Abstract
AbstractAimUnderstanding the evolution of the latitudinal diversity gradient (i.e. increase in species diversity towards the tropics) is a prominent issue in ecology and biogeography. Disentangling the relative contributions of environment and evolutionary history in shaping this gradient remains a major challenge because their relative importance has been found to vary across regions and taxa. Here, using the global distributions and a molecular phylogeny of Rhododendron, one of the largest genera of flowering plants, we aim to compare the relative contributions of contemporary environment, evolutionary time and diversification rates in generating extant species diversity patterns.LocationGlobal.Time periodUndefined.Major taxa studiedRhododendron.MethodsWe compiled the global distributions of all Rhododendron species, and constructed a dated molecular phylogeny using nine chloroplast genes and seven nuclear regions. By integrating these two datasets, we estimated the temporal trends of Rhododendron diversification, and explored the global patterns of its species diversity, net diversification rates, and species ages. Next, we reconstructed the geographical ancestral area of the clade. Finally, we compared the relative contribution of contemporary environment, time‐for‐speciation, and diversification rates on the species diversity pattern of Rhododendron.ResultsIn contrast to the predictions of the time‐for‐speciation hypothesis, we found that although Rhododendron originated at a temperate latitude, its contemporary species diversity is highest in the tropics/subtropics, suggesting an into‐the‐tropics colonization for this genus. We found that the elevated diversification induced by heterogeneous environmental conditions in the tropics/subtropics shapes the global pattern of Rhododendron diversity.Main conclusionsOur findings support tropical and subtropical mountains as not only biodiversity and endemism hotspots, but also as cradles for the diversification of Rhododendron. Our study emphasizes the need of unifying ecological and evolutionary approaches in order to gain comprehensive understanding of the mechanisms underlying the global patterns of plant diversity.
Highlights
The latitudinal gradient of species diversity has intrigued biologists continuously since the early 19th century
In contrast to the predictions of the time-for-speciation hypothesis, we found that Rhododendron originated at a temperate latitude, its contemporary species diversity is highest in the tropics/subtropics, suggesting an into-the-tropics colonization for this genus
We found that the elevated diversification induced by heterogeneous environmental conditions in the tropics/subtropics shapes the global pattern of Rhododendron diversity
Summary
The latitudinal gradient of species diversity (i.e. the increase in species diversity from the poles to the equator) has intrigued biologists continuously since the early 19th century The mechanisms underlying this pattern remain one of the most debated issues in ecology and biogeography (Rosenzweig, 1995). One notable environmental hypothesis explains species diversity patterns as being predicted by habitat heterogeneity (Kerr & Packer, 1997; Pianka, 1967; Stein et al, 2014, 2015), in that species’ coexistence largely depends upon the availability of different habitats. There is ample empirical evidence from different taxonomic groups supporting positive diversity–habitat heterogeneity relationships (Hortal, Triantis, Meiri, Thebault, & Sfenthourakis, 2009; Tews et al, 2004), the mechanisms underlying these relationships remain controversial (Stein et al, 2014)
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