Contrasted patterns of hyperdiversification in Mediterranean hotspots

Abstract

Dating the Tree of Life has now become central to relating patterns of biodiversity to key processes in Earth history such as plate tectonics and climate change. Regions with a Mediterranean climate have long been noted for their exceptional species richness and high endemism. How and when these biota assembled can only be answered with a good understanding of the sequence of divergence times for each of their components. A critical aspect of dating by using molecular sequence divergence is the incorporation of multiple suitable age constraints. Here, we show that only rigorous phylogenetic analysis of fossil taxa can lead to solid calibration and, in turn, stable age estimates, regardless of which of 3 relaxed clock-dating methods is used. We find that Proteaceae, a model plant group for the Mediterranean hotspots of the Southern Hemisphere with a very rich pollen fossil record, diversified under higher rates in the Cape Floristic Region and Southwest Australia than in any other area of their total distribution. Our results highlight key differences between Mediterranean hotspots and indicate that Southwest Australian biota are the most phylogenetically diverse but include numerous lineages with low diversification rates.

Fig. 1.

Dated phylogeny of Proteaceae: maximum clade credibility tree with mean ages from the Bayesian uncorrelated lognormal method (implemented in BEAST) using nucleotide sequence data from 8 loci. Nodes on which fossil age constraints were applied (as uniform prior distributions) are identified with black dots (see Table S1). White dots identify additional, redundant, or uninformative age constraints obtained from the fossil analyses. Branches are colored according to absolute net diversification rate by stem age of their subtending clade. Taxa present in Mediterranean hotspots are identified with either SWA (Southwest Australia) or CFR (Cape Floristic Region). If not endemic to these hotspots, the percentage of hotspot species is mentioned in brackets. Total species numbers are indicated in brackets after the name of each taxon. Absolute ages are in million years.

Fig. 2.

Confidence intervals on expected clade diversity (number of extant species) according to stem group age, given a fixed background diversification rate (r) equal to that of Proteaceae as a whole. The black lines are the 95% confidence intervals in the absence of extinction (r0 = 0.066) and the gray lines under a model of high relative extinction rate (r0.9 = 0.046). (A) All clades on the phylogeny; clades with >50% of their species in Mediterranean hotspots are marked with filled diamonds. (B) Clades with >50% of their species in Southwest Australia; horizontal dashed lines are the 95% highest probability densities of ages estimated in BEAST. (C) Clades with >50% of their species in the Cape Floristic Region; horizontal lines as above.