Do Mediterranean-type ecosystems have a common history?-Insights from the Buckthorn family (Rhamnaceae)

Abstract

Mediterranean-type ecosystems (MTEs) are remarkable in their species richness and endemism, but the processes that have led to this diversity remain enigmatic. Here, we hypothesize that continent-dependent speciation and extinction rates have led to disparity in diversity between the five MTEs of the world: the Cape, California, Mediterranean Basin, Chile, and Western Australia. To test this hypothesis, we built a phylogenetic tree for 280 Rhamnaceae species, estimated divergence times using eight fossil calibrations, and used Bayesian methods and simulations to test for differences in diversification rates. Rhamnaceae lineages in MTEs generally show higher diversification rates than elsewhere, but speciation and extinction dynamics show a pattern of continent-dependence. We detected high speciation and extinction rates in California and significantly lower extinction rates in the Cape and Western Australia. The independent colonization of four of five MTEs may have occurred conterminously in the Oligocene/Early Miocene, but colonization of the Mediterranean Basin happened later, in the Late Miocene. This suggests that the in situ radiations of these clades were initiated before the onset of winter rainfall in these regions. These results indicate independent evolutionary histories of Rhamnaceae in MTEs, possibly related to the intensity of climate oscillations and the geological history of the regions.

Flowering after disaster: Early Danian buckthorn (Rhamnaceae) flowers and leaves from Patagonia

“…The Hauenschild et al topology is based on sequence data from one chloroplast marker ( trnL-trnF ) and one nuclear marker (ITS) for more than 400 species. Then, we constrained the tree searches such that the final topology is consistent with the relationships reported by Onstein et al [29]. The Onstein et al topology is based sequence data from six chloroplast markers and one nuclear marker (ITS) for 280 species.…”

“…Rhamnaceae are distributed throughout tropical and temperate environments worldwide [21,116], but highest diversity is associated with seasonally dry Mediterranean-type environments [29,56]. The broad distribution of the family hindered early efforts to draw conclusions about their biogeographic history [117].…”

“…Gentry [55] proposed a Laurasian origin for the family and subsequent expansion into the Southern Hemisphere; however, recent work supports an alternate hypothesis. Richardson et al and Onstein et al showed that many of the groups associated with tropical and subtropical forests are early-divergent lineages within Rhamnaceae [26,29,53,56]. Richardson et al [53] suggested a Gondwanan origin for the family but did not specify the forest type, and Onstein et al [29,56] emphasized the tropical rainforest aspect of the likely ancestral habitat.…”

“…[51,52]. These fossils provide minimum age estimates for the diversification of crown-group Rhamnaceae, and several have been used to calibrate trees in recent molecular phylogenetic analyses [29,53,54]. …”

Are there many different routes to becoming a global biodiversity hotspot?

“…The Cape Floristic hotspot of South Africa is dominated by a small number of exceptionally species-rich plant clades that have diversified largely in situ over an extended period at relatively slow and constant rates (9). In California, high species richness has also been attributed to in situ diversification dynamics rather than accumulation of diversity from elsewhere, albeit with much variation among individual lineages in speciation and extinction rates (10,18). These are all preliminary snapshots of individually large, complex, and very incompletely understood systems, but they do suggest striking differences in the balance between colonization and in situ diversification and especially in the timing and rates of these parameters.…”