MAGALLON, SUSANA A.1* and MICHAEL J. SANDERSON2. 1Instituto de Biologia, Universidad Nacional Autonoma de Mexico, A.P.70-233, Mexico D.F. 04510, Mexico; 2Section of Evolution and Ecology, University of California, One Shields Ave., Davis, CA 95616. - Absolute diversification rates in angiosperm clades.
Angiosperms include an exceedingly high taxonomic richness, which
emerged in a relatively brief period of time. We examine the
evolutionary diversification of angiosperms by estimating the absolute
rate of diversification for angiosperms as a whole, and for a large
set of angiosperm clades. We also identify angiosperm clades with a
standing diversity that is either much higher or much lower than
expected, given the estimated background diversification rate.
Identification of angiosperm clades, their relationships and taxonomic
composition are based on an empirical compilation of recent primary
studies. Ages of clades were obtained from an integrative and critical
use of the paleobotanical record. Diversification was modeled as a
stochastic time-homogeneous birth and death process that depends on
the rate of diversification (r), and significantly, on the
relative extinction rate (e). The process
also considers if the age of any given clade corresponds to its stem
group or to its crown group. Absolute diversification rates were
estimated under e=0 (no extinction) and
e=0.9 (a high relative extinction rate).
The absolute diversification rate of the angiosperms as a whole, and
of 58 angiosperm clades, were obtained. Confidence intervals (95%) of
the expected number of species in a clade that diversifies at the rate
estimated for angiosperms as a whole (under e=0 and e=0.9) were
obtained, and the standing diversity of each angiosperm clade was
compared to the expected number of species of a clade its age. Ten
clades fall above the confidence intervals. These exceedingly
species-rich clades include Asterales, Lamiales and Cyperales.
Thirteen clades fall below the confidence intervals, and were
recognized as unexpectedly species-poor. Among these are
Chloranthaceae, Nymphaeales and Platanaceae. The phylogenetic
distribution of exceedingly species-rich clades suggests that traits
that confer high rates of diversification evolved independently in
different lineages, and most likely do not characterize angiosperms as
a whole.
Key words: birth-and-death process, crown-group, extinction, phylogeny, speciation, stem-group