Global patterns of intraspecific leaf trait responses to elevation

Elevational gradients are often used to quantify how traits of plant species respond to abiotic and biotic environmental variations. Yet, such analyses are frequently restricted spatially and applied along single slopes or mountain ranges. Since we know little on the response of intraspecific leaf traits to elevation across the globe, we here perform a global meta‐analysis of leaf traits in 109 plant species located in 4 continents and reported in 71 studies published between 1983 and 2018. We quantified the intraspecific change in seven morpho‐ecophysiological leaf traits along global elevational gradients: specific leaf area (SLA), leaf mass per area (LMA), leaf area (LA), nitrogen concentration per unit of area (Narea), nitrogen concentration per unit mass (Nmass), phosphorous concentration per unit mass (Pmass) and carbon isotope composition (δ¹³C). We found LMA, Narea, Nmass and δ¹³C to significantly increase and SLA to decrease with increasing elevation. Conversely, LA and Pmass showed no significant pattern with elevation worldwide. We found significantly larger increase in Narea, Nmass, Pmass and δ¹³C with elevation in warmer regions. Larger responses to increasing elevation were apparent for SLA of herbaceous compared to woody species, but not for the other traits. Finally, we also detected evidences of covariation across morphological and physiological traits within the same elevational gradient. In sum, we demonstrate that there are common cross‐species patterns of intraspecific leaf trait variation across elevational gradients worldwide. Irrespective of whether such variation is genetically determined via local adaptation or attributed to phenotypic plasticity, the leaf trait patterns quantified here suggest that plant species are adapted to live on a range of temperature conditions. Since the distribution of mountain biota is predominantly shifting upslope in response to changes in environmental conditions, our results are important to further our understanding of how plants species of mountain ecosystems adapt to global environmental change.     

Elevational differentiation accelerates trait evolution but not speciation rates in Amazonian birds

The importance of ecologically mediated divergent selection in accelerating trait evolution has been poorly studied in the most species‐rich biome of the planet, the continental Neotropics. We performed macroevolutionary analyses of trait divergence and diversification rates across closely related pairs of Andean and Amazonian passerine birds, to assess whether the difference in elevational range separating species pairs – a proxy for the degree of ecological divergence – influences the speed of trait evolution and diversification rates. We found that elevational differentiation is associated with faster divergence of song frequency, a trait important for pre‐mating isolation, and several morphological traits, which may contribute to extrinsic post‐mating isolation. However, elevational differentiation did not increase recent speciation rates, possibly due to early bursts of diversification during the uplift of the eastern Andes followed by a slow‐down in speciation rate. Our results suggest that ecological differentiation may speed up trait evolution, but not diversification of Neotropical birds.     

Isozyme polymorphisms provide evidence of clinal variation with elevation in Nothofagus pumilio

Variable physical conditions along elevational gradients strongly influence patterns of genetic differentiation in tree species. Here, the hypothesis is tested that different growth forms of Nothofagus pumilio, which characterizes the subalpine forests in the southern Andes, will display continuous genetic variation with elevation. At each of four elevational strips in three different mountain ranges, fresh leaf tissue was sampled from 30 randomly selected individuals to be analyzed by protein electrophoresis. Allelic frequencies were used to test for heterogeneity across populations and to classify populations into different elevational strips by discriminant analysis. The degree of population divergence was estimated by F(ST). Clinical variation on within-population genetic characteristics was analyzed by linear regressions against elevation. Seven enzyme systems coded for 14 putative isozyme loci, 57% of which were polymorphic in at least one population. Allele frequencies significantly varied with elevation and discriminant analysis separated populations at different elevational strips. Among-population divergence within any mountain range was small, but greater than among different mountain ranges. Overall, low-elevation populations were more variable than high-elevation populations, and regression analyses suggested continuous variation in populations of N. pumilio 100 m apart. Marked stepwise phenological differences on mountain slopes are most probably responsible for the isolation of nearby populations.     

Disentangling the historical routes to community assembly in the global epicentre of biodiversity

Aim

The exceptional turnover in biota with elevation and number of species coexisting at any elevation makes tropical mountains hotspots of biodiversity. However, understanding the historical processes through which species arising in geographical isolation (i.e. allopatry) assemble along the same mountain slope (i.e. sympatry) remains a major challenge. Multiple models have been proposed including (1) the sorting of already elevationally divergent species, (2) the displacement of elevation upon secondary contact, potentially followed by convergence, or (3) elevational conservatism, in which ancestral elevational ranges are retained. However, the relative contribution of these processes to generating patterns of elevational overlap and turnover is unknown.

Location

Tropical mountains of Central- and South-America.

Time Period

The last 12 myr.

Major Taxa Studied

Birds.

Methods

We collate a dataset of 165 avian sister pairs containing estimates of phylogenetic age, geographical and regional elevational range overlap. We develop a framework based on continuous-time Markov models to infer the relative frequency of different historical pathways in explaining present-day overlap and turnover of sympatric species along elevational gradients.

Results

We show that turnover of closely related bird species across elevation can predominantly be explained by displacement of elevation ranges upon contact (81%) rather than elevational divergence in allopatry (19%). In contrast, overlap along elevation gradients is primarily (88%) explained by conservatism of elevational ranges rather than displacement followed by elevational expansion (12%).

Main Conclusions

Bird communities across elevation gradients are assembled through a mix of processes, including the sorting, displacement and conservatism of species elevation ranges. The dominant role of conservatism in explaining co-occurrence of species on mountain slopes rejects more complex scenarios requiring displacement followed by expansion. The ability of closely related species to coexist without elevational divergence provides a direct and faster pathway to sympatry and helps explain the exceptional species richness of tropical mountains.     

Speciation in ninespine stickleback: reproductive isolation and phenotypic divergence among cryptic species of Japanese ninespine stickleback

Although similar patterns of phenotypic diversification are often observed in phylogenetically independent lineages, differences in the magnitude and direction of phenotypic divergence have been also observed among independent lineages, even when exposed to the same ecological gradients. The stickleback family is a good model with which to explore the ecological and genetic basis of parallel and nonparallel patterns of phenotypic evolution, because there are a variety of populations and species that are locally adapted to divergent environments. Although the patterns of phenotypic divergence as well as the genetic and ecological mechanisms have been well characterized in threespine sticklebacks, Gasterosteus aculeatus, we know little about the patterns of phenotypic diversification in other stickleback lineages. In eastern Hokkaido, Japan, there are three species of ninespine sticklebacks, Pungitius tymensis and the freshwater type and the brackish‐water type of the P. pungitius–P. sinensis species complex. They utilize divergent habitats along coast–stream gradients of rivers. Here, we investigated genetic, ecological and phenotypic divergence among three species of Japanese ninespine sticklebacks. Divergence in trophic morphology and salinity tolerance occurred in the direction predicted by the patterns observed in threespine sticklebacks. However, the patterns of divergence in armour plate were different from those previously found in threespine sticklebacks. Furthermore, the genetic basis of plate variation may differ from that in threespine sticklebacks. Because threespine sticklebacks are well‐established model for evolutionary research, the sympatric trio of ninespine sticklebacks will be an invaluable resource for ecological and genetic studies on both common and lineage‐specific patterns of phenotypic diversification.     

Ecological speciation along an elevational gradient in a tropical passerine bird?

Local adaptation of populations along elevational gradients is well known, but conclusive evidence that such divergence has resulted in the origin of distinct species in parapatry remains lacking. We integrated morphological, vocal, genetic and behavioural data to test predictions pertaining to the hypothesis of parapatric ecological speciation associated with elevation in populations of a tropical montane songbird, the Grey‐breasted Wood‐wren (Henicorhina leucophrys: Troglodytidae), from the Sierra Nevada de Santa Marta, Colombia. We confirmed that two distinct populations exist along the elevational gradient. Phylogenetic analyses tentatively indicate that the two populations are not sister taxa, suggesting they did not differentiate from a single ancestor along the gradient, but rather resulted from separate colonization events. The populations showed marked divergence in morphometrics, vocalizations and genetic variation in mitochondrial and nuclear loci, and little to no evidence of hybridization. Individuals of both populations responded more strongly to their own local songs than to songs from another elevation. Although the two forms do not appear to have differentiated locally in parapatry, morphological and vocal divergence along the elevational gradient is consistent with adaptation, suggesting a possible link between adaptive evolution in morphology and songs and the origin of reproductive isolation via a behavioural barrier to gene flow. The adaptive value of phenotypic differences between populations requires additional study.     

Testing for local adaptation and evolutionary potential along altitudinal gradients in rainforest Drosophila: beyond laboratory estimates

Predicting how species will respond to the rapid climatic changes predicted this century is an urgent task. Species distribution models (SDMs) use the current relationship between environmental variation and species’ abundances to predict the effect of future environmental change on their distributions. However, two common assumptions of SDMs are likely to be violated in many cases: (i) that the relationship of environment with abundance or fitness is constant throughout a species’ range and will remain so in future and (ii) that abiotic factors (e.g. temperature, humidity) determine species’ distributions. We test these assumptions by relating field abundance of the rainforest fruit fly Drosophila birchii to ecological change across gradients that include its low and high altitudinal limits. We then test how such ecological variation affects the fitness of 35 D. birchii families transplanted in 591 cages to sites along two altitudinal gradients, to determine whether genetic variation in fitness responses could facilitate future adaptation to environmental change. Overall, field abundance was highest at cooler, high‐altitude sites, and declined towards warmer, low‐altitude sites. By contrast, cage fitness (productivity) increased towards warmer, lower‐altitude sites, suggesting that biotic interactions (absent from cages) drive ecological limits at warmer margins. In addition, the relationship between environmental variation and abundance varied significantly among gradients, indicating divergence in ecological niche across the species’ range. However, there was no evidence for local adaptation within gradients, despite greater productivity of high‐altitude than low‐altitude populations when families were reared under laboratory conditions. Families also responded similarly to transplantation along gradients, providing no evidence for fitness trade‐offs that would favour local adaptation. These findings highlight the importance of (i) measuring genetic variation in key traits under ecologically relevant conditions, and (ii) considering the effect of biotic interactions when predicting species’ responses to environmental change.     

Consequences of divergence and introgression for speciation in Andean cloud forest birds

Divergence with gene flow is well documented and reveals the influence of ecological adaptation on speciation. Yet, it remains intuitive that gene exchange inhibits speciation in many scenarios, particularly among ecologically similar populations. The influence of gene flow on the divergence of populations facing similar selection pressures has received less empirical attention than scenarios where differentiation is coupled with local environmental adaptation. I used a paired study design to test the influence of genomic divergence and introgression on plumage differentiation between ecologically similar allopatric replacements of Andean cloud forest birds. Through analyses of short‐read genome‐wide sequences from over 160 individuals in 16 codistributed lineages, I found that plumage divergence is associated with deep genetic divergence, implicating a prominent role of geographic isolation in speciation. By contrast, lineages that lack plumage divergence across the same geographic barrier are more recently isolated or exhibit a signature of secondary genetic introgression, indicating a negative relationship between gene flow and divergence in phenotypic traits important to speciation. My results suggest that the evolutionary outcomes of cycles of isolation and divergence in this important theatre of biotic diversification are sensitive to time spent in the absence of gene flow.     

Testing the role of interspecific competition in the evolutionary origin of elevational zonation: an example with Buarremon brush-finches (Aves, Emberizidae) in the neotropical mountains

Interspecific competition might drive the evolution of ecological niches and result in pairs of formerly competing species segregating along ecological gradients following a process of character displacement. This mechanism has been proposed to account for replacement of related species along gradients of elevation in many areas of the world, but the fundamental issue of whether competition is responsible for the origin of elevational replacements has not been tested. To test hypotheses about the role of interspecific competition in the origin of complementary elevational ranges, I combined molecular phylogenetics, phylogeography, and population genetic analyses on Buarremon torquatus and B. brunneinucha (Aves, Emberizidae), whose patterns of elevational distribution suggest character displacement or ecological release. The hypothesis that elevational distributions in these species changed in opposite directions as a result of competition is untenable because: (1) a historical expansion of the range of B. brunneinucha into areas occupied by B. torquatus was not accompanied by a shift in the elevational range of the former species; (2) when B. brunneinucha colonized the range of B. torquatus, lineages of the latter distributions had already diverged; and (3) historical trends in effective population size do not suggest populations with elevational ranges abutting those of putative competitors have declined as would be expected if competition caused range contractions. However, owing to uncertainty in coalescent estimates of historical population sizes, the hypothesis that some populations of B. torquatus have declined cannot be confidently rejected, which suggests asymmetric character displacement might have occurred. I suggest that the main role of competition in elevational zonation may be to act as a sorting mechanism that allows the coexistence along mountain slopes only of ecologically similar species that differ in elevational distributions prior to attaining sympatry. The contrasting biogeographic histories of B. brunneinucha and B. torquatus illustrate how present-day ecological interactions can have recent origins, and highlights important challenges for testing the hypothesis of character displacement in the absence of data on population history and robust reconstructions of the evolution of traits and geographic ranges.     

Tallow tree allocates contrasting secondary chemicals in response to varying environments along elevational gradients

了解树木如何沿海拔梯度调节多种类型的次生代谢产物对于阐明植物如何采用生理和生态策略来应对各种生物及非生物环境变化至关重要。本研究旨在探索乌桕如何分配不同的次生代谢产物来响应海拔梯度上的环境变化。我们在中国沿海拔梯度对乌桕不同取食类型的植食性昆虫及其对乌桕叶片的危害率进行了野外实地调查，并对健康叶片和虫害叶片中的次生代谢产物(单宁和黄酮类)进行了测定分析。研究结果表明，乌桕叶片被危害(咀嚼或潜叶式危害)的可能性随海拔的升高而减小。遭受虫害叶片和未遭受虫害的健康叶片中黄酮类化合物的浓度随海拔梯度变化呈现相反的趋势，即随着海拔的升高，健康叶片和遭受虫害叶片受不同的生物和非生物因素驱动，健康叶片中黄酮类化合物浓度增加，而遭受虫害叶片中黄酮类化合物浓度下降，其中槲皮苷对黄酮类化合物随海拔梯度变化的贡献最大。单宁浓度随海拔梯度的变化未发生显著变化，高海拔地区乌桕健康叶片中单宁与黄酮类化合物的比例与低海拔相比相对较低。我们的研究揭示了沿海拔梯度昆虫植食性以及不同植物次生代谢产物的变化趋势，并强调了在理解植物的生理和生态策略中同时考虑多种次生代谢物质的重要性。     

Testing alternative hypotheses for evolutionary diversification in an African songbird: rainforest refugia versus ecological gradients

Geographic isolation in rainforest refugia and local adaptation to ecological gradients may both be important drivers of evolutionary diversification. However, their relative importance and the underlying mechanisms of these processes remain poorly understood because few empirical studies address both putative processes in a single system. A key question is to what extent is divergence in signals that are important in mate and species recognition driven by isolation in rainforest refugia or by divergent selection across ecological gradients? We studied the little greenbul, Andropadus virens, an African songbird, in Cameroon and Uganda, to determine whether refugial isolation or ecological gradients better explain existing song variation. We then tested whether song variation attributable to refugial or ecological divergence was biologically meaningful using reciprocal playback experiments to territorial males. We found that much of the existing song variation can be explained by both geographic isolation and ecological gradients, but that divergence across the gradient, and not geographic isolation, affects male response levels. These data suggest that ecologically divergent traits, independent of historical isolation during glacial cycles, can promote reproductive isolation. Our study provides further support for the importance of ecology in explaining patterns of evolutionary diversification in ecologically diverse regions of the planet.     

Fine-scale spatial genetic structure across a strong environmental gradient in the saltmarsh plant <Emphasis Type="Italic">Puccinellia maritima</Emphasis>

Saltmarsh forms the transition between maritime and terrestrial environments where biotic and abiotic conditions vary substantially along a gradient in elevation. Theoretical and empirical population genetics studies have focused on the influence of environmental gradients on intra-specific genetic variation. Contrastingly, only a few studies have focused on genetic variation in saltmarsh plants, despite the potentially strong influence of environmental gradients shaping diversity in these species. In the present paper, we assess the genetic structure of the saltmarsh plant Puccinellia maritima collected across an elevation gradient in restored and natural saltmarsh. Both spatial autocorrelograms of genetic variation and spatial analysis of principal components detected genetic structure in the natural saltmarsh organized along the gradient in elevation, yet no such pattern was identified considering distance between individuals without taking elevation into account. In combination with previous phenotypic analyses, our results imply that ecological divergence likely plays a key role in shaping genetic structure within saltmarsh species. Comparison of restored and natural saltmarsh indicated that interspecific competition plays an important role in shaping the genetic structure observed on the natural saltmarsh. The results of this study demonstrate that saltmarshes are valuable models in which to test effects of ecological differentiation and, by extension, provide a better understanding of the functioning of this threatened environment.     

Long‐term acclimation to reciprocal light conditions suggests depth‐related selection in the marine foundation species Posidonia oceanica

Phenotypic differences among populations of the same species reflect selective responses to ecological gradients produced by variations in abiotic and biotic factors. Moreover, they can also originate from genetic differences among populations, due to a reduced gene flow. In this study, we examined the extent of differences in photo‐acclimative traits of Posidonia oceanica (L.) Delile clones collected above and below the summer thermocline (i.e., ?5 and ?25 m) in a continuous population extending along the water depth gradient. During a reciprocal light exposure and subsequent recovery in mesocosms, we assessed degree of phenotypic plasticity and local adaptation of plants collected at different depths, by measuring changes in several traits, such as gene expression of target genes, photo‐physiological features, and other fitness‐related traits (i.e., plant morphology, growth, and mortality rates). Samples were also genotyped, using microsatellite markers, in order to evaluate the genetic divergence among plants of the two depths. Measures collected during the study have shown a various degree of phenotypic changes among traits and experimental groups, the amount of phenotypic changes observed was also dependent on the type of light environments considered. Overall plants collected at different depths seem to be able to acclimate to reciprocal light conditions in the experimental time frame, through morphological changes and phenotypic buffering, supported by the plastic regulation of a reduced number of genes. Multivariate analyses indicated that plants cluster better on the base of their depth origin rather than the experimental light conditions applied. The two groups were genetically distinct, but the patterns of phenotypic divergence observed during the experiment support the hypothesis that ecological selection can play a role in the adaptive divergence of P. oceanica clones along the depth gradient.     

Plant invasion and speciation along elevational gradients on the oceanic island La Palma,Canary Islands

Ecosystems that provide environmental opportunities but are poor in species and functional richness generally support speciation as well as invasion processes. These processes are expected not to be equally effective along elevational gradients due to specific ecological, spatial, and anthropogenic filters, thus controlling the dispersal and establishment of species. Here, we investigate speciation and invasion processes along elevational gradients. We assess the vascular plant species richness as well as the number and percentage of endemic species and non‐native species systematically along three elevational gradients covering large parts of the climatic range of La Palma, Canary Islands. Species richness was negatively correlated with elevation, while the percentage of Canary endemic species showed a positive relationship. However, the percentage of Canary–Madeira endemics did not show a relationship with elevation. Non‐native species richness (indicating invasion) peaked at 500 m elevation and showed a consistent decline until about 1,200 m elevation. Above that limit, no non‐native species were present in the studied elevational gradients. Ecological, anthropogenic, and spatial filters control richness, diversification, and invasion with elevation. With increase in elevation, richness decreases due to species–area relationships. Ecological limitations of native ruderal species related to anthropogenic pressure are in line with the absence of non‐native species from high elevations indicating directional ecological filtering. Increase in ecological isolation with elevation drives diversification and thus increased percentages of Canary endemics. The best preserved eastern transect, including mature laurel forests, is an exception. The high percentage of Canary–Madeira endemics indicates the cloud forest's environmental uniqueness—and thus ecological isolation—beyond the Macaronesian islands.     

Evaluation of genetic isolation within an island flora reveals unusually widespread local adaptation and supports sympatric speciation

It is now recognized that speciation can proceed even when divergent natural selection is opposed by gene flow. Understanding the extent to which environmental gradients and geographical distance can limit gene flow within species can shed light on the relative roles of selection and dispersal limitation during the early stages of population divergence and speciation. On the remote Lord Howe Island (Australia), ecological speciation with gene flow is thought to have taken place in several plant genera. The aim of this study was to establish the contributions of isolation by environment (IBE) and isolation by community (IBC) to the genetic structure of 19 plant species, from a number of distantly related families, which have been subjected to similar environmental pressures over comparable time scales. We applied an individual-based, multivariate, model averaging approach to quantify IBE and IBC, while controlling for isolation by distance (IBD). Our analyses demonstrated that all species experienced some degree of ecologically driven isolation, whereas only 12 of 19 species were subjected to IBD. The prevalence of IBE within these plant species indicates that divergent selection in plants frequently produces local adaptation and supports hypotheses that ecological divergence can drive speciation in sympatry.     

Multiple strong postmating and intrinsic postzygotic reproductive barriers isolate florally diverse species of Jaltomata (Solanaceae)

Divergence in phenotypic traits often contributes to premating isolation between lineages, but could also promote isolation at postmating stages. Phenotypic differences could directly result in mechanical isolation or hybrids with maladapted traits; alternatively, when alleles controlling these trait differences pleiotropically affect other components of development, differentiation could indirectly produce genetic incompatibilities in hybrids. Here, we determined the strength of nine postmating and intrinsic postzygotic reproductive barriers among 10 species of Jaltomata (Solanaceae), including species with highly divergent floral traits. To evaluate the relative importance of floral trait diversification for the strength of these postmating barriers, we assessed their relationship to floral divergence, genetic distance, geographical context, and ecological differences, using conventional tests and a new linear‐mixed modeling approach. Despite close evolutionary relationships, all species pairs showed moderate to strong isolation. Nonetheless, floral trait divergence was not a consistent predictor of the strength of isolation; instead this was best explained by genetic distance, although we found evidence for mechanical isolation in one species, and a positive relationship between floral trait divergence and fruit set isolation across species pairs. Overall, our data indicate that intrinsic postzygotic isolation is more strongly associated with genome‐wide genetic differentiation, rather than floral divergence.     

Cryptic species and hidden ecological interactions of halictine bees along an elevational gradient

Changes of abiotic and biotic conditions along elevational gradients represent serious challenges to organisms which may promote the turnover of species, traits and biotic interaction partners. Here, we used molecular methods to study cuticular hydrocarbon (CHC) profiles, biotic interactions and phylogenetic relationships of halictid bees of the genus Lasioglossum along a 2,900 m elevational gradient at Mt. Kilimanjaro, Tanzania. We detected a strong species turnover of morphologically indistinguishable taxa with phylogenetically clustered cryptic species at high elevations, changes in CHC profiles, pollen resource diversity, and a turnover in the gut and body surface microbiome of bees. At high elevations, increased proportions of saturated compounds in CHC profiles indicate physiological adaptations to prevent desiccation. More specialized diets with higher proportions of low‐quality Asteraceae pollen imply constraints in the availability of food resources. Interactive effects of climatic conditions on gut and surface microbiomes, CHC profiles, and pollen diet suggest complex feedbacks among abiotic conditions, ecological interactions, physiological adaptations, and phylogenetic constraints as drivers of halictid bee communities at Mt. Kilimanjaro.     

Drivers of genetic differentiation in a generalist insect‐pollinated herb across spatial scales

The isolation‐by‐distance model (IBD) predicts that genetic differentiation among populations increases with geographic distance. Yet, empirical studies show that a variety of ecological, topographic and historical factors may override the effect of geographic distance on genetic variation. This may particularly apply to species with narrow but highly heterogeneous distribution ranges, such as those occurring along elevational gradients. Using nine SSR markers, we study the genetic differentiation of the montane pollination‐generalist herb, Erysimum mediohispanicum. Because the effects of any given factor may depend on the geographic scale considered, we investigate the contribution of different environmental and historical factors at three different spatial scales. We evaluate five competing models that put forward the role of geographic distance, local environmental factors [biotic interactions (IBEb) and climatic variables (IBEa)], landscape resistance (IBR) and phylogeographic patterns (IBP), respectively. We find significant IBD regardless of the spatial scale and the genetic distance estimator considered. However, IBEa and IBP also play a prominent role in shaping genetic differentiation patterns at the larger spatial scales, and IBR is significant at the fine spatial scale. Overall, our results highlight the importance of combining different estimators, statistical approaches and spatial scales to disentangle the relative importance of the various ecological factors contributing to the shaping of genetic divergence patterns in natural populations.     

Isolation by distance,not incipient ecological speciation,explains genetic differentiation in an Andean songbird (Aves: Furnariidae: Cranioleuca antisiensis,Line‐cheeked Spinetail) despite near threefold body size change across an environmental gradient

During the process of ecological speciation, reproductive isolation results from divergent natural selection and leads to a positive correlation between genetic divergence and adaptive phenotypic divergence, that is, isolation by adaptation (IBA). In natural populations, phenotypic differentiation is often autocorrelated with geographic distance, making IBA difficult to distinguish from the neutral expectation of isolation by distance (IBD). We examined these two alternatives in a dramatic case of clinal phenotypic variation in an Andean songbird, the Line‐cheeked Spinetail (Cranioleuca antisiensis). At its geographic extremes, this species shows a near threefold difference in body mass (11.5 to 31.0 g) with marked plumage differences. We analysed phenotypic, environmental and genetic data (5,154 SNPs) from 172 individuals and 19 populations sampled along its linear distribution in the Andes. We found that body mass was tightly correlated with environmental temperature, consistent with local adaptation as per Bergmann's rule. Using a P_ST–F_ST analysis, we found additional support for natural selection driving body mass differentiation, but these results could also be explained by environment‐mediated phenotypic plasticity. When we assessed the relative support for patterns of IBA and IBD using variance partitioning, we found that IBD was the best explanation for genetic differentiation along the cline. Adaptive phenotypic or environmental divergence can reduce gene flow, a pattern interpreted as evidence of ecological speciation's role in diversification. Our results provide a counterexample to this interpretation. Despite conditions conducive to ecological speciation, our results suggest that dramatic size and environmental differentiation within C. antisiensis are not limiting gene flow.     

Exploring the role of physiology and biotic interactions in determining elevational ranges of tropical animals

Tropical mountains contain some of the world’s richest animal communities as a result of high turnover of species along elevational gradients. We describe an approach to study the roles of biotic and abiotic factors in establishing elevational ranges, and to improve our ability to predict the effects of climate change on these communities. As a framework we use Hutchinson’s concept of the fundamental niche (determined by the match between the physical environment and the organism’s physiological and biophysical characteristics) and realized niche (the subset of the fundamental niche determined by biotic interactions). Using tropical birds as an example, we propose a method for estimating fundamental niches and discuss five biotic interactions that we expect to influence distributions of tropical montane animals: predation, competition, parasites and pathogens, mutualisms, and habitat associations. The effects of biotic factors on elevational ranges have been studied to some extent, but there is little information on physiological responses of tropical montane animals. It will be necessary to understand all of these ecological constraints in concert to predict current and future elevational ranges and potential threats to montane species. Given the importance of tropical mountains as global biodiversity hotspots, we argue that this area of research requires urgent attention.