Sarraceniaceae formed the sister group Dilleniidae Ochnaceae taxon of Linales. Dilleniaceae and Theaceae form a monophyletic group with Paracryphia placed as the sister group of Rhizophoraceae and Anisophylleaceae, hama-Rosidae are central to understanding phyloge-particularly among the so-called 'iower netic patterns among nonmagnoliid dicotyledons. melids, such as Hamamehdaceae and Platanaceae rcumscribed , , , , (Hufford & Crane, 1989). Certain so-cafled ''highand Cronquist (1981, 1988), Rosidae may be para-er" hamamelids, such as Fagales and Juglandales, phyletic with respect to Asteridae, Dilleniidae, and have been suggested to be more closely related o some Hamamelidae, To understand better the evo-Rosidae than to "lower" Hamamelidae (Wolte, lution and diversification of nonmagnoliid dicoty-1973, Hickey & Wolfe, 1975 Rickey & ledons, more precise and viable hypotheses of re-Doyle, 1977;Thorne, 1976;Nixon, 1989). lationships among these major groups are necessry.Rosidae have been considered "more advanced" be com elpful suggestions.
Rapid, ancient radiations pose one of the most difficult challenges for phylogenetic estimation. We used DNA sequence data of 9,006 aligned base pairs from five genes (chloroplast atpB, matK, rbcL, and 18S and 26S nrDNA) to elucidate relationships among major lineages of Saxifragales (angiosperms, eudicots). These relationships were poorly supported in previous studies, apparently because the lineages originated in rapid succession. Using an array of methods that explicitly incorporate assumptions about evolutionary process (weighted maximum parsimony, maximum likelihood, LogDet/paralinear transformed distances), we show that the initial diversification of Saxifragales was indeed rapid. We suggest that the poor resolution of our best phylogenetic estimate is not due to violations of assumptions or to combining data partitions having conflicting histories or processes. We show that estimated branch lengths during the initial diversification are exceedingly short, and we estimate that acquiring sufficient sequence data to resolve these relationships would require an extraordinary effort (approximately 10(7) bp), assuming a linear increase in branch support with branch length. However, our simulation of much larger data sets containing a distribution of phylogenetic signal similar to that of the five sampled gene sequences suggests a limit to achievable branch support. Using statistical tests of differences in the likelihoods of topologies, we evaluated whether the initial radiation of Saxifragales involved the simultaneous origin of major lineages. Our results are consistent with predictions that resolving the branching order of rapid, ancient radiations requires sampling characters that evolved rapidly at the time of the radiation but have since experienced a slower evolutionary rate.
ENDRESS, P. K. & HUFFORD, L. D., 1989. The diversity of stamen structures and dehiscence patterns among Magnoliidae. Structure of stamens, particularly the patterns of anther dehiscence were studied over a wide range of families of the Magnoliidae with emphasis on the Magnoliales and Laurales as the most conservative orders of the angiosperms. Valvate dehiscence by proximal and distal stomial bifurcation was found (in addition to the already known Sarcandra and Polyalthia) for the first time in Degeneriaceae, Himantandraceae, Eupomatiaceae, in some additional Annonaceae, and in Peumus of the Monimioideae sensu lata. At least proximal bifurcations of the stomia occur in some Magnoliaceae and Ranunculaceae. An endothecial‐like connective hypodermis was found (in addition to the already known Chloranthaceae and Magnoliaceae) in some Annonaceae, in Pseudowintera (Winteraceae), and in Thalictrum (Ranunculaceae). In the Annonaceae an endothecial‐like connective hypodermis is partly correlated with valvate dehiscence by stomial bifurcations (as in many Hamamelididae). However, in many Magnoliidae stamens with this valvate pattern the anther is massive, especially in ‘laminar’ stamens, and the counterforce to the opening valves is therefore provided on the morphological and not on the histological level. Concomitant with valvate dehiscence by circular or elliptic flaps in the Laurales is often structural and functional dissocation of the two pollen sacs of a thcca, which is expressed by: (1) independent opening of each pollen sac, (2) lack of disruption of the interlocular zone of a theca, (3) frequent occurrence of asymmetry of the two pollen sacs of the theca, (4) frequent loss of one pollen sac per theca. In Berberidaceae with similar flaps asymmetry of the two pollen sacs of a theca is also common. These finds, together with the detection by paleobotanists of valvate anthers from the Lower Cretaceous, point to the probability that valvate anthers were more common in primitive angiosperms than previously thought.
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