SIMS, HALLIE J.1* and KEVIN J. MCCONWAY2. 1Dept. of Paleobiology, Smithsonian Institution, Washington, D.C., U.S.A.; 2Dept. of Statistics, The Open University, Milton Keynes, U.K.. - Non-stochastic variation of species-level diversification rates within angiosperms.
Variations in the origination and extinction rates of species over
geologic time often are linked with a range of factors, including the
evolution of key innovations, changes in ecosystem structure, and
environmental factors such as changes in climate and physical
geography. Before hypothesizing causality of a single factor, it is
critical to demonstrate that the observed variation in diversification
is significantly greater than one would expect due to natural
stochasticity in the evolutionary branching process. We use a
likelihood ratio test to compare taxonomic rate heterogeneity to a
birth-death model, using data on well-supported sister pairs of taxa
and their species richness. Simulations indicate that this metric has
greater power and more accurate Type I error rates than previous
relative rate tests using sister taxa. We assess the likelihood that
the distribution of extant species among angiosperm genera and
families could be the result of stochastically constant
diversification rates. Results strongly support the conclusion that
there is significantly more heterogeneity in diversification at the
species-level within angiosperms than would be expected if the process
occurred at a constant, stochastically varying rate. This result is
consistent in independent data sets of genus pairs (S Li = 203.543, d.f. = 146, p =
0.0012) and family pairs (S Li
77.527, d.f. = 44, p = 0.0013) and is not affected significantly by
degrading pairs to simulate inaccuracy in the assumption of
simultaneous origin of sister taxa. When we parse taxon pairs into
higher groups of angiosperms, results indicate less non-stochastic
rate heterogeneity within rosids and basal eudicots than within
asterids and eumagnoliids. These results provide strong support for
the hypothesis that species-level rates of origination and/or
extinction have varied non-stochastically within angiosperms, and that
the magnitude of heterogeneity varies among major groups within
angiosperms.
Key words: angiosperms, diversity, evolutionary rates, likelihood, sister pairs