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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.

Aims Patterns of species richness, such as the remarkable biodiversity of tropical regions, have been documented and studied for centuries. However, their underlying evolutionary and ecological causes are still incompletely understood. A commonly stated paradigm in the literature is that high richness in some habitats is directly caused by one of three competing explanations: (1) greater time-for-speciation (earlier colonization), (2) more rapid diversification rates (faster speciation relative to extinction) or (3) higher carrying capacity. However, these three explanations have been relatively little studied using theoretical approaches (especially in terms of comparing all three). Furthermore, empirical studies give conflicting results about their relative importance. Here, we use simulations to study the processes that drive richness patterns along environmental gradients. Location Globally applicable. Methods We use individual-based and trait-based modelling of eco-evolutionary dynamics to simulate the evolutionary radiation of a clade across five habitats with differing ecological conditions, and track patterns of species richness within and between habitats over time. We specifically address the roles of time and diversification rates in explaining richness patterns and the potential impact of carrying capacity. Main results and conclusions Contrary to the widespread paradigm, we find that variation in carrying capacity can underlie differences in diversification rates and time-for-speciation among habitats. Therefore, carrying capacity is not a competing, alternative explanation for richness patterns. We also find that the time-for-speciation effect dominates richness patterns over short time-scales, whereas diversification rates dominate over longer time-scales. These latter observations can help reconcile the seemingly conflicting results of many empirical studies, which find that some patterns are explained by time and others by differences in diversification rates.

Aim: The Sino-Himalayas have higher species richness than adjacent regions, making them a global biodiversity hotspot. Various mechanisms, including ecological constraints, energetic constraints, diversification rate (DivRate) variation, time-for-speciation effect and multiple colonizations, have been posited to explain this pattern. We used pheasants (Aves: Phasianidae) as a model group to test these hypotheses and to understand the ecological and evolutionary processes that have generated the extraordinary diversity in these mountains. Location: Sino-Himalayas and adjacent regions. Taxon: Pheasants. Methods: Using distribution maps predicted by species distribution models (SDMs) and a time-calibrated phylogeny for pheasants, we examined the relationships between species richness and predictors including net primary productivity (NPP), niche diversity (NicheDiv), DivRate, evolutionary time (EvolTime) and colonization frequency using Pearson's correlations and structural equation modelling (SEM). We reconstructed ancestral ranges at nodes and examined basal/derived species patterns to reveal the mechanisms underlying species richness gradients in the Sino-Himalayas. Results: We found that ancestral pheasants originated in Africa in the early Oligocene (∼33 Ma), and then colonized the Sino-Himalayan mountains and other regions. In the Sino-Himalayas, species richness was strongly related to DivRate, NPP, NicheDiv and colonization frequency, but weakly correlated with EvolTime. The direct effects of NicheDiv and DivRate on richness were stronger than NPP and EvolTime. NPP indirectly influenced species richness via DivRate, but its effect on richness via NicheDiv was relatively weak. Main conclusions: Higher species diversity in the Sino-Himalayas was generated by both ecological and evolutionary mechanisms. An increase in available niches, rapid diversifications and multiple colonizations was found to be key direct processes for the build-up of the diversity hotspots of pheasants in the Sino-Himalayan mountains. Productivity had an important but indirect effect on species richness, which worked through increased DivRate. Our study offers new insights on species accumulation in the Sino-Himalayas and provides a useful model for understanding other biodiversity hotspots.

A major goal of ecology and evolutionary biology is to explain geographic patterns of species richness. Richness is often correlated with climatic variables. However, the processes underlying these climate‐diversity relationships remain poorly understood. Two potential hypotheses to explain these relationships involve: (i) faster diversification rates (speciation minus extinction) in high‐richness climates and (ii) earlier colonization of high‐richness climates, allowing more time for speciation to build up richness. Few studies have tested these hypotheses directly, and most focused on animal clades with limited richness. In this study, we test these hypotheses in Chinese angiosperms, encompassing ~10% of Earth's plant species, using large‐scale phylogenetic, climatic, and distributional data including 26,977 species. We find that climatic zones that were colonized earlier have higher species richness. By contrast, relationships between diversification rates and richness of climatic zones are often nonsignificant or negative. Our study reveals that even when richness is strongly correlated with climate, the underlying explanation may still be rooted in phylogenetic history. Thus, climate may not be a competing explanation for richness patterns relative to colonization times and diversification rates. We also show that the timing of colonization can be crucial for explaining richness patterns. Yet, many recent studies have ignored this explanation and instead have focused solely on rates of speciation and diversification as drivers of diversity gradients. Species richness is often strongly correlated with climatic variables, but what explains these strong climate‐richness relationships? Here, we address this question by analyzing climatic, distributional, and phylogenetic data from ~27,000 species of Chinese plants (~10% of the world's species). We find that climates with high species richness are explained by longer time in those climatic zones, not by faster rates of diversification.

Climatic niche conservatism shapes patterns of diversity in many taxonomic groups, while ecological opportunity (EO) can trigger rapid speciation that is less constrained by the amount of time a lineage has occupied a given habitat. These two processes are well studied, but limited research has considered their joint and relative roles in shaping diversity patterns. We characterized climatic and biogeographic variables for 102 species of arvicoline rodents (Arvicolinae, Cricetidae), testing the effects of climatic niche conservatism and EO on arvicoline diversification as lineages transitioned between biogeographic regions. We found that the amount of time a lineage has occupied a precipitation niche is positively correlated with diversity along a precipitation gradient, suggesting climatic niche conservatism. In contrast, shift in diversification rate explained diversity patterns along a temperature gradient. Our results suggest that an indirect relationship exists between temperature and diversification that is associated with EO as arvicoline rodents colonized warm Palearctic environments. Climatic niche conservatism alone did not fully explain diversity patterns under density-dependence, highlighting the additional importance of EO-related processes in promoting the explosive radiation in arvicoline rodents and shaping diversity pattern among biogeographic regions and along climatic gradients.

Aim Marine macroalgae provide an excellent opportunity to test hypotheses about latitudinal diversity gradients because macroalgal richness decreases towards the tropics, contrary to classic patterns, and because three evolutionarily distinct macroalgal clades (Rhodophyta, Chlorophyta, Phaeophyceae) have converged ecologically. Specifically, we determine the extent to which environmental conditions can predict genus richness in macroalgae. We also evaluate whether the magnitude or direction of the effect of environmental factors, or their ability to explain variation in macroalgal diversity, varies geographically. Location Global oceans. Methods We formulated and fitted global spatial regression models and geographically weighted regression (GWR) models to determine the extent to which environmental conditions could predict genus richness in macroalgae. GWR allowed us to determine how the role of environmental conditions varied amongst geographical regions. Results The global regression model showed that sea surface temperature and nutrients were important predictors of macroalgal genus richness at a global scale. However, GWR revealed that environmental factors explained less variability in richness in the tropics than elsewhere. Main conclusions Our results show that whilst environmental conditions influence marine macroalgal diversity, the strength of this influence shows considerable geographical variation. In particular, environmental conditions explain more of the observed variation in diversity at high latitudes than at low latitudes. This finding is consistent with the hypothesis that environmental tolerances influence species distributions more strongly at high latitudes, whereas other factors, such as biotic interactions, play a more prominent role in the tropics.

AIM: Patterns of species richness are often closely linked with climate, but the specific mechanisms by which species' climatic niches underlie large‐scale richness patterns remain poorly understood. It has been hypothesized that reduced temperature seasonality in the tropics promotes the evolution of species with narrow temperature niche breadths, and that this hypothesis helps explain high tropical richness. However, the relationship between species' climatic niche breadths and species richness has yet to be tested. We have addressed this issue using treefrogs (Hylidae) in eastern North America. LOCATION: Eastern North America. METHODS: We characterized climatic niches and niche breadths for all 24 hylid species in eastern North America using temperature and precipitation variables. We then examined the relationships between species richness, climatic niche positions and climatic niche breadths using phylogenetic comparative methods. RESULTS: Species richness was negatively associated with mean climatic niche breadth, such that high‐richness climates had species with narrower climatic niches. Our results also supported the roles of niche conservatism and the time‐for‐speciation effect in generating the relationship between climate and species richness in the region (more species in warm, wet regions that have been inhabited longer). Importantly, we show that the invasion of low‐richness climates has occurred primarily through recent intraspecific niche expansion into these climates rather than evolution of species that are narrowly specialized for these conditions (although the two hylid clades studied showed somewhat different patterns). MAIN CONCLUSIONS: We found that climatic zones with high species richness contain more species with narrower climatic niche breadths. Our results suggest that this pattern arose because narrow climatic niche breadths restricted the dispersal of most hylid species out of the ancestral, warm, moist climatic zones, allowing more time for speciation to build up higher species richness in these zones.

The Tropical Andes make up Earth’s most species-rich biodiversity hotspot for both animals and plants. Nevertheless, the ecological and evolutionary processes underlying this extraordinary richness remain uncertain. Here, we examine the processes that generate high richness in the Tropical Andes relative to other regions in South America and across different elevations within the Andes, using frogs as a model system. We combine distributional data, a newly generated time-calibrated phylogeny for 2,318 frog species, and phylogenetic comparative methods to test the relative importance of diversification rates and colonization times for explaining Andean diversity at different scales. At larger scales (among regions and families), we find that faster diversification rates in Andean clades most likely explain high Andean richness. In contrast, at smaller temporal and spatial scales (within family-level clades within the Andes), diversification rates rarely explain richness patterns. Instead, we show that colonization times are important for shaping elevational richness patterns within the Andes, with more species found in habitats colonized earlier. We suggest that these scale-dependent patterns might apply to many other richness gradients. Recognition of this scale dependence may help to reconcile conflicting results among studies of richness patterns across habitats, regions, and organisms.

Most groups of organisms occur in multiple regions and have different numbers of species in different regions. These richness patterns are directly explained by speciation, extinction, and dispersal. Thus, regional richness patterns may be explained by differences in when regions were colonized (more time for speciation in regions colonized earlier), differences in how often they were colonized, or differences in diversification rates (speciation minus extinction) among regions (with diversification rates potentially influenced by area, climate, and/or many other variables). Few studies have tested all three factors, and most that did examined them only in individual clades. Here, we analyze a diverse set of 15 clades of plants and animals to test the causes of regional species richness patterns within clades. We find that time was the sole variable significantly explaining richness patterns in the best-fitting models for most clades (10/15), whereas time combined with other factors explained richness in all others. Time was the most important factor explaining richness in 13 of 15 clades, and it explained 72%of the variance in species richness among regions across all 15 clades (on average). Surprisingly, time was increasingly important in older and larger clades. In contrast, the area of the regions was relatively unimportant for explaining these regional richness patterns. A systematic review yielded 15 other relevant studies, which also overwhelmingly supported time over diversification rates (13 to 1, with one study supporting both diversification rates and time). Overall, our results suggest that colonization time is a major factor explaining regional-scale richness patterns within clades (e.g., families).

For most marine organisms, species richness peaks in the Central Indo-Pacific region and declines longitudinally, a striking pattern that remains poorly understood. Here, we used phylogenetic approaches to address the causes of richness patterns among global marine regions, comparing the relative importance of colonization time, number of colonization events, and diversification rates (speciation minus extinction). We estimated regional richness using distributional data for almost all percomorph fishes (17 435 species total, including approximately 72% of all marine fishes and approximately 33% of all freshwater fishes). The high diversity of the Central Indo-Pacific was explained by its colonization by many lineages 5.3–34 million years ago. These relatively old colonizations allowed more time for richness to build up through in situ diversification compared to other warm-marine regions. Surprisingly, diversification rates were decoupled from marine richness patterns, with clades in low-richness cold-marine habitats having the highest rates. Unlike marine richness, freshwater diversity was largely derived from a few ancient colonizations, coupled with high diversification rates. Our results are congruent with the geological history of the marine tropics, and thus may apply to many other organisms. Beyond marine biogeography, we add to the growing number of cases where colonization and time-for-speciation explain large-scale richness patterns instead of diversification rates.