Nonstochastic Variation of Species-Level Diversification Rates Within Angiosperms

Abstract

Variations in the origination and extinction rates of species over geological time often are linked with a range of factors, including the evolution of key innovations, changes in ecosystem structure, and environmental factors such as shifts 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. Here, we use a likelihood-ratio test to compare taxonomic rate heterogeneity to a neutral birth-death model, using data on well-supported sister pairs of taxa and their species richness. We test the likelihood that the distribution of extant species among angiosperm genera and families could be the result of constant diversification rates. Results strongly support the conclusion that there is significantly more heterogeneity in diversity at the species level within angiosperms than would be expected due to stochastic processes. This result is consistent in datasets of genus pairs and family pairs 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 among higher groups of angiosperms, results indicate that a constant rates model is not rejected by rosid and basal eudicot pairs but is rejected by asterid and eumagnoliid pairs. These results provide strong support for the hypothesis that species-level rates of origination and/or extinction have varied nonrandomly within angiosperms and that the magnitude of heterogeneity varies among major groups within angiosperms.

The Relationship Between Global and Regional Distribution Diminishes Among Phylogenetically Basal Species

“…Across a purely central European phylogeny, species from such lineages will be ranked too low; too basal. Moreover, clade rank may be strongly biased toward species from only a few species-rich lineages (Sims and McConway 2003;Magallon and Sanderson 2001); hence clade rank becomes indiscernible from clade membership. Overall, clade rank may be a biased parameter of the phylogenetic position of species.…”

“…For example, among birds, basal species are marginally distributed ecologically or geographically, and may thus suffer little competition from more actively diversifying clades (Ricklefs 2003). The low number of close relatives reflects either low speciation or high extinction rates (Sims and McConway 2003;Magallon and Sanderson 2001;Ricklefs and Renner 1994), which may themselves affect the distribution of species, even though there is little consensus on the mechanisms involved. A low speciation rate might result in a high age of many species and a long period of global range expansion from the locality of origin (Vermeij 1978;Levin 2000).…”

Detecting the Historical Signature of Key Innovations Using Stochastic Models of Character Evolution and Cladogenesis

“…A popular approach has been to compare the relative sizes of sister groups using nonparametric sign tests, for example, in studies of insect phytophagy (Mitter et al 1988), plant latex and resin canals (Farrell et al 1991), floral nectar spurs (Hodges 1997), and flower symmetry (Sargent 2004). Alternatively, parametric approaches make it possible to identify unusually large clades by fitting data to predictions of null models of stochastic cladogenesis, and this can increase the statistical power associated with trait correlations (Slowinski and Guyer 1993;Sims and McConway 2003;McConway and Sims 2004).In contrast, it is more difficult to evaluate unique origins of key innovations. If a trait arises only once, distinguishing its effect on diversification from the effects of other, coincidental, factors becomes problematic (see Discussion).…”

“…Despite this, general methods for detecting shifts in diversification rate on phylogenies can be used to associate an inferred rate shift with the origin of the trait of interest. These use stochastic branching models to evaluate the observed data, either the distribution of clade sizes on a phylogenetic tree (e.g., Slowinski and Guyer 1989;Sanderson and Donoghue 1994;Sims and McConway 2003;McConway and Sims 2004;Moore et al 2004), or the distribution of the number of lineages through time (e.g., Nee et al 1992Nee et al , 1994Paradis 1997). The dichotomy of approaches has led to them being termed topological and temporal methods, respectively (Chan and Moore 2002).…”

Elatinaceae are sister to Malpighiaceae; Peridiscaceae belong to Saxifragales

“…Neotropical Malpighiaceae appear to have coevolved with these insect pollinators, and this may partially account for the greater diversity of New World species relative to Old World species. Until the sister-group relationships of the family are clarified we cannot begin to evaluate the hypothesis that a shift in species diversification was associated with these novel floral structures associated with the family (e.g., Guyer and Slowinski, 1993;Sanderson and Donoghue, 1994;Heard, 1997, 2002;Sims and McConway, 2003).…”