DOYLE, JAMES A.*, MICHAEL J. SANDERSON, and SUSANA MAGALLON. Section of Evolution and Ecology, University of California, Davis, CA 95616. - Integrating fossil and molecular data on the age of angiosperms: effects of fossil age constraints and rate smoothing methods.
Most molecular estimates of the age of angiosperms, based on the
assumption of a molecular clock, have predated the oldest fossil
records of crown-group angiosperms in the Early Cretaceous (135 MA).
Previous experiments with rbcL showed marked variation in evolutionary
rates among lineages and suggested that ages based on this gene had
been overestimated due to reliance on high-rate herbaceous taxa and
failure to consider rate variation across sites, and they revealed
large, unexplained differences between ages based on different codon
positions. Proposed solutions include use of more fossil age
constraints on nodes and new methods that allow deviation from a
clock. Using all positions of rbcL and assuming a clock, addition of
18 minimum ages within angiosperms and outgroups, which pushes back
the ages of some ancient clades (e.g., Proteales) before their
inferred molecular ages, increases the estimated age of angiosperms
from 141 to 201 MA. Assuming a maximum age of 125 MA for eudicots
lowers this only slightly, to 184 MA. Nonparametric rate smoothing
(NPRS), which assumes that rates vary gradually, results in ages that
conflict more with the fossil record, not less: 207 MA with no
constraints, 277 MA with minimum constraints, and 215 MA with maximum
and minimum constraints. A new penalized likelihood method designed to
overcome possible shortcomings of NPRS gives ages similar to those
based on NPRS. These results may mean either that angiosperms are much
older than their known fossil record, or that rates of molecular
evolution did not change smoothly but rather accelerated during the
radiation of angiosperms and slowed in "living fossils" such
as Platanus, Nelumbo, and Winteraceae. Plastid photosystem genes show
less conflict between dates based on different codon partitions,
suggesting that rbcL may not be a good choice for molecular age
estimates.
Key words: angiosperms, fossil record, molecular clock, rate smoothing, rbcL