The past and future influence of geographic information systems on hybrid zone, phylogeographic and speciation research

Ecological niche contributes to the persistence of the Western x Glaucous-winged Gull hybrid zone

Hybrid zones occur in nature when populations with limited reproductive barriers overlap in space. Many hybrid zones persist over time, and different models have been proposed to explain how selection can maintain hybrid zone stability. More empirical studies are needed to elucidate the role of ecological adaptation in maintaining stable hybrid zones. Here, we investigated the role of exogenous factors in maintaining a hybrid zone between western gulls (Larus occidentalis) and glaucous-winged gulls (L. glaucescens). We used ecological niche models (ENMs) and niche similarity tests to quantify and examine the ecological niches of western gulls, glaucous-winged gulls, and their hybrids. We found evidence of niche divergence between all three groups. Our results best support the bounded superiority model, providing further evidence that exogenous selection favoring hybrids may be an important factor in maintaining this stable hybrid zone.

The role of the environment in the spatial dynamics of an extensive hybrid zone between two neotropical cats

Identifying factors that create and maintain a hybrid zone is of great interest to ecology, evolution and, more recently, conservation biology. Here, we investigated the role of environmental features in shaping the spatial dynamics of a hybrid zone between the southern tigrina, Leopardus guttulus, and Geoffroy's cat, L. geoffroyi, testing for exogenous selection as the main force acting on its maintenance. These Neotropical felid species are mainly allopatric, with a restricted area of sympatry in the ecotone between the Atlantic Forest and Pampa biomes. As both biomes have experienced high rates of anthropogenic habitat alteration, we also analysed the influence of habitat conversion on the hybrid zone structure. To do this, we used 13 microsatellite loci to identify potential hybrids and generated ecological niche models for them and their parental species. We compared the influence of variables on parental species and hybrid occurrence and calculated the amount of niche overlap among them. Parental species showed different habitat requirements and predicted co-occurrence was restricted to the forest-grassland mosaic of the ecotone. However, hybrids were found beyond this area, mainly in the range of L. geoffroyi. Hybrids demonstrated higher tolerance to habitat alteration than parental types, with a probability of occurrence that was positively related with mosaics of cropland areas and remnants of natural vegetation. These results indicate that exogenous selection alone does not drive the dynamics of the hybrid zone, and that habitat conversion influences its structure, potentially favouring hybrids over parental species.

Population dynamics of sugarcane borers, Diatraea spp., under different climatic scenarios in Colombia

Seasonal temperature and precipitation patterns on a global scale are main factors to which insects and plants adapt through natural selection, although periodic outbreaks in insect populations may occur in areas where they had not been previously reported, a phenomenon considered as a consequence of global warming. In this study, we estimate the distribution of sugarcane borers, Diatraea spp., under different climate scenarios (rcp26, rcp45, rcp60 and rcp85.) Insects were collected weekly in four sugarcane fields from four different towns in the department of Caldas (Colombia) during 2017, and also in several sugarcane fields in the Cauca River Valley (CRV) between 2010 and 2017. The influence of climatic variables on different agro-ecological zones of the CRV sugarcane fields was defined by climatic data between 2010 and 2017 (maximum and minimum daily temperatures, and accumulated precipitation). The estimate of an optimal niche for Diatraea spp. includes temperatures between 20°C and 23°C, accumulated annual rainfall between 1200 and 1500 mm, dry months with precipitations below 50 mm, slopes of less than 0.05 degrees, crop heterogeneity with an index of 0.2 and primary production values of 1.0. Data suggests Diatraea population is considerably influenced by adverse climate change effects, under the premise of an increase in local and global temperatures, reducing its population niches as well as the number of individuals.

Historical climate changes and hybridization shaped the evolution of Atlantic Forest spinetails (Aves: Furnariidae)

Combining phylogeographic approaches and hybrid zone inference in a single framework is a robust way to depict respectively the biogeographic history of lineages and the evolutionary processes responsible for speciation. Here, we studied the spatiotemporal patterns of diversification and characterize the hybrid zone between two Atlantic Forest spinetails (Synallaxis ruficapilla and Synallaxis cinerea) using mitochondrial DNA and nuclear (autosomal and Z-linked) genes. We consistently recovered divergence between and within the two species during the late Pliocene and Pleistocene using an isolation with migration model. Also, our results indicate distinct levels of introgression among lineages. Ecological niche models and demographic inferences, used to infer range distributions throughout the late Quaternary, were not consistent with the hypothesis of a large river as a primary barrier responsible for the divergence of the two species. Instead, a scenario of isolation and divergence followed by geographic expansion and admixture as a consequence of Quaternary climatic oscillations was supported. Paleomodels also were not consistent with the idea that the hybrid zone originated in primary differentiation and favor a secondary contact scenario. Model fitting indicated that clines of different loci spanning the hybrid zone are coincident and concordant. The narrow cline for one Z-linked locus could be indicative of some form of post-zygotic selection hindering genetic homogenization between the two species.

Origin and cross-century dynamics of an avian hybrid zone

Background

Characterizations of the dynamics of hybrid zones in space and time can give insights about traits and processes important in population divergence and speciation. We characterized a hybrid zone between tanagers in the genus Ramphocelus (Aves, Thraupidae) located in southwestern Colombia. We evaluated whether this hybrid zone originated as a result of secondary contact or of primary differentiation, and described its dynamics across time using spatial analyses of molecular, morphological, and coloration data in combination with paleodistribution modeling.

Results

Models of potential historical distributions based on climatic data and genetic signatures of demographic expansion suggested that the hybrid zone likely originated following secondary contact between populations that expanded their ranges out of isolated areas in the Quaternary. Concordant patterns of variation in phenotypic characters across the hybrid zone and its narrow extent are suggestive of a tension zone, maintained by a balance between dispersal and selection against hybrids. Estimates of phenotypic cline parameters obtained using specimens collected over nearly a century revealed that, in recent decades, the zone appears to have moved to the east and to higher elevations, and may have become narrower. Genetic variation was not clearly structured along the hybrid zone, but comparisons between historical and contemporary specimens suggested that temporal changes in its genetic makeup may also have occurred.

Conclusions

Our data suggest that the hybrid zone likey resulted from secondary contact between populations. The observed changes in the hybrid zone may be a result of sexual selection, asymmetric gene flow, or environmental change.

Electronic supplementary material

The online version of this article (doi: 10.1186/s12862-017-1096-7) contains supplementary material, which is available to authorized users.

Environmental (in)dependence of a hybrid zone: Insights from molecular markers and ecological niche modeling in a hybrid zone of Origanum (Lamiaceae) on the island of Crete

The role of environment and the relative significance of endogenous versus exogenous selection in shaping hybrid zones have been crucial issues in the studies of hybridization. Recent advances in ecological niche modeling (ENM) offer new methodological tools, especially in combination with the genotyping of individuals in the hybrid zone. Here, we study the hybrid zone between the widely known spices Origanum onites and Origanum vulgare ssp. hirtum in Crete. We analyze the genetic structure of both parental taxa and their hybrid Origanum × intercendens using AFLP markers on 15 sympatric and 12 allopatric populations and employ ecological niche modeling and niche similarity tests to study their niche patterns. We complement these analyses with seed viability measurements. Our study revealed that the hybridizing taxa O. onites and O. vulgare ssp. hirtum and the resulting genotypic classes showed geographical and environmental niche similarities based on the predictions of ENMs and the subsequent similarity tests. The occurrence of the hybrid zone is not directly dependent on environmental factors which favor the fitness of the hybrid compared to the parental taxa, but rather on aspects such as historical factors and management practices, which may contribute to the localization and maintenance of the contact zone between parental species. Our results suggest that if a minimum required niche differentiation between genotypic classes is not achieved, environmental dependence might not have a prominent role on the outcome of the hybridization.

Variation in complex mating signals in an "island" hybrid zone between Stenobothrus grasshopper species

Two grasshopper species Stenobothrus rubicundus and S. clavatus were previously shown to meet in a narrow hybrid zone on Mount Tomaros in northern Greece. The species are remarkable for their complex courtship songs accompanied by conspicuous movements of antennae and wings. We analyzed variations in forewing morphology, antenna shape, and courtship song across the hybrid zone using a geographic information system, and we documented three contact zones on Mount Tomaros. All male traits and female wings show abrupt transitions across the contact zones, suggesting that these traits are driven by selection rather than by drift. Male clines in antennae are displaced toward S. clavatus, whereas all clines in wings are displaced toward S. rubicundus. We explain cline discordance as depending on sexual selection via female choice. The high covariance between wings and antennae found in the centers of all contact zones results from high levels of linkage disequilibria among the underlying loci, which in turn more likely results from assortative mating than from selection against hybrids. The covariance is found to be higher in clavatus-like than rubicundus-like populations, which implies asymmetric assortative mating in parental-like sites of the hybrid zone and a movement of the hybrid zone in favor of S. clavatus.

Ecological differentiation and habitat unsuitability maintaining a ground beetle hybrid zone

Exogenous selection via interactions between organisms and environments may influence the dynamics of hybrid zones between species in multiple ways. Two major models of a hybrid zone allowed us to hypothesize that environmental conditions influence hybrid zone dynamics in two ways. In the first model, an environmental gradient determines the mosaic distribution at the boundary between ecologically differentiated species (mosaic hybrid zone model). In the second model, a patch of unsuitable habitat traps a hybrid zone between species whose hybrids are unfit (tension zone model). To test these, we examined the environmental factors influencing the spatial structure of a hybrid zone between the ground beetles Carabus maiyasanus and C. iwawakianus using GIS‐based quantification of environmental factors and a statistical comparison of species distribution models (SDMs). We determined that both of the hypothetical processes can be important in the hybrid zone. We detected interspecific differences in the environmental factors in presence localities and their relative contribution in SDMs. SDMs were not identical between species even within contact areas, but tended to be similar within the range of each species. These results suggest an association between environments and species, and provide evidence that ecological differentiation between species plays a role in the maintenance of the hybrid zone. Contact areas were characterized by a relatively high temperature, low precipitation, and high topological wetness. Thus, the contact areas were regarded as being located in an unsuitable habitat with a drier climate, where those populations are likely to occur in patches with limited precipitation concentrated. A comparison of spatial scales suggests that exogenous selection via environmental factors may be weaker than endogenous selection via genitalic incompatibility.

Differential effects of climate and species interactions on range limits at a hybrid zone: potential direct and indirect impacts of climate change

The relative contributions of climate versus interspecific interactions in shaping species distributions have important implications for closely related species at contact zones. When hybridization occurs within a contact zone, these factors regulate hybrid zone location and movement. While a hybrid zone's position may depend on both climate and interactions between the hybridizing species, little is known about how these factors interact to affect hybrid zone dynamics. Here, we utilize SDM (species distribution modeling) both to characterize the factors affecting the current location of a moving North American avian hybrid zone and to predict potential direct and indirect effects of climate change on future distributions. We focus on two passerine species that hybridize where their ranges meet, the Black‐capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadee. Our contemporary climate models predict the occurrence of climatically suitable habitat extending beyond the hybrid zone for P. atricapillus only, suggesting that interspecific interactions primarily regulate this range boundary in P. atricapillus, while climatic factors regulate P. carolinensis. Year 2050 climate models predict a drastic northward shift in suitable habitat for P. carolinensis. Because of the greater importance of interspecific interactions for regulating the southern range limit of P. atricapillus, these climate‐mediated shifts in the distribution of P. carolinensis may indirectly lead to a range retraction in P. atricapillus. Together, our results highlight the ways climate change can both directly and indirectly affect species distributions and hybrid zone location. In addition, our study lends support to the longstanding hypothesis that abiotic factors regulate species' poleward range limits, while biotic factors shape equatorial range limits.

Genetic roadmap of the Arctic: plant dispersal highways, traffic barriers and capitals of diversity

We provide the first comparative multispecies analysis of spatial genetic structure and diversity in the circumpolar Arctic using a common strategy for sampling and genetic analyses. We aimed to identify and explain potential general patterns of genetic discontinuity/connectivity and diversity, and to compare our findings with previously published hypotheses. We collected and analyzed 7707 samples of 17 widespread arctic-alpine plant species for amplified fragment length polymorphisms (AFLPs). Genetic structure, diversity and distinctiveness were analyzed for each species, and extrapolated to cover the geographic range of each species. The resulting maps were overlaid to produce metamaps. The Arctic and Atlantic Oceans, the Greenlandic ice cap, the Urals, and lowland areas between southern mountain ranges and the Arctic were the strongest barriers against gene flow. Diversity was highest in Beringia and gradually decreased into formerly glaciated areas. The highest degrees of distinctiveness were observed in Siberia. We conclude that large-scale general patterns exist in the Arctic, shaped by the Pleistocene glaciations combined with long-standing physical barriers against gene flow. Beringia served as both refugium and source for interglacial (re)colonization, whereas areas further west in Siberia served as refugia, but less as sources for (re)colonization.

Species distribution models contribute to determine the effect of climate and interspecific interactions in moving hybrid zones

Climate is a major factor delimiting species' distributions. However, biotic interactions may also be prominent in shaping geographical ranges, especially for parapatric species forming hybrid zones. Determining the relative effect of each factor and their interaction of the contact zone location has been difficult due to the lack of broad scale environmental data. Recent developments in species distribution modelling (SDM) now allow disentangling the relative contributions of climate and species' interactions in hybrid zones and their responses to future climate change. We investigated the moving hybrid zone between the breeding ranges of two parapatric passerines in Europe. We conducted SDMs representing the climatic conditions during the breeding season. Our results show a large mismatch between the realized and potential distributions of the two species, suggesting that interspecific interactions, not climate, account for the present location of the contact zone. The SDM scenarios show that the southerly distributed species, Hippolais polyglotta, might lose large parts of its southern distribution under climate change, but a similar gain of novel habitat along the hybrid zone seems unlikely, because interactions with the other species (H. icterina) constrain its range expansion. Thus, whenever biotic interactions limit range expansion, species may become 'trapped' if range loss due to climate change is faster than the movement of the contact zone. An increasing number of moving hybrid zones are being reported, but the proximate causes of movement often remain unclear. In a global context of climate change, we call for more interest in their interactions with climate change.

Predicting the potential distribution ofVexillata(Nematoda: Ornithostrongylidae) and its hosts (Mammalia: Rodentia) within America

Species distribution modelling has been a powerful tool to explore the potential distribution of parasites in wildlife, being the basis of studies on biogeography.Vexillataspp. are intestinal nematodes found in several species of mammalian hosts, such as rodents (Geomyoidea) and hares (Leporidae) in the Nearctic and northern Neotropical regions. In the present study, we modelled the potential distribution ofVexillataspp. and their hosts, using exclusively species from the Geomyidae and Heteromyidae families, in order to identify their distributional patterns. Bioclimatic and topographic variables were used to identify and predict suitable habitats forVexillataand its hosts. Using these models, we identified that temperature seasonality is a significant environmental factor that influences the distribution of the parasite genus and its host. In particular, the geographical distribution is estimated to be larger than that predicted for its hosts. This suggests that the nematode has the potential to extend its geographical range and also its spectrum of host species. Increasing sample size and geographical coverage will contribute to recommendations for conservation of this host–parasite system.

The role of physical barriers in the location of avian suture zones in the Guiana Shield, northern Amazonia

Suture zones represent natural forums in which to examine the role of geography and ecology in the speciation process. Here, we conduct a comparative analysis designed to investigate the location of avian phylogeographic breaks and contact zones in the Guiana Shield, northern Amazonia. We use distributional and genetic data from 78 pairs of avian taxa to address whether phylogeographic breaks and contact zones are associated with contemporary landscape features. Using spatially explicit statistical models, we found that phylogeographic breaks and contact zones are not randomly distributed throughout the landscape. In general, geographic breaks cluster along physical barriers (rivers, nonforested habitats, and small mountain ranges), whereas contact zones aggregate where these barriers either break down or are easier to overcome, such as around rivers' headwaters. Our results indicate that although major Amazonian rivers are often key determinants of taxon boundaries, the "riverine barrier effect" is a synergistic consequence of the wide lower reaches of some rivers, coupled with nonriverine landscape features at the headwaters. Our data suggest that ancestral refugia are not necessary to explain current distribution patterns and that pairs of codistributed taxa do not seem to be the result of simultaneous diversification processes.

Distinct niche divergence characterizes the homoploid hybrid speciation of Pinus densata on the Tibetan plateau

Ecological divergence and selection for novel adaptations to new habitats have been theoretically proposed to play important roles in promoting homoploid hybrid speciation (HHS). The successful establishment of Pinus densata on the Tibetan Plateau is one of the few known examples of HHS. In this study, we carried out extensive field expeditions to obtain representative coverage of occurrence sites of P. densata and its two putative parents. We then applied a series of geographic information system-based analyses to define the patterns of environmental variation within and among the three pine species, to remove potentially confounding effects of spatial autocorrelation in the environmental data due to allopatric ranges, and to build species distribution models. All results consistently indicated that the ecological preferences of P. densata and its parental species have diverged, and they identified candidate ecological factors associated with habitat-specific adaptation. Projections from niche modeling indicated that P. densata could extend across a vast range along the parallel valley systems of the southeastern Tibetan Plateau. Our findings provide evidence of a distinct niche shift in P. densata and support the hypothesis that local adaptation and geographic isolation help maintain and reinforce between-species differences and reproductive isolation in the species complex.

Extreme environments select for reproductive assurance: evidence from evening primroses (Oenothera)

Competing evolutionary forces shape plant breeding systems (e.g. inbreeding depression, reproductive assurance). Which of these forces prevails in a given population or species is predicted to depend upon such factors as life history, ecological conditions, and geographical context. Here, we examined two such predictions: that self-compatibility should be associated with the annual life history or extreme climatic conditions. We analyzed data from a clade of plants remarkable for variation in breeding system, life history and climatic conditions (Oenothera, sections Anogra and Kleinia, Onagraceae). We used a phylogenetic comparative approach and Bayesian or hybrid Bayesian tests to account for phylogenetic uncertainty. Geographic information system (GIS)-based climate data and ecological niche modeling allowed us to quantify climatic conditions. Breeding system and reproductive life span are not correlated in Anogra and Kleinia. Instead, self-compatibility is associated with the extremes of temperature in the coldest part of the year and precipitation in the driest part of the year. In the 60 yr since this pattern was anticipated, this is the first demonstration of a relationship between the evolution of self-compatibility and climatic extremes. We discuss possible explanations for this pattern and possible implications with respect to anthropogenic climate change.

Mapping the suturing of a continental biota

The present-day spatial distribution of interspecific contact zones and intraspecific phylogeographical breaks provides a window into the past ecological and evolutionary processes that underlie speciation and species ranges. The clustering of contact zones and/or phylogeographical breaks in space indicates the suturing of diverged biotas. The presence of such suture zones indicates that similar ecological and historical factors have influenced the past and present distributions of populations and their divergence. Thus, suture zones are ideal natural laboratories for studying divergence, secondary contact and speciation across many different taxa. The concept of suture zones was formalized decades ago by Remington (1968), but only a few detailed and quantitative investigations of suture zones exist (Swenson & Howard 2004, 2005; Whinnett et al. 2005; Moritz et al. 2009). This limited number of investigations is largely because of a lack of detailed geographical data and sophisticated analytical tools. In this issue of Molecular Ecology, Rissler & Smith (2010) have accomplished a detailed investigation into the suturing of amphibian lineages in the United States which uses both detailed geographical data and sophisticated analytical methods. The work greatly enhances our knowledge of suture zones by extending previous work that has focused less on amphibians and by explicitly considering the relationship between species richness and suture zones.

Patterns of differential introgression in a salamander hybrid zone: inferences from genetic data and ecological niche modelling

Hybrid zones have yielded considerable insight into many evolutionary processes, including speciation and the maintenance of species boundaries. Presented here are analyses from a hybrid zone that occurs among three salamanders -Plethodon jordani, Plethodon metcalfi and Plethodon teyahalee- from the southern Appalachian Mountains. Using a novel statistical approach for analysis of non-clinal, multispecies hybrid zones, we examined spatial patterns of variation at four markers: single-nucleotide polymorphisms (SNPs) located in the mtDNA ND2 gene and the nuclear DNA ILF3 gene, and the morphological markers of red cheek pigmentation and white flecks. Concordance of the ILF3 marker and both morphological markers across four transects is observed. In three of the four transects, however, the pattern of mtDNA is discordant from all other markers, with a higher representation of P. metcalfi mtDNA in the northern and lower elevation localities than is expected given the ILF3 marker and morphology. To explore whether climate plays a role in the position of the hybrid zone, we created ecological niche models for P. jordani and P. metcalfi. Modelling results suggest that hybrid zone position is not determined by steep gradients in climatic suitability for either species. Instead, the hybrid zone lies in a climatically homogenous region that is broadly suitable for both P. jordani and P. metcalfi. We discuss various selective (natural selection associated with climate) and behavioural processes (sex-biased dispersal, asymmetric reproductive isolation) that might explain the discordance in the extent to which mtDNA and nuclear DNA and colour-pattern traits have moved across this hybrid zone.

Ecological niche modelling as an exploratory tool for identifying species limits: an example based on Mexican muroid rodents

Niche conservatism theory suggests that recently diverged sister species share the same ecological niche. However, if the ecological niche evolves as part of the speciation process, the ecological pattern could be useful for recognizing cryptic species. In a broad sense systematists agree that the niche characters could be used for species differentiation. However, to date such characters have been ignored. We used the genetic algorithm for rule-set production for modelling the ecological niche as a means of inferring ecological divergence in allopatric populations of muroid rodents for which taxonomic identity is uncertain. Our results show that niche differentiation is significant in most of the identified phylogroups. The differentiation is likely associated with natural evolutionary units, which can be identified by applying species concepts based on phylogenetic and ecological patterns (e.g. phylogenetic, cohesive, evolutionary). Even so, the role of the niche partition within phylogenetic reconstruction may be a limited one.

Divergence in an obligate mutualism is not explained by divergent climatic factors

Adaptation to divergent environments creates and maintains biological diversity, but we know little about the importance of different agents of ecological divergence. Coevolution in obligate mutualisms has been hypothesized to drive divergence, but this contention has rarely been tested against alternative ecological explanations. Here, we use a well-established example of coevolution in an obligate pollination mutualism, Yucca brevifolia and its two pollinating yucca moths, to test the hypothesis that divergence in this system is the result of mutualists adapting to different abiotic environments as opposed to coevolution between mutualists. We used a combination of principal component analyses and ecological niche modeling to determine whether varieties of Y. brevifolia associated with different pollinators specialize on different environments. Yucca brevifolia occupies a diverse range of climates. When the two varieties can disperse to similar environments, they occupy similar habitats. This suggests that the two varieties have not specialized on distinct habitats. In turn, this suggests that nonclimatic factors, such as the biotic interaction between Y. brevifolia and its pollinators, are responsible for evolutionary divergence in this system.

Modeling environmentally associated morphological and genetic variation in a rainforest bird, and its application to conservation prioritization

To better understand how environment shapes phenotypic and genetic variation, we explore the relationship between environmental variables across Ecuador and genetic and morphological variation in the wedge-billed woodcreeper (Glyphorynchus spirurus), a common Neotropical rainforest bird species. Generalized dissimilarity models show that variation in amplified fragment length polymorphism markers was strongly associated with environmental variables on both sides of the Andes, but could also partially be explained by geographic distance on the western side of the Andes. Tarsus, wing, tail, and bill lengths and bill depth were well explained by environmental variables on the western side of the Andes, whereas only tarsus length was well explained on the eastern side. Regions that comprise the highest rates of genetic and phenotypic change occur along steep elevation gradients in the Andes. Such environmental gradients are likely to be particularly important for maximizing adaptive diversity to minimize the impacts of climate change. Using a framework for conservation prioritization based on preserving ecological and evolutionary processes, we found little overlap between currently protected areas in Ecuador and regions we predicted to be important in maximizing adaptive variation.

Identification and dynamics of a cryptic suture zone in tropical rainforest

Suture zones, shared regions of secondary contact between long-isolated lineages, are natural laboratories for studying divergence and speciation. For tropical rainforest, the existence of suture zones and their significance for speciation has been controversial. Using comparative phylogeographic evidence, we locate a morphologically cryptic suture zone in the Australian Wet Tropics rainforest. Fourteen out of 18 contacts involve morphologically cryptic phylogeographic lineages, with mtDNA sequence divergences ranging from 2 to 15 per cent. Contact zones are significantly clustered in a suture zone located between two major Quaternary refugia. Within this area, there is a trend for secondary contacts to occur in regions with low environmental suitability relative to both adjacent refugia and, by inference, the parental lineages. The extent and form of reproductive isolation among interacting lineages varies across species, ranging from random admixture to speciation, in one case via reinforcement. Comparative phylogeographic studies, combined with environmental analysis at a fine-scale and across varying climates, can generate new insights into suture zone formation and to diversification processes in species-rich tropical rainforests. As arenas for evolutionary experimentation, suture zones merit special attention for conservation.