Diversification of peatmosses: a phylogenetic approach

Phylogenetic tree shape can provide insights into temporal patterns of biodiversification. Lineage-through-time (LTT) plots provide a quantitative description of tree shape allowing for statistical comparisons of alternative diversification models. We generated LTT plots from phylogenetic trees for Sphagnum, based on nrITS and cpDNA trnL-trnF sequences.

Analyses of LTT plots suggest that the diversification rate of Sphagnum has been decreasing (Fig. 1; A; based on 10 MP trees; B: based on 20 Bayesian trees). The application of survival models (Paradis 1997) indicate that a decreasing rate of diversification in Sphagnum has a significantly higher likelihood than alternative models (including constant diversification rate). The impact of phylogenetic uncertainty on LTT plots were investigated and the effects on inferences about diversification were insignificant (Fig. 1).

Nevertheless, incomplete sampling, even if random, can yield LTT plots that appear to represent a decreasing rate of diversification. To test the possibility that the decreasing rate of diversification observed in Sphagnum was an artifact of incomplete sampling, simulations were run in which 240 terminals (the size of our data set) were randomly sampled from simulated phylogenies, generated under a constant diversification model, with 300, 400, . 1000 actual extant taxa. The question was, at what level of total species richness would our sample of 240 taxa be so incomplete that an artifactual pattern of decreasing diversification would result?

The simulations (Fig. 2) showed that unless Sphagnum is twice as diverse at the species level as any recent systematist has suggested, the apparent decreasing diversification rate is unlikely to be an artifact of incomplete sampling.

Reference:

Shaw, A. J., C. J. Cox, & I. Melosik. 2004. Diversification of peatmosses: a phylogenetic approach. In Goffinet, B., V.C. Hollowell, and R.E. Magill [eds.]. Molecular systematics of bryophytes: progress, problems & perspectives, pp. 240-254. Monographs in Systematic Botany from the Missouri Botanical Garden. St. Louis, Missouri, USA.

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