Global angiosperm family richness revisited: linking ecology and evolution to climate

Aim The global richness gradient of angiosperm families is correlated with current climate, and it has been claimed that historical processes are not necessary to understand patterns of plant family richness. This claim has drawn criticism, and there have been doubts about the quality of the data used to quantify the pattern. We revisit this issue using the Angiosperm Phylogeny Group (APG) III classification and revised range maps, and we incorporate an evolutionary variable, family age, to explore covariation between evolution and ecology and their links to climate via the tropical conservatism hypothesis (TCH). Location Global. Methods The richness pattern for 408 families was derived from range maps, and family ages were derived from a dated angiosperm phylogeny. Patterns were generated for all families, 143 families composed of trees, and 149 families composed of herbs. We also examined family range size patterns to test the extent to which extratropical floras are nested subsets of tropical floras. Ordinary least squares (OLS) multiple and partial regressions were used to generate climate models for richness, mean range size and mean age for each plant dataset and to evaluate the covariation between contemporary climate and clade age as correlates of family richness. Results We confirmed the strong association between contemporary climate and family richness. Age patterns predicted by TCH were also found for families comprising trees. The richness of herbaceous families, in contrast, was correlated with climate but the age pattern was not as predicted by TCH. Floras in cold and dry areas are strongly nested within richer tropical floras. Main conclusions Phylogenetic niche conservatism at the family level offers a likely explanation for the global diversity gradient of trees, but not for non‐desert herbs, probably because of the faster evolutionary rates for herbs and less constrained evolutionary responses to climate change. Thus, it appears that multiple processes account for the overall angiosperm family gradient. Our analysis also demonstrates that even very strong associations of taxon richness and climate do not preclude evolutionary processes, as has been widely argued, and that climatic and evolutionary hypotheses for richness gradients are not mutually exclusive.     

Post-Eocene climate change, niche conservatism, and the latitudinal diversity gradient of New World birds

Aim The aim of this study was to test a variant of the evolutionary time hypothesis for the bird latitudinal diversity gradient derived from the effects of niche conservatism in the face of global climate change over evolutionary time. Location The Western Hemisphere. Methods We used digitized range maps of breeding birds to estimate the species richness at two grain sizes, 756 and 12,100 km². We then used molecular phylogenies resolved to family to quantify the root distance (RD) of each species as a measure of its level of evolutionary development. Birds were classified as ‘basal’ or ‘derived’ based on the RD of their family, and richness patterns were contrasted for the most basal and most derived 30% of species. We also generated temperature estimates for the Palaeogene across the Western Hemisphere to examine how spatial covariation between past and present climates might make it difficult to distinguish between ecological and evolutionary hypotheses for the current richness gradient. Results The warm, wet tropics support many species from basal bird clades, whereas the northern temperate zone and cool or dry tropics are dominated by species from more recent, evolutionarily derived clades. Furthermore, crucial to evaluating how niche conservatism among birds may drive the hemispherical richness gradient, the spatial structure of the richness gradient for basal groups is statistically indistinguishable from the overall gradient, whereas the richness gradient for derived groups is much shallower than the overall gradient. Finally, modern temperatures and the pattern of climate cooling since the Eocene are indistinguishable as predictors of bird species richness. Main conclusions Differences in the richness gradients of basal vs. derived clades suggest that the hemispherical gradient has been strongly influenced by the differential extirpation of species in older, warm‐adapted clades from parts of the world that have become cooler in the present. We propose that niche conservatism and global‐scale climate change over evolutionary time provide a parsimonious explanation for the contemporary bird latitudinal diversity gradient in the New World, although dispersal limitation of some highly derived clades probably plays a secondary role.     

Phylogenetic comparative approaches for studying niche conservatism

Analyses of phylogenetic niche conservatism (PNC) are becoming increasingly common. However, each analysis makes subtly different assumptions about the evolutionary mechanism that generates patterns of niche conservatism. To understand PNC, analyses should be conducted with reference to a clear underlying model, using appropriate methods. Here, we outline five macroevolutionary models that may underlie patterns of PNC (drift, niche retention, phylogenetic inertia, niche filling/shifting and evolutionary rates) and link these to published phylogenetic comparative methods. For each model, we give recent examples from the literature and suggest how the methods can be practically applied. We hope that this will help clarify the niche conservatism literature and encourage people to think about the evolutionary models underlying niche conservatism in their study group.     

Ecology and macroevolution – evolutionary niche monopolisation as a mechanisms of niche conservatism

Explaining macroevolution from microevolution is a key issue in contemporary evolutionary theory. A recurrent macroevolutionary pattern is that some niche‐related traits consistently evolve slower than others, so called niche conservatism. Despite a growing amount of data, the underlying evolutionary processes are not fully understood. I here analyse adaptive radiations in an individual‐based eco‐evolutionary model. I find a coevolutionary mechanism – evolutionary niche monopolisation – as a possibly important generator of niche conservatism. A single lineage of a radiating clade can monopolise, and later diversify within, a substantial part of the available niche space – much larger than what can be explained by limiting similarity. This leads to niche conservatism, since no species evolves into or out of the monopolised region. The region can in this sense also be described as an adaptive zone. The model indicates that evolutionary niche monopolisation is operative in a large part of parameter space, underlining its possible importance. The mechanism is driven by competitive interactions and differences in niche widths in alternative niche dimensions. I discuss plausible examples of evolutionary niche monopolisation in well‐studied natural systems.     

Evolutionary constraints on regional faunas: whom, but not how many

The latitudinal diversity gradient has been hypothesized to reflect past evolutionary dynamics driven by climatic niche conservation during cladogenesis, i.e. the tropical conservatism hypothesis. Here we show that the species diversity of treefrogs (Hylidae) across the western hemisphere is actually independent of evolutionary niche dynamics. We evaluated three key predictions of the tropical conservatism hypothesis that relate to the relationships between climate, species richness and the phylogenetic structure of regional treefrog faunas across the continental Americas. Species composition was dependent on the inability of some lineages to evolve cold tolerance, but the actual number of species in a region was strongly predicted by precipitation, not temperature. Moreover, phylogenetic structure was independent of precipitation. Thus, species in low-richness areas were no more closely related than species in highly diverse regions. These results provide no support for the tropical conservatism hypothesis. Instead, they show that regional species composition and richness are constrained by different climatic components, demonstrating that global biodiversity gradients can be independent of niche stasis during cladogenesis.     

The niche of higher plants: evidence for phylogenetic conservatism.

A species' ecological niche depends on the species' adaptations to its present habitat, but also on the legacy from its ancestors. Most authors argue that such a phylogenetic niche conservatism is of minor importance, although no quantitative analyses across a major taxon is available. Higher plants from central Europe offer a unique opportunity for such an exercise, as the niche positions along various environmental gradients are available for most species. We quantified niche conservatism by two approaches. First, we used a phylogenetic tree and quantified the degree of retention of niches across the tree. Depending on the gradient, the values ranged from 0.43 to 0.22. This was significantly greater than the null expectation. Second, we used a taxonomy and quantified the amount of variance among species that could be explained at higher taxonomic levels. The values ranged from 25 to 72%. Again, this was significantly higher than the null expectation. Thus, both approaches indicated a clear niche conservatism. The distribution of conservatism across taxonomic levels differed considerably among environmental gradients. The differences among environmental gradients could be correlated with the palaeoenvironmental conditions during the radiation of the phylogenetic lineages. Thus, niche conservatism among extant plant species may reflect the opportunities of their ancestors during their diversification.     

Conservatism of responses to environmental change is rare under natural conditions in a native grassland

Whether or not niche conservatism is common is widely debated. Despite this uncertainty, closely related species are often assumed to be ecologically similar. This principle has led to the proposed use of phylogenetic information in forecasting species responses to environmental change. Tests of niche conservatism often focus on ‘functional traits’ and environmental tolerances, but there have been limited tests for conservatism in species’ responses to changes in the environment, especially in the field. The prevalence of functional convergence and the likelihood of functional trade-offs in a heterogeneous environment suggest that conservatism of the response niche is unlikely to be detectable under natural conditions. To test the relevance of evolutionary information in predicting ecological responses, we tested for conservatism (measured as phylogenetic signal) of grassland plant population responses to 14 treatments (e.g. light, nutrients, water, enemies, mutualists), each manipulated for 2–3 years, and 4 treatment categories (aboveground, belowground, resource, and herbivory) at a single site. Individual treatment responses showed limited evidence of conservatism, with only weak conservatism in plant responses to mycorrhizae and grazing. Aspects of the response niche were conserved among monocots both aboveground and belowground, although the pattern varied. Conservatism was limited to grazing aboveground, but belowground responses were conserved as a group, suggesting fundamental differences in how selection has led to niche conservatism in aboveground and belowground environments. Overall, our results suggest that conservatism of the response niche is not common, but is actually rare. As such, evolutionary relationships are likely to be of limited relevance for predicting species responses under field conditions, at least over the short time scales used in this study.     

An inverse latitudinal gradient of diversity of peracarid crustaceans along the Pacific Coast of South America: out of the deep south

Aim To understand the ecological and historical/evolutionary processes underlying an inverse latitudinal gradient of richness (LGR) using crustacean peracarid species as a model group. Location The Pacific coast of South America, along the Chilean coast between 18° S and 56° S. Methods The LGR was evaluated using a dataset including 320 marine peracarid species reported for the coasts of Chile. Five ecological hypotheses invoking a relationship between species richness and present‐day conditions were tested: species–energy, species–area, Rapoport rescue effect, mid‐domain geometric constraint and niche breadth. Historical/evolutionary hypotheses (i.e. biogeographic conservatism, and diversification rates) were indirectly tested by analysing the latitudinal variation in the taxonomic distinctness, the taxonomic conservatism of the midpoint of the latitudinal range and the degree of nestedness at different taxonomic levels. Results Richness increased poleward, varying approximately eightfold, following an inverse LGR coupled with an increase in bathymetric distribution. Overall this inverse LGR seems robust to uncertainties in the completeness of the species inventory. We found support for only two of the five ecological hypotheses tested: species–area and Rapoport rescue effect. Historical/evolutionary hypotheses seemed important in structuring the richness pattern, as indicated by the higher taxonomic distinctness in the southern region, the strong taxonomic inertia in the mean range size and the high degree of nestedness of assemblages at different taxonomic levels. Conclusions When combined, these results underscore the importance of long‐term processes and historical/evolutionary explanations for the inverse LGR, conceptualized in what we term the ‘out of the deep south’ hypothesis that involves the effects of both biogeographic niche conservatism and evolutionary rates. We propose that the southern region may be a source of evolutionary novelties and/or exhibit higher diversification rates (i.e. higher speciation/lower extinction rates). Furthermore, phylogenetic conservatism of latitudinal range may limit the geographic expansion of these new taxa towards the depauperated northern region.     

Phylogenetic conservatism of environmental niches in mammals

Phylogenetic niche conservatism is the pattern where close relatives occupy similar niches, whereas distant relatives are more dissimilar. We suggest that niche conservatism will vary across clades in relation to their characteristics. Specifically, we investigate how conservatism of environmental niches varies among mammals according to their latitude, range size, body size and specialization. We use the Brownian rate parameter, σ(2), to measure the rate of evolution in key variables related to the ecological niche and define the more conserved group as the one with the slower rate of evolution. We find that tropical, small-ranged and specialized mammals have more conserved thermal niches than temperate, large-ranged or generalized mammals. Partitioning niche conservatism into its spatial and phylogenetic components, we find that spatial effects on niche variables are generally greater than phylogenetic effects. This suggests that recent evolution and dispersal have more influence on species' niches than more distant evolutionary events. These results have implications for our understanding of the role of niche conservatism in species richness patterns and for gauging the potential for species to adapt to global change.     

What causes latitudinal gradients in species diversity? Evolutionary processes and ecological constraints on swallowtail biodiversity

The latitudinal diversity gradient (LDG) is one of the most striking ecological patterns on our planet. Determining the evolutionary causes of this pattern remains a challenging task. To address this issue, previous LDG studies have usually relied on correlations between environmental variables and species richness, only considering evolutionary processes indirectly. Instead, we use a phylogenetically integrated approach to investigate the ecological and evolutionary processes responsible for the global LDG observed in swallowtail butterflies (Papilionidae). We find evidence for the 'diversification rate hypothesis' with different diversification rates between two similarly aged tropical and temperate clades. We conclude that the LDG is caused by (1) climatically driven changes in both clades based on evidence of responses to cooling and warming events, and (2) distinct biogeographical histories constrained by tropical niche conservatism and niche evolution. This multidisciplinary approach provides new findings that allow better understanding of the factors that shape LDGs.     

Phylogenetic niche conservatism, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity among species

Ecologists are increasingly adopting an evolutionary perspective, and in recent years, the idea that closely related species are ecologically similar has become widespread. In this regard, phylogenetic signal must be distinguished from phylogenetic niche conservatism. Phylogenetic niche conservatism results when closely related species are more ecologically similar that would be expected based on their phylogenetic relationships; its occurrence suggests that some process is constraining divergence among closely related species. In contrast, phylogenetic signal refers to the situation in which ecological similarity between species is related to phylogenetic relatedness; this is the expected outcome of Brownian motion divergence and thus is necessary, but not sufficient, evidence for the existence of phylogenetic niche conservatism. Although many workers consider phylogenetic niche conservatism to be common, a review of case studies indicates that ecological and phylogenetic similarities often are not related. Consequently, ecologists should not assume that phylogenetic niche conservatism exists, but rather should empirically examine the extent to which it occurs.     

Climatic niche conservatism and ecological opportunity in the explosive radiation of arvicoline rodents (Arvicolinae,Cricetidae)

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.     

Ecological niche conservatism: a time‐structured review of evidence

Aim To evaluate the evolutionary conservatism of coarse‐resolution Grinnellian (or scenopoetic) ecological niches. Location Global. Methods I review a broad swathe of literature relevant to the topic of niche conservatism or differentiation, and illustrate some of the resulting insights with examplar analyses. Results Ecological niche characteristics are highly conserved over short‐to‐moderate time spans (i.e. from individual life spans up to tens or hundreds of thousands of years); little or no ecological niche differentiation is discernible as part of the processes of invasion or speciation. Main conclusions Although niche conservatism is widespread, many methodological complications obscure this point. In particular, niche models are frequently over‐interpreted: too often, they are based on limited occurrence data in high‐dimensional environmental spaces, and cannot be interpreted robustly to indicate niche differentiation.     

Phylogenetic properties of Tertiary relict flora in the east Asian continental islands: imprint of climatic niche conservatism and in situ diversification

Understanding biodiversity patterns on islands has long been a central aim in ecology and conservation biology. Island‐specific biogeographical processes play substantial roles in the formation of endemic biota. Here, we examined how climate niche conservatism and geohistorical factors are interactively associated with in situ diversification of Tertiary relict flora in the east Asian continental islands. We generated two novel datasets for species distribution and phylogeny that included all of the known vascular plant species in Japan (5575). Then we tested phylogenetic signal of climatic tolerance, in terms of absolute minimum temperature and water balance, and explored environmental predictors of phylogenetic structure (evolutionary derivedness and clustering) of species assemblages. Although phylogenetic signal of climatic tolerance was significant across the phylogeny of most species, the strength of climatic niche conservatism differed among ferns, gymnosperms, angiosperm trees, and angiosperm herbs. For angiosperm trees, cold temperatures acted as environmental filters that generated phylogenetic derivedness/clustering of species assemblages. For fern and angiosperm herb species, however, phylogenetic properties were not associated with climatic harshness. These contrasting patterns among groups reflected climate niche evolution in vascular plants with different growth forms and traits; for example, diversification of angiosperm trees (but not fern and herb) occurred in response to historical climatic cooling. More importantly, geographical constraints contributed to evolutionary radiation that resulted from isolation by distance from the continent or by elevation. Quaternary climate change was also associated with clade‐specific radiation in refugial habitats. The degree to which geographical, geological, and palaeoclimatic variables explain the phylogenetic structure underscores the importance of isolation‐ and habitat‐stability‐related geohistorical processes in driving in situ diversification despite climatic niche conservatism. We propose that the highly endemic flora of the east Asian islands resulted from the interplay of idiosyncratic regional factors, and ecological and evolutionary processes, such as climate niche assembly and adaptive/nonadaptive radiation.     

Investigating niche and lineage diversification in widely distributed taxa: phylogeography and ecological niche modeling of the Peromyscus maniculatus species group

The relationship between lineage formation and variation in the ecological niche is a fundamental evolutionary question. Two prevailing hypotheses reflect this relationship: niche conservatism and niche divergence. Niche conservatism predicts a pattern where sister taxa will occupy similar niche spaces; whereas niche divergence predicts that sister taxa will occupy different niche spaces. Widely distributed species often show distinct phylogeographic structure, but little research has been conducted on how the environment may be related to these phylogenetic patterns. We investigated the relationship between lineage divergence and environmental space for the closely related species Peromyscus maniculatus and P. polionotus utilizing phylogenetic techniques and ecological niche modeling (ENM). We estimated the phylogenetic relationship among individuals based on complete cytochrome b sequences that represent individuals from a majority of the species ranges. Niche spaces that lineages occupy were estimated by using 12 environmental layers. Differences in niche space were tested using multivariate statistics based on location data, and ENMs were employed using maximum entropy algorithms. Two similarity indices estimated significant divergence in environmental space based on the ENM. Six geographically structured lineages were identified within P. maniculatus. Nested within P. maniculatus we found that P. polionotus recently diverged from a clade occupying central and western United States. We estimated that the majority of the genetic lineages occupy distinct environmental niches, which supports a pattern of niche divergence. Two sister taxa showed niche divergence and represent different ecomorphs, suggesting morphological, genetic and ecological divergence between the two lineages. Two other sister taxa were observed in the same environmental space based on multivariate statistics, suggesting niche conservatism. Overall our results indicate that a widely distributed species may exhibit both niche conservatism and niche divergence, and that most lineages seem to occupy distinct environmental niches.     

Climatic niche conservatism and the evolutionary dynamics in species range boundaries: global congruence across mammals and amphibians

Aim Comparative evidence for phylogenetic niche conservatism – the tendency for lineages to retain their ancestral niches over long time scales – has so far been mixed, depending on spatial and taxonomic scale. We quantify and compare conservatism in the climatic factors defining range boundaries in extant continental mammals and amphibians in order to identify those factors that are most evolutionarily conserved, and thus hypothesized to have played a major role in determining the geographic distributions of many species. We also test whether amphibians show stronger signals of climatic niche conservatism, as expected from their greater physiological sensitivity and lower dispersal abilities. Location Global; continental land masses excluding Antarctica. Methods We used nearly complete global distributional databases to estimate the climatic niche conservatism in extant continental mammals and amphibians. We characterized the climatic niche of each species by using a suite of variables and separately investigate conservatism in each variable using both taxonomic and phylogenetic approaches. Finally, we explored the spatial, taxonomic and phylogenetic patterns in recent climatic niche evolution. Results Amphibians and mammals showed congruent patterns of conservatism in cold tolerance, with assemblages of escapee species (i.e. those escaping most from the climatic constraints of their ancestors) aggregated in the North Temperate Zone. Main conclusions The relative strength of climatic niche conservatism varies across the variables tested, but is strongest for cold tolerance in both mammals and amphibians. Despite the apparent conservatism in this variable, there is also a strong signal of recent evolutionary shifts in cold tolerance in assemblages inhabiting the North Temperate Zone. Our results thus indicate that distribution patterns of both taxa are influenced by both niche conservatism and niche evolution.     

Phylogenetic conservatism and biogeographic affinity influence woody plant species richness–climate relationships in eastern Eurasia

Mechanisms underlying species richness patterns remain a central yet controversial issue in biology. Climate has been regarded as a major determinant of species richness. However, the relative influences of different evolutionary processes, (i.e. niche conservatism, diversification rate and time for speciation) on species richness–climate relationships remain to be tested. Here, using newly compiled distribution maps for 11 422 woody plant species in eastern Eurasia, we estimated species richness patterns for all species and for families with tropical and temperate affinities separately, and explored the phylogenetic signals in species richness patterns of different families and their relationships with contemporary climate and climate change since the Last Glacial Maximum (LGM). We further compared the effects of niche conservatism (represented by contemporary-ancestral climatic niches differences), diversification rate and time for speciation (represented by family age) on variation in the slopes of species richness–climate relationships. We found that winter coldness was the best predictor for species richness patterns of most tropical families while Quaternary climate change was the best predictor for those of most temperate families. Species richness patterns of closely-related families were more similar than those of distantly-related families within eudicots, and significant phylogenetic signals characterized the slopes of species richness–climate relationships across all angiosperm families. Contemporary-ancestral climatic niche differences dominated variation in the relationships between family-level species richness and most climate variables. Our results indicate significant phylogenetic conservatism in family-level species richness patterns and their relationships with contemporary climate within eudicots. These findings shed light on the mechanisms underlying large-scale species richness patterns and suggest that ancestral climatic niche may influence the evolution of species richness–climate relationships in plants through niche conservatism.     

Environmental niche equivalency versus conservatism: quantitative approaches to niche evolution

Environmental niche models, which are generated by combining species occurrence data with environmental GIS data layers, are increasingly used to answer fundamental questions about niche evolution, speciation, and the accumulation of ecological diversity within clades. The question of whether environmental niches are conserved over evolutionary time scales has attracted considerable attention, but often produced conflicting conclusions. This conflict, however, may result from differences in how niche similarity is measured and the specific null hypothesis being tested. We develop new methods for quantifying niche overlap that rely on a traditional ecological measure and a metric from mathematical statistics. We reexamine a classic study of niche conservatism between sister species in several groups of Mexican animals, and, for the first time, address alternative definitions of "niche conservatism" within a single framework using consistent methods. As expected, we find that environmental niches of sister species are more similar than expected under three distinct null hypotheses, but that they are rarely identical. We demonstrate how our measures can be used in phylogenetic comparative analyses by reexamining niche divergence in an adaptive radiation of Cuban anoles. Our results show that environmental niche overlap is closely tied to geographic overlap, but not to phylogenetic distances, suggesting that niche conservatism has not constrained local communities in this group to consist of closely related species. We suggest various randomization tests that may prove useful in other areas of ecology and evolutionary biology.     

Phylogenetic diversity,structure and diversification patterns of endemic plants along the elevational gradient in the Eastern Himalaya

ABSTRACT

Background: Several studies have documented the variation in species diversity patterns along elevational gradients in the Himalaya, but few have reported the evolutionary and biogeographic processes behind these patterns.

Aims: To understand whether evolutionary history and phylogeny have any role in structuring plant species communities along an elevational gradient in the Sikkim Himalaya.

Methods: We used data on endemic plant species occurrence from primary and secondary sources to construct family-level phylogenetic supertrees for different growth forms with the help of Phylomatic tool of Phylocom. These phylogenetic supertrees were used as a base for testing phylogenetic diversity (PD), niche conservatism, diversification time patterns and phylogenetic structure of various plant growth forms along an elevational gradient.

Results: PD was the highest at mid-elevations for all growth forms and PD had a significant positive correlation with endemic species richness. Species at mid-elevations were dominated by the ancestral/primitive taxa. There was phylogenetic clustering at higher elevations and phylogenetic overdispersion at lower and mid-elevations for the majority of the growth forms.

Conclusions: Time-for-speciation effect and niche conservatism along elevation (retention of niche-related ancestral elevational distribution over evolutionary time scale by species) together determine plant species diversity patterns in the Himalaya.     

Evolutionary and ecological causes of the latitudinal diversity gradient in hylid frogs: treefrog trees unearth the roots of high tropical diversity

Why are there more species in the tropics than in temperate regions? In recent years, this long-standing question has been addressed primarily by seeking environmental correlates of diversity. But to understand the ultimate causes of diversity patterns, we must also examine the evolutionary and biogeographic processes that directly change species numbers (i.e., speciation, extinction, and dispersal). With this perspective, we dissect the latitudinal diversity gradient in hylid frogs. We reconstruct a phylogeny for 124 hylid species, estimate divergence times and diversification rates for major clades, reconstruct biogeographic changes, and use ecological niche modeling to identify climatic variables that potentially limit dispersal. We find that hylids originated in tropical South America and spread to temperate regions only recently (leaving limited time for speciation). There is a strong relationship between the species richness of each region and when that region was colonized but not between the latitudinal positions of clades and their rates of diversification. Temperature seasonality seemingly limits dispersal of many tropical clades into temperate regions and shows significant phylogenetic conservatism. Overall, our study illustrates how two general principles (niche conservatism and the time-for-speciation effect) may help explain the latitudinal diversity gradient as well as many other diversity patterns across taxa and regions.