A simple dynamic model explains the diversity of island birds worldwide.

Colonization, speciation and extinction are dynamic processes that influence global patterns of species richness1-6. Island biogeography theory predicts that the contribution of these processes to the accumulation of species diversity depends on the area and isolation of the island7,8. Notably, there has been no robust global test of this prediction for islands where speciation cannot be ignored9, because neither the appropriate data nor the analytical tools have been available. Here we address both deficiencies to reveal, for island birds, the empirical shape of the general relationships that determine how colonization, extinction and speciation rates co-vary with the area and isolation of islands. We compiled a global molecular phylogenetic dataset of birds on islands, based on the terrestrial avifaunas of 41 oceanic archipelagos worldwide (including 596 avian taxa), and applied a new analysis method to estimate the sensitivity of island-specific rates of colonization, speciation and extinction to island features (area and isolation). Our model predicts-with high explanatory power-several global relationships. We found a decline in colonization with isolation, a decline in extinction with area and an increase in speciation with area and isolation. Combining the theoretical foundations of island biogeography7,8 with the temporal information contained in molecular phylogenies10 proves a powerful approach to reveal the fundamental relationships that govern variation in biodiversity across the planet.

Demographic responses of oceanic island birds to local and regional ecological disruptions revealed by whole-genome sequencing.

Disentangling the effects of ecological disruptions operating at different spatial and temporal scales in shaping past species' demography is particularly important in the current context of rapid environmental changes driven by both local and regional factors. We argue that volcanic oceanic islands provide useful settings to study the influence of past ecological disruptions operating at local and regional scales on population demographic histories. We investigate potential drivers of past population dynamics for three closely related species of passerine birds from two volcanic oceanic islands, Reunion and Mauritius (Mascarene archipelago), with distinct volcanic history. Using ABC and PSMC inferences from complete genomes, we reconstructed the demographic history of the Reunion Grey White-eye (Zosterops borbonicus (Pennant, 1781)), the Reunion Olive White-eye (Z. olivaceus (Linnaeus, 1766)) and the Mauritius Grey White-eye (Z. mauritianus (Gmelin, 1789)) and searched for possible causes underlying similarities or differences between species living on the same or different islands. Both demographic inferences strongly support ancient and long-term expansions in all species. They also reveal different trajectories between species inhabiting different islands, but consistent demographic trajectories in species or populations from the same island. Species from Reunion appear to have experienced synchronous reductions in population size during the Last Glacial Maximum, a trend not seen in the Mauritian species. Overall, this study suggests that local events may have played a role in shaping population trajectories of these island species. It also highlights the potential of our conceptual framework to disentangle the effects of local and regional drivers on past species' demography and long-term population processes.

Diversification of the terrestrial frog genus Anomaloglossus (Anura, Aromobatidae) in the Guiana Shield proceeded from highlands to lowlands, with successive loss and reacquisition of endotrophy.

Two main landscapes emerge from the Guiana Shield: the highlands to the west called the Pantepui region and the Amazonian lowlands to the east, both harbouring numerous endemic species. With 32 currently recognized species, the genus Anomaloglossus stands out among Neotropical frogs as one that diversified only within the Guiana Shield both in the highlands and the lowlands. We present a time-calibrated phylogeny obtained by using combined mitogenomic and nuclear DNA, which suggests that the genus originates from Pantepui where extant lineages started diversifying around 21 Ma, and subsequently (ca. 17 Ma) dispersed during the Miocene Climatic Optimum to the lowlands of the eastern Guiana Shield where the ability to produce endotrophic tadpoles evolved. Further diversification within the lowlands in the A. stepheni group notably led to an evolutionary reversal toward exotrophy in one species group during the late Miocene, followed by reacquisition of endotrophy during the Pleistocene. These successive shifts of reproductive mode seem to have accompanied climatic oscillations. Long dry periods might have triggered evolution of exotrophy, whereas wetter climates favoured endotrophic forms, enabling colonization of terrestrial habitats distant from water. Acquisition, loss, and reacquisition of endotrophy makes Anomaloglossus unique among frogs and may largely explain the current species diversity. The micro evolutionary processes involved in these rapid shifts of reproductive mode remain to be revealed.

Collective and harmonized high throughput barcoding of insular arthropod biodiversity: Toward a Genomic Observatories Network for islands.

Current understanding of ecological and evolutionary processes underlying island biodiversity is heavily shaped by empirical data from plants and birds, although arthropods comprise the overwhelming majority of known animal species, and as such can provide key insights into processes governing biodiversity. Novel high throughput sequencing (HTS) approaches are now emerging as powerful tools to overcome limitations in the availability of arthropod biodiversity data, and hence provide insights into these processes. Here, we explored how these tools might be most effectively exploited for comprehensive and comparable inventory and monitoring of insular arthropod biodiversity. We first reviewed the strengths, limitations and potential synergies among existing approaches of high throughput barcode sequencing. We considered how this could be complemented with deep learning approaches applied to image analysis to study arthropod biodiversity. We then explored how these approaches could be implemented within the framework of an island Genomic Observatories Network (iGON) for the advancement of fundamental and applied understanding of island biodiversity. To this end, we identified seven island biology themes at the interface of ecology, evolution and conservation biology, within which collective and harmonized efforts in HTS arthropod inventory could yield significant advances in island biodiversity research.

Beyond morphs: Inter-individual colour variation despite strong genetic determinism of colour morphs in a wild bird.

Categorizing individuals into discrete forms in colour polymorphic species can overlook more subtle patterns in coloration that can be of functional significance. Thus, quantifying inter-individual variation in these species at both within- and between-morph levels is critical to understand the evolution of colour polymorphisms. Here we present analyses of inter-individual colour variation in the Reunion grey white-eye (Zosterops borbonicus), a colour polymorphic wild bird endemic to the island of Reunion in which all highland populations contain two sympatric colour morphs, with birds showing predominantly grey or brown plumage, respectively. We first quantified colour variation across multiple body areas by using a continuous plumage colour score to assess variation in brown-grey coloration as well as smaller scale variation in light patches. To examine the possible causes of among-individual variation, we tested if colour variation in plumage component elements could be explained by genotypes at two markers near a major-effect locus previously related to back coloration in this species, and by other factors such as age, sex and body condition. Overall, grey-brown coloration was largely determined by genetic factors and was best described by three distinct clusters that were associated to genotypic classes (homozygotes and heterozygote), with no effect of age or sex, whereas variation in smaller light patches was primarily related to age and sex. Our results highlight the importance of characterizing subtle plumage variation beyond morph categories that are readily observable since multiple patterns of colour variation may be driven by different mechanisms, have different functions and will likely respond in different ways to selection.

Within-island diversification in a passerine bird.

The presence of congeneric taxa on the same island suggests the possibility of in situ divergence, but can also result from multiple colonizations of previously diverged lineages. Here, using genome-wide data from a large population sample, we test the hypothesis that intra-island divergence explains the occurrence of four geographical forms meeting at hybrid zones in the Reunion grey white-eye (Zosterops borbonicus), a species complex endemic to the small volcanic island of Reunion. Using population genomic and phylogenetic analyses, we reconstructed the population history of the different forms. We confirmed the monophyly of the complex and found that one of the lowland forms is paraphyletic and basal relative to others, a pattern highly consistent with in situ divergence. Our results suggest initial colonization of the island through the lowlands, followed by expansion into the highlands, which led to the evolution of a distinct geographical form, genetically and ecologically different from the lowland ones. Lowland forms seem to have experienced periods of geographical isolation, but they diverged from one another by sexual selection rather than niche change. Overall, low dispersal capabilities in this island bird combined with both geographical and ecological opportunities seem to explain how divergence occurred at such a small spatial scale.

Differential divergence in autosomes and sex chromosomes is associated with intra-island diversification at a very small spatial scale in a songbird lineage.

Recently diverged taxa showing marked phenotypic and ecological diversity provide optimal systems to understand the genetic processes underlying speciation. We used genome-wide markers to investigate the diversification of the Reunion grey white-eye (Zosterops borbonicus) on the small volcanic island of Reunion (Mascarene archipelago), where this species complex exhibits four geographical forms that are parapatrically distributed across the island and differ strikingly in plumage colour. One form restricted to the highlands is separated by a steep ecological gradient from three distinct lowland forms which meet at narrow hybrid zones that are not associated with environmental variables. Analyses of genomic variation based on single nucleotide polymorphism data from genotyping-by-sequencing and pooled RAD-seq approaches show that signatures of selection associated with elevation can be found at multiple regions across the genome, whereas most loci associated with the lowland forms are located on the Z sex chromosome. We identified TYRP1, a Z-linked colour gene, as a likely candidate locus underlying colour variation among lowland forms. Tests of demographic models revealed that highland and lowland forms diverged in the presence of gene flow, and divergence has progressed as gene flow was restricted by selection at loci across the genome. This system holds promise for investigating how adaptation and reproductive isolation shape the genomic landscape of divergence at multiple stages of the speciation process.

Recovering the evolutionary history of crowned pigeons (Columbidae: Goura): Implications for the biogeography and conservation of New Guinean lowland birds.

Assessing the relative contributions of immigration and diversification into the buildup of species diversity is key to understanding the role of historical processes in driving biogeographical and diversification patterns in species-rich regions. Here, we investigated how colonization, in situ speciation, and extinction history may have generated the present-day distribution and diversity of Goura crowned pigeons (Columbidae), a group of large forest-dwelling pigeons comprising four recognized species that are all endemic to New Guinea. We used a comprehensive geographical and taxonomic sampling based mostly on historical museum samples, and shallow shotgun sequencing, to generate complete mitogenomes, nuclear ribosomal clusters and independent nuclear conserved DNA elements. We used these datasets independently to reconstruct molecular phylogenies. Divergence time estimates were obtained using mitochondrial data only. All analyses revealed similar genetic divisions within the genus Goura and recovered as monophyletic groups the four species currently recognized, providing support for recent taxonomic changes based on differences in plumage characters. These four species are grouped into two pairs of strongly supported sister species, which were previously not recognized as close relatives: Goura sclaterii with Goura cristata, and Goura victoria with Goura scheepmakeri. While the geographical origin of the Goura lineage remains elusive, the crown age of 5.73 Ma is consistent with present-day species diversity being the result of a recent diversification within New Guinea. Although the orogeny of New Guinea's central cordillera must have played a role in driving diversification in Goura, cross-barrier dispersal seems more likely than vicariance to explain the speciation events having led to the four current species. Our results also have important conservation implications. Future assessments of the conservation status of Goura species should consider threat levels following the taxonomic revision proposed by del Hoyo and Collar (HBW and BirdLife International illustrated checklist of the birds of the world 1: non-passerines, 2014), which we show to be fully supported by genomic data. In particular, distinguishing G. sclaterii from G. scheepmakeri seems to be particularly relevant.

MtDNA metagenomics reveals large-scale invasion of belowground arthropod communities by introduced species.

Using a series of standardized sampling plots within forest ecosystems in remote oceanic islands, we reveal fundamental differences between the structuring of aboveground and belowground arthropod biodiversity that are likely due to large-scale species introductions by humans. Species of beetle and spider were sampled almost exclusively from single islands, while soil-dwelling Collembola exhibited more than tenfold higher species sharing among islands. Comparison of Collembola mitochondrial metagenomic data to a database of more than 80 000 Collembola barcode sequences revealed almost 30% of sampled island species are genetically identical, or near identical, to individuals sampled from often very distant geographic regions of the world. Patterns of mtDNA relatedness among Collembola implicate human-mediated species introductions, with minimum estimates for the proportion of introduced species on the sampled islands ranging from 45% to 88%. Our results call for more attention to soil mesofauna to understand the global extent and ecological consequences of species introductions.

Cryptic diversity in Amazonian frogs: Integrative taxonomy of the genus Anomaloglossus (Amphibia: Anura: Aromobatidae) reveals a unique case of diversification within the Guiana Shield.

Lack of resolution on species boundaries and distribution can hamper inferences in many fields of biology, notably biogeography and conservation biology. This is particularly true in megadiverse and under-surveyed regions such as Amazonia, where species richness remains vastly underestimated. Integrative approaches using a combination of phenotypic and molecular evidence have proved extremely successful in reducing knowledge gaps in species boundaries, especially in animal groups displaying high levels of cryptic diversity like amphibians. Here we combine molecular data (mitochondrial 16S rRNA and nuclear TYR, POMC, and RAG1) from 522 specimens of Anomaloglossus, a frog genus endemic to the Guiana Shield, including 16 of the 26 nominal species, with morphometrics, bioacoustics, tadpole development mode, and habitat use to evaluate species delineation in two lowlands species groups. Molecular data reveal the existence of 18 major mtDNA lineages among which only six correspond to described species. Combined with other lines of evidence, we confirm the existence of at least 12 Anomaloglossus species in the Guiana Shield lowlands. Anomaloglossus appears to be the only amphibian genus to have largely diversified within the eastern part of the Guiana Shield. Our results also reveal strikingly different phenotypic evolution among lineages. Within the A. degranvillei group, one subclade displays acoustic and morphological conservatism, while the second subclade displays less molecular divergence but clear phenotypic divergence. In the A. stepheni species group, a complex evolutionary diversification in tadpole development is observed, notably with two closely related lineages each displaying exotrophic and endotrophic tadpoles.

Colour polymorphism is associated with lower extinction risk in birds.

Colour polymorphisms have played a major role in enhancing current understanding of how selection and demography can impact phenotypes. Because different morphs often display alternative strategies and exploit alternative ecological niches, colour polymorphism can be expected to promote adaptability to environmental changes. However, whether and how it could influence populations' and species' response to global changes remains debated. To address this question, we built an up-to-date and complete database on avian colour polymorphism based on the examination of available data from all 10,394 extant bird species. We distinguished between true polymorphism (where different genetically determined morphs co-occur in sympatry within the same population) and geographic variation (parapatric or allopatric colour variation), because these two patterns of variation are expected to have different consequences on populations' persistence. Using the IUCN red list, we then showed that polymorphic bird species are at lesser risk of extinction than nonpolymorphic ones, after controlling for a range of factors such as geographic range size, habitat breadth, life history, and phylogeny. This appears consistent with the idea that high genetic diversity and/or the existence of alternative strategies in polymorphic species promotes the ability to adaptively respond to changing environmental conditions. In contrast, polymorphic species were not less vulnerable than nonpolymorphic ones to specific drivers of extinction such as habitat alteration, direct exploitation, climate change, and invasive species. Thus, our results suggest that colour polymorphism acts as a buffer against environmental changes, although further studies are now needed to understand the underlying mechanisms. Developing accurate quantitative indices of sensitivity to specific threats is likely a key step towards a better understanding of species response to environmental changes.

Candidate Gene Analysis Suggests Untapped Genetic Complexity in Melanin-Based Pigmentation in Birds.

Studies on melanin-based color variation in a context of natural selection have provided a wealth of information on the link between phenotypic and genetic variation. Here, we evaluated associations between melanic plumage patterns and genetic polymorphism in the Réunion grey white-eye (Zosterops borbonicus), a species in which mutations on MC1R do not seem to play any role in explaining melanic variation. This species exhibits 5 plumage color variants that can be grouped into 3 color forms which occupy discrete geographic regions in the lowlands of Réunion, and a fourth high-elevation form which comprises 2 color morphs (grey and brown) and represents a true color polymorphism. We conducted a comprehensive survey of sequence variation in 96 individuals at a series of 7 candidate genes other than MC1R that have been previously shown to influence melanin-based color patterns in vertebrates, including genes that have rarely been studied in a wild bird species before: POMC, Agouti, TYR, TYRP1, DCT, Corin, and SLC24A5 Of these 7 genes, 2 (Corin and TYRP1) displayed an interesting shift in allele frequencies between lowland and highland forms and a departure from mutation-drift equilibrium consistent with balancing selection in the polymorphic highland form only. Sequence variation at Agouti, a gene frequently involved in melanin-based pigmentation patterning, was not associated with color forms or morphs. Thus, we suggest that functionally important changes in loci other than those classically studied are involved in the color polymorphism exhibited by the Réunion grey white-eye and possibly many other nonmodel species.

Islands as model systems in ecology and evolution: prospects fifty years after MacArthur-Wilson.

The study of islands as model systems has played an important role in the development of evolutionary and ecological theory. The 50th anniversary of MacArthur and Wilson's (December 1963) article, 'An equilibrium theory of insular zoogeography', was a recent milestone for this theme. Since 1963, island systems have provided new insights into the formation of ecological communities. Here, building on such developments, we highlight prospects for research on islands to improve our understanding of the ecology and evolution of communities in general. Throughout, we emphasise how attributes of islands combine to provide unusual research opportunities, the implications of which stretch far beyond islands. Molecular tools and increasing data acquisition now permit re-assessment of some fundamental issues that interested MacArthur and Wilson. These include the formation of ecological networks, species abundance distributions, and the contribution of evolution to community assembly. We also extend our prospects to other fields of ecology and evolution - understanding ecosystem functioning, speciation and diversification - frequently employing assets of oceanic islands in inferring the geographic area within which evolution has occurred, and potential barriers to gene flow. Although island-based theory is continually being enriched, incorporating non-equilibrium dynamics is identified as a major challenge for the future.

Multihost experimental evolution of the pathogen Ralstonia solanacearum unveils genes involved in adaptation to plants.

Ralstonia solanacearum, the causal agent of a lethal bacterial wilt plant disease, infects an unusually wide range of hosts. These hosts can further be split into plants where R. solanacearum is known to cause disease (original hosts) and those where this bacterium can grow asymptomatically (distant hosts). Moreover, this pathogen is able to adapt to many plants as supported by field observations reporting emergence of strains with enlarged pathogenic properties. To investigate the genetic bases of host adaptation, we conducted evolution experiments by serial passages of a single clone of the pathogen on three original and two distant hosts over 300 bacterial generations and then analyzed the whole-genome of nine evolved clones. Phenotypic analysis of the evolved clones showed that the pathogen can increase its fitness on both original and distant hosts although the magnitude of fitness increase was greater on distant hosts. Only few genomic modifications were detected in evolved clones compared with the ancestor but parallel evolutionary changes in two genes were observed in independent evolved populations. Independent mutations in the regulatory gene efpR were selected for in three populations evolved on beans, a distant host. Reverse genetic approaches confirmed that these mutations were associated with fitness gain on bean plants. This work provides a first step toward understanding the within-host evolutionary dynamics of R. solanacearum during infection and identifying bacterial genes subjected to in planta selection. The discovery of EfpR as a determinant conditioning host adaptation of the pathogen illustrates how experimental evolution coupled with whole-genome sequencing is a potent tool to identify novel molecular players involved in central life-history traits.

The evolutionary history of Antirrhinum in the Pyrenees inferred from phylogeographic analyses.

BACKGROUND: The origin and colonisation history after the Quaternary ice ages remain largely unresolved for many plant lineages, mainly owing to a lack of fine-scale studies. Here, we present a molecular phylogeny and a phylogeographic analysis of Antirrhinum, an important model system in plant biology, in the Pyrenees range. Our goal was to reconstruct the evolutionary and colonisation history of four taxa endemic to this region (A. majus subsp. majus, A. majus. subsp. striatum, A. molle, and A. sempervirens) by using a dense sampling strategy, with a total of 452 individuals from 99 populations whose collective distribution spans nearly the entirety of the Pyrenees and adjacent mountains. RESULTS: Phylogenetic and phylogeographic analyses of the sequences of two plastid (trnS-trnG and trnK-matK) regions revealed the following: (i) historical relationship between the Pyrenees and Iberia (but not with the Alps); (ii) the long persistence of populations in the Pyrenees, at least since the Late Pleistocene; (iii) three different colonisation histories for populations from the Western, Central, and Eastern Pyrenees; (iv) the deep phylogeographic separation of the eastern and western populations; and (v) the colonisation of southern France from the Eastern Pyrenees. CONCLUSIONS: The present study underlines the enormous influence of the glacial history of the mountain ranges on the current configuration of intra- and inter-specific genetic diversity in Antirrhinum, as well as the importance of periglacial areas for the survival of species during glacial periods of the Quaternary.

Timing and number of colonizations but not diversification rates affect diversity patterns in hemosporidian lineages on a remote oceanic archipelago.

Parasite diversity on remote oceanic archipelagos is determined by the number and timing of colonizations and by in situ diversification rate. In this study, we compare intra-archipelago diversity of two hemosporidian parasite genera, Plasmodium and Leucocytozoon, infecting birds of the Mascarene archipelago. Despite the generally higher vagility of Plasmodium parasites, we report a diversity of Plasmodium cytochrome b haplotypes in the archipelago much lower than that of Leucocytozoon. Using phylogenetic data, we find that this difference in diversity is consistent with differences in the timing and number of colonizations, while rates of diversification do not vary significantly between the two genera. The prominence of immigration history in explaining current diversity patterns highlights the importance of historical contingencies in driving disparate biogeographic patterns in potentially harmful blood parasites infecting island birds.

The role of immigration and in-situ radiation in explaining blood parasite assemblages in an island bird clade.

Parasite communities on islands are assembled through multiple immigrations and/or in-situ diversification. In this study, we used a phylogenetic approach to investigate the role of such processes in shaping current patterns of diversity in Leucocytozoon, a group of haemosporidian blood parasites infecting whites eyes (Zosterops) endemic to the Mascarene archipelago (south-western Indian Ocean). We found that this parasite community arose through a combination of multiple immigrations and in-situ diversification, highlighting the importance of both processes in explaining island diversity. Specifically, two highly diverse parasite clades appear to have been present in the Mascarenes for most of their evolutionary history and have diversified within the archipelago, while another lineage apparently immigrated more recently, probably with human-introduced birds. Interestingly, the evolutionary histories of one clade of parasites and Indian Ocean Zosterops seem tightly associated with a significant signal for phylogenetic congruence, suggesting that host-parasite co-divergence may have occurred in this system.

Island songbirds as windows into evolution in small populations.

Due to their limited ranges and inherent isolation, island species have long been recognized as crucial systems for tackling a range of evolutionary questions, including in the early study of speciation.1,2 Such species have been less studied in the understanding of the evolutionary forces driving DNA sequence evolution. Island species usually have lower census population sizes (N) than continental species and, supposedly, lower effective population sizes (Ne). Given that both the rates of change caused by genetic drift and by selection are dependent upon Ne, island species are theoretically expected to exhibit (1) lower genetic diversity, (2) less effective natural selection against slightly deleterious mutations,3,4 and (3) a lower rate of adaptive evolution.5-8 Here, we have used a large set of newly sequenced and published whole-genome sequences of Passerida species (14 insular and 11 continental) to test these predictions. We confirm that island species exhibit lower census size and Ne, supporting the hypothesis that the smaller area available on islands constrains the upper bound of Ne. In the insular species, we find lower nucleotide diversity in coding regions, higher ratios of non-synonymous to synonymous polymorphisms, and lower adaptive substitution rates. Our results provide robust evidence that the lower Ne experienced by island species has affected both the ability of natural selection to efficiently remove weakly deleterious mutations and also the adaptive potential of island species, therefore providing considerable empirical support for the nearly neutral theory. We discuss the implications for both evolutionary and conservation biology.

Exploring the vertebrate fauna of the Bird's Head Peninsula (Indonesia, West Papua) through DNA barcodes.

Biodiversity knowledge is widely heterogeneous across the Earth's biomes. Some areas, due to their remoteness and difficult access, present large taxonomic knowledge gaps. Mostly located in the tropics, these areas have frequently experienced a fast development of anthropogenic activities during the last decades and are therefore of high conservation concerns. The biodiversity hotspots of Southeast Asia exemplify the stakes faced by tropical countries. While the hotspots of Sundaland (Java, Sumatra, Borneo) and Wallacea (Sulawesi, Moluccas) have long attracted the attention of biologists and conservationists alike, extensive parts of the Sahul area, in particular the island of New Guinea, have been much less explored biologically. Here, we describe the results of a DNA-based inventory of aquatic and terrestrial vertebrate communities, which was the objective of a multidisciplinary expedition to the Bird's Head Peninsula (West Papua, Indonesia) conducted between 17 October and 20 November 2014. This expedition resulted in the assembly of 1005 vertebrate DNA barcodes. Based on the use of multiple species-delimitation methods (GMYC, PTP, RESL, ABGD), 264 molecular operational taxonomic units (MOTUs) were delineated, among which 75 were unidentified and an additional 48 were considered cryptic. This study suggests that the diversity of vertebrates of the Bird's Head is severely underestimated and considerations on the evolutionary origin and taxonomic knowledge of these biotas are discussed.

The geographic scale of diversification on islands: genetic and morphological divergence at a very small spatial scale in the Mascarene grey white-eye (Aves: Zosterops borbonicus).

BACKGROUND: Oceanic islands provide unique scenarios for studying the roles of geography and ecology in driving population divergence and speciation. Assessing the relative importance of selective and neutral factors in driving population divergence is central to understanding how such divergence may lead to speciation in small oceanic islands, where opportunities for gene flow and population mixing are potentially high. Here we report a case of genetic and morphological structure in the Mascarene grey white-eye (Zosterops borbonicus) a species that shows a striking, geographically structured plumage polymorphism on the topographically and ecologically complex island of Réunion, yet is monotypic on the relatively uniform neighbouring island of Mauritius. RESULTS: Analysis of 276 AFLP loci in 197 individuals revealed prolonged independent evolution of Réunion and Mauritius populations, which is congruent with previous mtDNA assessments. Furthermore, populations on Réunion showed significant differentiation into three main genetic groups separating lowland from highland areas despite the small geographic distances involved. Genetic differentiation along the altitudinal gradient is consistent with morphometric analysis of fitness-related traits. Birds in the highlands were larger, yet had relatively smaller beaks than in the lowlands, suggesting the role of selection in shaping morphology and restricting gene flow along the gradient. No genetic differentiation between plumage morphs was detected in neutral markers, suggesting that plumage differences are of recent origin. CONCLUSIONS: Our results suggest a dual role of vicariance and natural selection in differentiating populations of a passerine bird in an oceanic island at very small spatial scales. We propose a combination of past microallopatry driven by volcanic activity and selection-constrained dispersal along steep ecological gradients to explain the striking levels of population structure found within the island, although the possibility that genetic differences evolved in situ along the gradient cannot be ruled out at present. The lack of congruence between genetic groups and plumage morphs suggests that the latter are of recent origin and likely due to social or sexual selection acting on few loci. The presence of sharp and stable contact zones between plumage morphs suggests that they could be on independent evolutionary trajectories, yet whether or not they represent incipient species will require further research to directly assess the degree of reproductive isolation among them.