Differences In Diversification Rates Research Articles

Explaining extreme differences in species richness among co-occurring palm clades in Madagascar

Abstract Imbalance in species richness among related clades is a pervasive, yet incompletely understood feature of biodiversity. Comparison of species-poor and species-rich clades that have evolved within the same region can shed light on the mechanisms underlying this phenomenon. The long-isolated island of Madagascar is an ideal place for doing this. Madagascar harbours at least ten clades of palms (Arecaceae) that have colonized the island independently and diversified to widely differing degrees, ranging from one to 180 known species. We estimated colonization times and diversification rates for these clades based on an extensive phylogenomic dataset and tested the degree to which clades that arrived in Madagascar earlier have more species (time-for-speciation effect), finding a moderate effect. For context, we tested for time-for-speciation effects in other plant and animal lineages, finding variable but qualitatively similar results. Our findings suggest that variation in diversification rate (i.e. speciation and/or extinction rate) is a major driver of species richness imbalance among Malagasy clades, both in palms and elsewhere. We demonstrate that in palms, differences in diversification rates originated long before colonization of the island, suggesting a minor role of classical ‘island radiation’ and a stronger role of heritable traits driving diversification rate. Ability to colonize new climates also appears to play a role. Future work should address the interplay between the dynamic environment of Madagascar and the inherited traits of colonizing lineages to fully explain the island’s intriguing mix of species-poor and species-rich clades.

Rise and fall of a continental mesic radiation in Australia: spine evolution, biogeography, and diversification of Cryptandra (Rhamnaceae: Pomaderreae)

Abstract The Australian continent has experienced progressive aridification since the Miocene, spurring recent radiations of arid-adapted lineages and the likely decline of mesic biotic groups. While examples of the former have been relatively well-documented, post-Miocene declines of non-arid sclerophyllous floras are less well understood. Here, we present a well-sampled time-calibrated nuclear phylogeny (140 accessions representing 60/65 species) of an Australian plant genus (Cryptandra Sm.: Rhamnaceae) and using ancestral range reconstructions and diversification analyses, elucidate its evolutionary history through space and time. We used high-throughput sequencing to recover 30 orthologous nuclear loci and BioGeoBEARS to infer ancestral areas. We show that the present-day distribution of Cryptandra can be explained by multiple vicariance events followed by in situ diversification with little exchange between regions. All diversification models show a speciation rate decline in Cryptandra after its radiation in the Miocene (c. 23 Mya). This coincides with aridification episodes across Australia and indicates that diversification of this genus has been negatively affected by the expansion of aridity. We also show that there were no significant differences in diversification rates between spinescent and non-spinescent Cryptandra lineages, suggesting that spinescent lineages may be the legacies of selection from extinct megaherbivores.

Linking Ecological Specialization to Its Macroevolutionary Consequences: An Example with Passerine Nest Type.

A long-standing hypothesis in evolutionary biology is that the evolution of resource specialization can lead to an evolutionary dead end, where specialists have low diversification rates and limited ability to evolve into generalists. In recent years, advances in comparative methods investigating trait-based differences associated with diversification have enabled more robust tests of this idea and have found mixed support. We test the evolutionary dead end hypothesis by estimating net diversification rate differences associated with nest-type specialization among 3224 species of passerine birds. In particular, we test whether the adoption of hole-nesting, a nest-type specialization that decreases predation, results in reduced diversification rates relative to nesting outside of holes. Further, we examine whether evolutionary transitions to the specialist hole-nesting state have been more frequent than transitions out of hole-nesting. Using diversification models that accounted for background rate heterogeneity and different extinction rate scenarios, we found that hole-nesting specialization was not associated with diversification rate differences. Furthermore, contrary to the assumption that specialists rarely evolve into generalists, we found that transitions out of hole-nesting occur more frequently than transitions into hole-nesting. These results suggest that interspecific competition may limit adoption of hole-nesting, but that such competition does not result in limited diversification of hole-nesters. In conjunction with other recent studies using robust comparative methods, our results add to growing evidence that evolutionary dead ends are not a typical outcome of resource specialization. [Cavity nesting; diversification; hidden-state models; passerines; resource specialization.].

Time for speciation and niche conservatism explain the latitudinal diversity gradient in clupeiform fishes

AbstractAimThe latitudinal diversity gradient of increasing species richness from poles to equator is one of the most striking and pervasive spatial patterns of biodiversity. Climate appears to have been key to the formation of the latitudinal diversity gradient, but the processes through which climate shaped species richness remain unclear. We tested predictions of the time for speciation, carrying capacity and diversification rate latitudinal diversity gradient hypotheses in a trans‐marine/freshwater clade of fishes.LocationGlobal in marine and freshwater environments.TaxonClupeiformes (anchovies, herrings, sardines and relatives).MethodsWe tested predictions of latitudinal diversity gradient hypotheses using a molecular phylogeny, species distribution data and phylogenetic comparative approaches. To test the time for speciation hypothesis, we conducted ancestral state reconstructions to infer the ages of temperate, subtropical and tropical lineages and frequency of evolutionary transitions between climates. We tested the carry capacity hypothesis by characterizing changes in net diversification rates through time. To test the diversification rate hypothesis, we qualitatively compared the diversification rates of temperate, subtropical and tropical lineages and conducted statistical tests for associations between latitude and diversification rates.ResultsWe identified four transitions to temperate climates and two transitions out of temperate climates. We found no differences in diversification rates among temperate and tropical clupeiforms. Net diversification rates remained positive in crown Clupeiformes since their origin ~150 Ma in both tropical and temperate lineages. Climate niche characters exhibited strong phylogenetic signal. All temperate clupeiform lineages arose <50 Ma, after the Early Eocene Climatic Optimum.Main conclusionsOur results support the time for speciation hypothesis, which proposes that climate niche conservatism and fluctuations in the extent of temperate climates limited the time for species to accumulate in temperate climates, resulting in the latitudinal diversity gradient. We found no support for the carrying capacity or diversification rate hypotheses.

Out of the temperate zone: A phylogenomic test of the biogeographical conservatism hypothesis in a contrarian clade of ants

AbstractAimThe standard latitudinal diversity gradient (LDG), in which species richness decreases from equator to pole, is a pervasive pattern observed in most organisms. Some lineages, however, exhibit inverse LDGs. Seemingly problematic, documenting and studying contrarian groups can advance understanding of LDGs generally. Here, we identify one such contrarian clade and use a historical approach to evaluate alternative hypotheses that might explain the group's atypical diversity pattern. We focus on the biogeographical conservatism hypothesis (BCH) and the diversification rate hypothesis (DRH).LocationGlobal.TaxonAnts (Hymenoptera: Formicidae: Stenammini).MethodsWe examined the shape of the LDG in Stenammini by plotting latitudinal midpoints for all extant, described species. We inferred a robust genome‐scale phylogeny using UCE data. We estimated divergence dates using beast2 and tested several biogeographical models in BioGeoBEARS. To examine diversification rates and test for a correlation between rate and latitude, we used the programs BAMM and STRAPP, respectively.ResultsStenammini has a skewed inverse LDG with a richness peak in the northern temperate zone. Phylogenomic analyses revealed five major clades and several instances of non‐monophyly among genera (Goniomma, Aphaenogaster). Stenammini and all its major lineages arose in the northern temperate zone. The tribe originated ~51 Ma during a climatic optimum and then diversified and dispersed southward as global climate cooled. Stenammini invaded the tropics at least seven times, but these events occurred more recently and were not linked with increased diversification. There is evidence for a diversification rate increase in Holarctic Aphaenogaster + Messor, but we found no significant correlation between latitude and diversification rate generally.Main ConclusionsOur results largely support the BCH as an explanation for the inverse latitudinal gradient in Stenammini. The clade originated in the Holarctic and likely became more diverse there due to center‐of‐origin, time‐for‐speciation and niche conservatism effects, rather than latitudinal differences in diversification rate.

What drives diversification? Range expansion tops climate, life history, habitat and size in lizards and snakes

AbstractAimA major challenge in ecology and evolutionary biology is to explain the dramatic differences in species richness among clades. Much variation in richness is explained by the differences in diversification rates among clades, and variation in diversification rates is often linked to various traits. But what types of traits are most important for explaining diversification? Here, we compared the impacts of different types of traits on diversification rates among lizard and snake families and tested predictions about the relative importance of ecology vs. morphology, static vs. dynamic traits and alpha vs. beta niche traits.LocationGlobal.TaxonSquamata.MethodsWe compared the relative impacts of traits related to biogeography (range size, range expansion), climate, life history (viviparity), microhabitat and morphology (body‐size) on diversification rates among all 72 family‐level clades of squamates. We compiled data on traits and tested for relationships between traits and diversification rates using phylogenetic multiple regression models.ResultsThe best‐fitting model explained ~60% of the variation in diversification rates across squamate families. This model included only microhabitat (proportion of arboreal species) and a novel, dynamic, ecological/biogeographic beta‐niche trait (rate of range expansion), which explained most variance. Other variables had more variable or non‐significant contributions, including rates of climatic‐niche change. Rates of range expansion were related to species richness, larger body size and faster rates of climatic‐niche change.Main conclusionsOverall, we provide possibly the most comprehensive comparison of the types of traits that can drive diversification. We also help explain diversity patterns in one of the largest vertebrate clades. We show that the rate of range expansion is the most important variable for explaining diversification rates and richness patterns in squamates. We also identify traits that help explain variation in rates of range expansion among clades.

Key roles for the freezing line and disturbance in driving the low plant species richness of temperate regions

AbstractAimAt the macroscale, climate strongly correlates with species richness gradients, resulting from differences in in‐situ diversification and dispersal. One historical explanation for the pattern is that regions spanning temperate climates contain few species because past disturbances have generated high extinction rates, and species from tropical regions are unable to easily colonize temperate regions. We test these postulates for Himalayan plants, which span subtropical to temperate climates over steep elevational gradients.LocationHimalaya.Time periodPresent day.Major taxa studiedAngiosperms.MethodsWe use a comprehensive survey of 31 floras to document the elevational and geographical distributions of native Himalayan plants, augmented by field studies of trees in both the east and west Himalaya. We use grade of membership models to cluster species according to locations shared and phylogenetic analysis to evaluate diversification rates.ResultsSpecies fall into four cohesive biotas, organized by climate. Points of turnover between biotas occur where the mean minimum temperature of the coldest month is approximately 0 °C (2,000–2,500 m), and at the point of occasional annual freezing (1,000–1,500 m); these boundaries run the length of the Himalaya. The patterns are retained when we consider whole clades rather than species. All plants (and the subsets trees, herbs and shrubs) belonging to the biota above the 2,000–2,500 m line have higher recent speciation rates than those lower down.Main conclusionsWe attribute the high rate of recent speciation in temperate climates to high rates of turnover, creating ecological and geographical opportunity. The high elevation biota has few species, but spans the largest area, implying species numbers are far from any carrying capacity, at least with respect to accumulation of allopatric forms. This study thus links climatic restrictions of clades to differences in diversification rates, and by inference species numbers.

Multicellularity and sex helped shape the Tree of Life.

Across the Tree of Life, there are dramatic differences in species numbers among groups. However, the factors that explain the differences among the deepest branches have remained unknown. We tested whether multicellularity and sexual reproduction might explain these patterns, since the most species-rich groups share these traits. We found that groups with multicellularity and sexual reproduction have accelerated rates of species proliferation (diversification), and that multicellularity has a stronger effect than sexual reproduction. Patterns of species richness among clades are then strongly related to these differences in diversification rates. Taken together, these results help explain patterns of biodiversity among groups of organisms at the very broadest scales. They may also help explain the mysterious preponderance of sexual reproduction among species (the 'paradox of sex') by showing that organisms with sexual reproduction proliferate more rapidly.

Comparing diversification rates in lakes, rivers, and the sea.

The diversity of species inhabiting freshwater relative to marine habitats is striking, given that freshwater habitats encompass <1% of Earth's water. The most commonly proposed explanation for this pattern is that freshwater habitats are more fragmented than marine habitats, allowing more opportunities for allopatric speciation and thus increased diversification rates in freshwater. However, speciation may be generally faster in sympatry than in allopatry, as illustrated by lacustrine radiations such as African cichlids. Such differences between rivers and lakes may be important to consider when comparing diversification broadly among freshwater and marine groups. Here I compared diversification rates of teleost fishes in marine, riverine and lacustrine habitats. I found that lakes had faster speciation and net diversification rates than other aquatic habitats. However, most freshwater diversity arose in rivers. Surprisingly, riverine and marine habitats had similar rates of net diversification on average. Biogeographic models suggest that lacustrine habitats are evolutionarily unstable, explaining the dearth of lacustrine species in spite of their rapid diversification. Collectively, these results suggest that strong diversification rate differences are unlikely to explain the freshwater paradox. Instead, this pattern may be attributable to the comparable amount of time spent in riverine and marine habitats over the 200-million-year history of teleosts.

Macroevolutionary stability predicts interaction patterns of species in seed dispersal networks.

Assessing deep-time mechanisms affecting the assembly of ecological networks is key to understanding biodiversity changes on broader time scales. We combined analyses of diversification rates with interaction network descriptors from 468 bird species belonging to 29 seed dispersal networks to show that bird species that contribute most to the network structure of plant-frugivore interactions belong to lineages that show higher macroevolutionary stability. This association is stronger in warmer, wetter, less seasonal environments. We infer that the macroevolutionary sorting mechanism acts through the regional pool of species by sorting species on the basis of the available relative differences in diversification rates, rather than absolute rates. Our results illustrate how the interplay between interaction patterns and diversification dynamics may shape the organization and long-term dynamics of ecological networks.

Out of the Mediterranean Region: Worldwide biogeography of snapdragons and relatives (tribe Antirrhineae, Plantaginaceae)

AbstractAimThe tribe Antirrhineae, including snapdragons, toadflaxes and relatives, is widely distributed across the Northern Hemisphere and the Neotropics. It displays an uneven distribution of diversity, with more than 50% of species and subspecies in the Mediterranean Region. Here we conducted the first detailed, worldwide biogeographic analysis of the Antirrhineae and tested two alternative hypotheses (time‐for‐speciation versus diversification rate differences) to explain the uneven distribution of diversity.LocationWorldwide, with a focus on the Mediterranean Region.TaxonTribe Antirrhineae (Plantaginaceae).MethodsA phylogenetic biogeographic approach was taken, accounting for area connections through time. Ancestral ranges, dispersal events, speciation and lineage accumulation within areas were estimated. Diversification rates for taxa present and absent in the Mediterranean Region were compared, accounting for the effect of a floral key innovation (nectar spur).ResultsA proto‐Mediterranean origin in the Late Eocene was estimated, and the Mediterranean Region stood out as the main centre for speciation and dispersal. Congruent patterns of long‐distance dispersal from the Mediterranean Region to North America were recovered for at least two amphiatlantic clades. A significant floristic exchange between the Mediterranean and south‐western Asia was detected. We found no evidence of different diversification rates between lineages inside and outside the Mediterranean Region.Main conclusionsThe Mediterranean Region played a key role in the origin of the current distribution of the Antirrhineae. However, the higher species richness found in this region appears to be the result of a time‐for‐speciation effect rather than that of increased diversification rates. The establishment of current Mediterranean climates in the Northern Hemisphere appears to have contributed to the recent diversification of the group, in combination with colonization of adjacent regions with arid and semi‐arid climates.

Species Selection Regime and Phylogenetic Tree Shape.

Species selection, the effect of heritable traits in generating between-lineage diversification rate differences, provides a valuable conceptual framework for understanding the relationship between traits, diversification, and phylogenetic tree shape. An important challenge, however, is that the nature of real diversification landscapes-curves or surfaces which describe the propensity of species-level lineages to diversify as a function of one or more traits-remains poorly understood. Here, we present a novel, time-stratified extension of the QuaSSE model in which speciation/extinction rate is specified as a static or temporally shifting Gaussian or skewed-Gaussian function of the diversification trait. We then use simulations to show that the generally imbalanced nature of real phylogenetic trees, as well as their generally greater than expected frequency of deep branching events, are typical outcomes when diversification is treated as a dynamic, trait-dependent process. Focusing on four basic models (Gaussian-speciation with and without background extinction; skewed-speciation; Gaussian-extinction), we also show that particular features of the species selection regime produce distinct tree shape signatures and that, consequently, a combination of tree shape metrics has the potential to reveal the species selection regime under which a particular lineage diversified. We evaluate this idea empirically by comparing the phylogenetic trees of plant lineages diversifying within climatically and geologically stable environments of the Greater Cape Floristic Region, with those of lineages diversifying in environments that have experienced major change through the Late Miocene-Pliocene. Consistent with our expectations, the trees of lineages diversifying in a dynamic context are less balanced, show a greater concentration of branching events close to the present, and display stronger diversification rate-trait correlations. We suggest that species selection plays an important role in shaping phylogenetic trees but recognize the need for an explicit probabilistic framework within which to assess the likelihoods of alternative diversification scenarios as explanations of a particular tree shape. [Cape flora; diversification landscape; environmental change; gamma statistic; species selection; time-stratified QuaSSE model; trait-dependent diversification; tree imbalance.].

Diatoms diversify and turn over faster in freshwater than marine environments.

Many clades that span the marine-freshwater boundary are disproportionately more diverse in the younger, shorter lived, and scarcer freshwater environments than they are in the marine realm. This disparity is thought to be related to differences in diversification rates between marine and freshwater lineages. However, marine and freshwaters are not ecologically homogeneous, so the study of diversification across the salinity divide should also account for other potentially interacting variables. In diatoms, freshwater and substrate-associated (benthic) lineages are several-fold more diverse than their marine and suspended (planktonic) counterparts. These imbalances provide an excellent system to understand whether these variables interact with diversification. Using multistate hidden-state speciation and extinction models, we found that freshwater lineages diversify faster than marine lineages regardless of whether they inhabit the plankton or the benthos. Freshwater lineages also had higher turnover rates (speciation + extinction), suggesting that habitat transitions impact speciation and extinction rates jointly. The plankton-benthos contrast was also consistent with state-dependent diversification, but with modest differences in diversification and turnover rates. Asymmetric and bidirectional transitions rejected hypotheses about the plankton and freshwaters as absorbing, inescapable habitats. Our results further suggest that the high turnover rate of freshwater diatoms is related to high turnover of freshwater systems themselves.

Time Explains Regional Richness Patterns within Clades More Often than Diversification Rates or Area.

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).

The Tortoise and the Finch: Testing for island effects on diversification using two iconic Galápagos radiations

AbstractAimsIslands are widely recognized as natural laboratories for evolutionary studies, but many questions about evolution on islands remain unresolved. Here we address two general questions from a macroevolutionary perspective. First, do lineages on islands have increased diversification rates relative to mainland lineages? Second, does the same geographical context (e.g., same archipelago) have similar effects on diversification in unrelated groups? We focused on Darwin's finches and Galápagos tortoises, two endemic radiations from the Galápagos Islands, and the larger families in which they are embedded.LocationGlobal.MethodsWe estimated a new time‐calibrated phylogeny for tortoises (Testudinidae). Then, we examined their macroevolutionary patterns and compared them to those of Darwin's finches and relatives (Thraupidae), using a published thraupid tree. Specifically, we estimated and compared diversification rates between islands and the mainland and between the Galapagos Islands and all other regions, using clade‐based and species‐based approaches.ResultsContrary to expectations, occurrence on islands in general did not significantly increase diversification rates in tanagers or tortoises. However, occurrence in the Galápagos Islands in particular was associated with increased speciation and diversification rates, and explained ~28% of the variation in diversification rates for thraupids and ~46% for testudinids. Both Darwin's finches and Galápagos tortoises were unique within each family in exhibiting the highest diversification rates. The congruence of these macroevolutionary patterns between both radiations supports a strong “place‐dependent” effect on diversification associated with the Galápagos. Finally, we found that Darwin's finches diversified ~2–8 times faster than Galápagos tortoises.Main conclusionsOur results show that occurring on islands in general did not increase diversification rates in these clades, but occurrence in the Galápagos did. We also show that dramatic local‐scale differences in diversification rates between clades in the Galápagos parallel global‐scale differences in diversification rates between these families and between birds and turtles overall.

Binary-state speciation and extinction method is conditionally robust to realistic violations of its assumptions

BackgroundPhylogenetic comparative methods allow us to test evolutionary hypotheses without the benefit of an extensive fossil record. These methods, however, make simplifying assumptions, among them that clades are always increasing or stable in diversity, an assumption we know to be false. This study simulates hypothetical clades to test whether the Binary State Speciation and Extinction (BiSSE) method can be used to correctly detect relative differences in diversification rate between ancestral and derived character states even as net diversification rates are declining overall. We simulate clades with declining but positive diversification rates, as well those in which speciation rates decline below extinction rates so that they are losing richness for part of their history. We run these analyses both with simulated symmetric and asymmetric speciation rates to test whether BiSSE can be used to detect them correctly.ResultsFor simulations with a neutral character, the fit for a BiSSE model with a neutral character is better than alternative models so long as net diversification rates remain positive. Once net diversification rates become negative, the BiSSE model with the greatest likelihood often has a non-neutral character, even though there is no such character in the simulation. BiSSE’s usefulness in detecting real asymmetry in speciation rates improves with clade age, even well after net diversification rates have become negative.ConclusionsBiSSE is most useful in analyzing clades of intermediate age, before they have reached peak diversity and gone into decline. After this point, users of BiSSE risk incorrectly inferring differential evolutionary rates when none exist. Fortunately, most studies using BiSSE and similar models focus on rapid, recent diversifications, and are less likely to encounter the biases BiSSE models are subject to for older clades. For extant groups that were once more diverse than now, however, caution should be taken in inferring past diversification patterns without fossil data.

Walk, swim or fly? Locomotor mode predicts genetic differentiation in vertebrates.

Limited dispersal is commonly used to explain differences in diversification rates. An obvious but unexplored factor affecting dispersal is the mode of locomotion used by animals. Whether individuals walk, swim or fly can dictate the type and severity of geographical barriers to dispersal, and determine the general range over which genetic differentiation might occur. We collated information on locomotion mode and genetic differentiation (FST ) among vertebrate populations from over 400 published articles. Our results showed that vertebrate species that walk tend to have higher genetic differentiation among populations than species that swim or fly. Within species that swim, vertebrates in freshwater systems have higher genetic differentiation than those in marine systems, which is consistent with the higher number of species in freshwater environments. These results show that locomotion mode can impact gene flow among populations, supporting at a broad-scale what has previously been proposed at smaller taxonomical scales.

An ant genus-group (Prenolepis) illuminates the biogeography and drivers of insect diversification in the Indo-Pacific

The Malay Archipelago and the tropical South Pacific (hereafter the Indo-Pacific region) are considered biodiversity hotspots, yet a general understanding of the origins and diversification of species-rich groups in the region remains elusive. We aimed to test hypotheses for the evolutionary processes driving insect species diversity in the Indo-Pacific using a higher-level and comprehensive phylogenetic hypothesis for an ant clade consisting of seven genera. We estimated divergence times and reconstructed the biogeographical history of ant species in the Prenolepis genus-group (Formicidae: Formicinae: Lasiini). We used a fossil-calibrated phylogeny to infer ancestral geographical ranges utilizing a biogeographic model that includes founder-event speciation. Ancestral state reconstructions of the ants' ecological preferences, and diversification rates were estimated for selected Indo-Pacific clades. Overall, we report that faunal interchange between Asia and Australia has occurred since at least 20–25 Ma, and early dispersal to the Fijian Basin happened during the early and mid-Miocene (ca. 10–20 Ma). Differences in diversification rates across Indo-Pacific clades may be related to ecological preference breadth, which in turn may have facilitated geographical range expansions. Ancient dispersal routes suggested by our results agree with the palaeogeography of the region. For this particular group of ants, the rapid orogenesis in New Guinea and possibly subsequent ecological shifts may have promoted their rapid diversification and widespread distribution across the Indo-Pacific.

The influence of wing morphology upon the dispersal, geographical distributions and diversification of the Corvides (Aves; Passeriformes).

New species are sometimes known to arise as a consequence of the dispersal and establishment of populations in new areas. It has nevertheless been difficult to demonstrate an empirical link between rates of dispersal and diversification, partly because dispersal abilities are challenging to quantify. Here, using wing morphology as a proxy for dispersal ability, we assess this relationship among the global radiation of corvoid birds. We found that species distributions are associated with wing shape. Widespread species (occurring on both islands and continents), and those that are migratory, exhibit wing morphologies better adapted to long-distance flight compared with sedentary continental or insular forms. Habitat preferences also strongly predict wing form, with species that occur in canopies and/or areas of sparse vegetation possessing dispersive morphologies. By contrast, we found no significant differences in diversification rates among either the migratory or habitat classifications, but species distributed in island settings diversify at higher rates than those found on continents. This latter finding may reflect the elevated dispersal capabilities of widespread taxa, facilitating the radiation of these lineages across insular areas. However, as the correlations between wing morphology and diversification rates were consistently weak throughout our dataset, this suggests that historical patterns of diversification are not particularly well reflected by present-day wing morphology.

The origin of species richness patterns along environmental gradients: uniting explanations based on time, diversification rate and carrying capacity

AbstractAimsPatterns 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.LocationGlobally applicable.MethodsWe 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 conclusionsContrary 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.