Aim Understanding 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. Location Global. Time period Undefined. Major taxa studied Rhododendron. Methods We 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. Results In 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 conclusions Our 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.

Aim: Sexual systems strongly influence angiosperm evolution and play important roles in community assembly and species responses to climate change. However, geographical variation in proportions of different sexual systems (dioecy, monoecy and hermaphroditism) in response to changes in climate, life-history traits and evolutionary age remains poorly understood. Here, we map the geographical variation in proportions of different sexual systems and hypothesize that the prevalence of hermaphrodites increases with aridity owing to their advantages in colonizing harsh environments, whereas dioecy is most successful in humid regions with tall-canopy vegetation and old floras. Location: China. Time period: Current. Major taxa studied: Woody angiosperms. Methods: Using data on sexual systems and distributions of 10,449 woody species in China, we estimated the proportions of different sexual systems in local floras (50 km × 50 km grid cells). Spatial linear models, phylogenetic general linear models and structural equation models were used to compare the relative influences of climate, plant height and evolutionary age on geographical variation in proportions of different sexual systems. Results: We found contrasting geographical patterns in the proportions of different sexual systems. The proportions of dioecy and monoecy increased with plant height | 547 WANG et Al.

The integrated contributions of climate and macroevolutionary processes to global patterns of species diversity are still controversial in spite of a long history of studies. The niche conservatism hypothesis and the net diversification rate hypothesis have gained wide attention in recent literature. Many studies have tested these two hypotheses for woody species in humid forests; however, the determinants of species diversity patterns for arid-adapted plants remain largely ignored. Here, using a molecular phylogeny and the global distributions of Zygophyllaceae, a typical arid-adapted plant family, we assessed the effects of contemporary climate and net diversification rates on species diversity patterns in drylands. We found the variables representing water availability to be the best predictors for Zygophyllaceae diversity. Specifically, Zygophyllaceae species diversity significantly decreased with the increase in water availability, probably owing to phylogenetic conservatism of water-related niches. The net diversification rates of Zygophyllaceae accelerated sharply in the recent 10 Myr, coinciding roughly with the period of global aridification. The species diversity of Zygophyllaceae significantly increased with the increase in mean net diversification rates per geographical unit, especially in the Old World, supporting the net diversification rate hypothesis. Our study provides a case exploring climatic and evolutionary mechanisms of dryland species diversity patterns, and suggests that the conservatism in water-related niches and elevated net diversification rates in drylands may have jointly determined the global patterns of dryland species diversity.

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