Molecular phylogenetics and diversification of the genus Sporophila (Aves: Passeriformes)

The evolutionary affinities within and among many groups of nine-primaried oscines remain unresolved. One such group is Sporophila, a large genus of New World tanager-finches. Our study focused particularly on clarifying the relationship between this genus and a closely related one, Oryzoborus, and on examining the phylogenetic affinities of the "capuchinos," a group of 11 Sporophila species that share a similar male plumage coloration pattern. Our phylogenetic analyses, based on 498 bp of mitochondrial DNA sequence, indicated that: (1) Oryzoborus is embedded within a well-supported clade containing all Sporophila species, which strongly suggests that both genera should be merged, (2) the species of capuchinos comprise a monophyletic group, implying that the plumage patterns common to all probably arose only once, and (3) the capuchinos clade is comprised of two sub-clades, one including two species that are distributed in northern South America and the other one containing eight species that are present south of the Amazon River. Mean sequence divergence among the southern capuchinos species was extremely low, suggesting a rapid radiation within the last half-million years that may be related to the high level of sexual selection present in the genus and might have been promoted by marine ingressions and egressions that occurred in some southern coastal regions of South America in the Late Pleistocene.     

Miocene and Pliocene dominated diversification of the lichen-forming fungal genus Melanohalea (Parmeliaceae, Ascomycota) and Pleistocene population expansions

ABSTRACT: BACKGROUND: Factors promoting diversification in lichen symbioses remain largely unexplored. While Pleistocene events have been important for driving diversification and affecting distributions in many groups, recent estimates suggest that major radiations within some genera in the largest clade of macrolichens (Parmeliaceae, Ascomycota) vastly predate the Pleistocene. To better understand the temporal placement and sequence of diversification events in lichens, we estimated divergence times in a common lichen-forming fungal genus, Melanohalea, in the Northern Hemisphere. Divergence times were estimated using both concatenated gene tree and coalescence-based multilocus species tree approaches to assess the temporal context of major radiation events within Melanohalea. In order to complement our understanding of processes impacting genetic differentiation, we also evaluated the effects of Pleistocene glacial cycles on population demographics of distinct Melanohalea lineages, differing in reproductive strategies. RESULTS: We found that divergence estimates, from both concatenated gene tree and coalescence-based multilocus species tree approaches, suggest that diversification within Melanohalea occurred predominantly during the Miocene and Pliocene, although estimated of divergence times differed by up to 8.3 million years between the two methods. These results indicate that, in some cases, taxonomically diagnostic characters may be maintained among divergent lineages for millions of years. In other cases, similar phenotypic characters among non-sister taxa, including reproductive strategies, suggest the potential for convergent evolution due to similar selective pressures among distinct lineages. Our analyses provide evidence of population expansions predating the last glacial maximum in the sampled lineages. These results suggest that Pleistocene glaciations were not inherently unfavorable or restrictive for some Melanohalea species, albeit with apparently different demographic histories between sexually and vegetatively reproducing lineages. CONCLUSIONS: Our results contribute to the understanding of how major changes during the Miocene and Pliocene have been important in promoting diversification within common lichen-forming fungi in the northern Hemisphere. Additionally, we provide evidence that glacial oscillations have influenced current population structure of broadly distributed lichenized fungal species throughout the Holarctic.     

Genetic divergence and migration patterns in a North American passerine bird: implications for evolution and conservation

Like many other migratory birds, the black-throated blue warbler (Dendroica caerulescens) shows pronounced differences in migratory behaviour and other traits between populations: birds in the southern part of the breeding range have darker plumage and migrate to the eastern Caribbean during the winter, whereas those in the north have lighter plumage and migrate to the western Caribbean. We examined the phylogeography of this species, using samples collected from northern and southern populations, to determine whether differentiation between these populations dates to the Pleistocene or earlier, or whether differences in plumage and migratory behaviour have arisen more recently. We analysed variation at 369 bp of the mitochondrial control region domain I and also at seven nuclear microsatellites. Analyses revealed considerable genetic variation, but the vast majority of this variation was found within rather than between populations, and there was little differentiation between northern and southern populations. Phylogeographic analyses revealed a very shallow phylogenetic tree, a star-like haplotype network, and a unimodal mismatch distribution, all indicative of a recent range expansion from a single refugium. Coalescent modelling approaches also indicated a recent common ancestor for the entire group of birds analysed, no split between northern and southern populations, and high levels of gene flow. These results show that Pleistocene or earlier events have played little role in creating differences between northern and southern populations, suggesting that migratory and other differences between populations have arisen very recently. The implications of these results for the evolution of migration and defining taxonomic groups for conservation efforts are discussed.     

On the Measurement of Ecological Novelty: Scale-Eating Pupfish Are Separated by 168 my from Other Scale-Eating Fishes

The colonization of new adaptive zones is widely recognized as one of the hallmarks of adaptive radiation. However, the adoption of novel resources during this process is rarely distinguished from phenotypic change because morphology is a common proxy for ecology. How can we quantify ecological novelty independent of phenotype? Our study is split into two parts: we first document a remarkable example of ecological novelty, scale-eating (lepidophagy), within a rapidly-evolving adaptive radiation of Cyprinodon pupfishes on San Salvador Island, Bahamas. This specialized predatory niche is known in several other fish groups, but is not found elsewhere among the 1,500 species of atherinomorphs. Second, we quantify this ecological novelty by measuring the time-calibrated phylogenetic distance in years to the most closely-related species with convergent ecology. We find that scale-eating pupfish are separated by 168 million years of evolution from the nearest scale-eating fish. We apply this approach to a variety of examples and highlight the frequent decoupling of ecological novelty from phenotypic divergence. We observe that novel ecology is not always tightly correlated with rates of phenotypic or species diversification, particularly within recent adaptive radiations, necessitating the use of additional measures of ecological novelty independent of phenotype.     

Recent diversification rates in North American tiger beetles estimated from a dated mtDNA phylogenetic tree

Species-level phylogenies derived from DNA sequence data provide a tool for estimating diversification rates and how these rates change over time, but to date there have been few empirical studies, particularly on insect groups. We use a densely sampled phylogenetic tree based on mitochondrial DNA to investigate diversification rates in the North American tiger beetles (genus Cicindela). Using node ages estimated from sequence data and calibrated by biogeographical evidence, we estimate an average per-lineage diversification rate of at least 0.22 +/- 0.08 species/Myr over the time interval since the most recent colonization that led to a radiation within the continent. In addition, we find evidence for a weak, recent increase in the net diversification rate. This is more consistent with a late Pleistocene increase in the speciation rate than with a constant rate of background extinction, but the results are sensitive to the dating method and taxon sampling. We discuss practical limitations to phylogenetic studies of diversification rates.     

Nuclear-mitochondrial discordance and gene flow in a recent radiation of toads

Natural hybridization among recently diverged species has traditionally been viewed as a homogenizing force, but recent research has revealed a possible role for interspecific gene flow in facilitating species radiations. Natural hybridization can actually contribute to radiations by introducing novel genes or reshuffling existing genetic variation among diverging species. Species that have been affected by natural hybridization often demonstrate patterns of discordance between phylogenies generated using nuclear and mitochondrial markers. We used Amplified Fragment Length Polymorphism (AFLP) data in conjunction with mitochondrial DNA in order to examine patterns of gene flow and nuclear-mitochondrial discordance in the Anaxyrus americanus group, a recent radiation of North American toads. We found high levels of gene flow between putative species, particularly in species pairs sharing similar male advertisement calls that occur in close geographic proximity, suggesting that prezygotic reproductive isolating mechanisms and isolation by distance are the primary determinants of gene flow and genetic differentiation among these species. Additionally, phylogenies generated using AFLP and mitochondrial data were markedly discordant, likely due to recent and/or ongoing natural hybridization events between sympatric populations. Our results indicate that the putative species in the A. americanus group have experienced high levels of gene flow, and suggest that their North American radiation could have been facilitated by the introduction of beneficial genetic variation from admixture between divergent populations coming into secondary contact after glacial retreats.     

Cryptic sexual dichromatism occurs across multiple types of plumage in the Green-backed Tit Parus monticolus

Several recent studies have found instances of cryptic sexual dichromatism within avian taxa. Although this dichromatism has been found in plumage produced through a variety of proximate mechanisms, little is known about how dichromatism varies across these types of plumage within a single species. We used a reflectance spectrometer to measure colour within the Green-backed Tit Parus monticolus , a species which displays multiple types of pigment and structural colours. We found significant differences in spectral measurements corresponding to hue, chroma, and brightness between male and female carotenoid, melanin, structural white, grey and structural blue plumage. The only plumage that did not appear to show sexual dichromatism was the olive plumage of the back. These findings suggest that the mechanism(s) producing cryptic dichromatism in the Green-backed Tit are non-specific and act across multiple types of plumage, rather than within a single type, such as carotenoid-based or structurally produced.     

Phylogeny of Philippine slender skinks (Scincidae: Brachymeles) reveals underestimated species diversity, complex biogeographical relationships, and cryptic patterns of lineage diversification

The spectacular, virtually endemic radiation of Philippine semi-fossorial skinks of the genus Brachymeles represent one of the few radiations of scincid lizards to possess both fully limbed and limbless species. And yet, nothing is known of the phylogenetic relationships of this exceptional group. Morphologically similar body plans have made it difficult to assess species-level diversity, and the genus has long been recognized as one of the more modest radiations of southeast Asian lizards. However, recent large-scale survey efforts have resulted in the discovery of numerous new species, and taxonomic studies indicate that the diversity within the genus Brachymeles is grossly underestimated. In this study we provide the first robust estimate of phylogenetic relationships within the genus Brachymeles using a multi-locus dataset and nearly complete taxonomic sampling. We provide statistical tests of monophyly for all polytypic species and two widespread limb-reduced species and our results indicate wholesale deviations from past summaries and taxonomic evaluations of the genus. With few exceptions, we are able to reject the monophyly of all polytypic and widespread species, thereby validating the need for large-scale taxonomic revisions. Our results reveal that the limbless, monotypic, genus Davewakeum is nested within Brachymeles. Mapping of body form on our preferred phylogenetic tree suggests that limb-reduction and digit loss has occurred on multiple occasions in the history of the genus. A Bayesian reconstruction of ancestral areas indicates strong statistical support for a minimum of five major dispersal events that have given rise to a major component of the observed species diversity on separate Pleistocene aggregate island platforms of the archipelago.     

Common Garden Experiment Reveals Genetic Control of Phenotypic Divergence between Swamp Sparrow Subspecies That Lack Divergence in Neutral Genotypes

Background

Adaptive divergence between populations in the face of strong selection on key traits can lead to morphological divergence between populations without concomitant divergence in neutral DNA. Thus, the practice of identifying genetically distinct populations based on divergence in neutral DNA may lead to a taxonomy that ignores evolutionarily important, rapidly evolving, locally-adapted populations. Providing evidence for a genetic basis of morphological divergence between rapidly evolving populations that lack divergence in selectively neutral DNA will not only inform conservation efforts but also provide insight into the mechanisms of the early processes of speciation. The coastal plain swamp sparrow, a recent colonist of tidal marsh habitat, differs from conspecific populations in a variety of phenotypic traits yet remains undifferentiated in neutral DNA.

Methods and Principal Findings

Here we use an experimental approach to demonstrate that phenotypic divergence between ecologically separated populations of swamp sparrows is the result of local adaptation despite the lack of divergence in neutral DNA. We find that morphological (bill size and plumage coloration) and life history (reproductive effort) differences observed between wild populations were maintained in laboratory raised individuals suggesting genetic divergence of fitness related traits.

Conclusions and Significance

Our results support the hypothesis that phenotypic divergence in swamps sparrows is the result of genetic differentiation, and demonstrate that adaptive traits have evolved more rapidly than neutral DNA in these ecologically divergent populations that may be in the early stages of speciation. Thus, identifying evolutionarily important populations based on divergence in selectively neutral DNA could miss an important level of biodiversity and mislead conservation efforts.     

Spatial and environmental correlates of intraspecific morphological variation in three species of passerine birds from the Purus–Madeira interfluvium,Central Amazonia

Biogeographic studies in Amazonia typically describe biodiversity across interfluvia, rarely within them, where geographic variability in morphological traits might be observed. We tested for intraspecific phenotypic variation in three bird species within the Purus–Madeira interfluvium (Central Amazon) and whether phenotypes were correlated with environmental heterogeneity or geographic distance among sites. We compared coloration indexes derived from reflectance spectra and morphometrics of up to five adult individuals of each sex among 11 sites within the interfluvium and contrasted them with proxies for geographic distance and environmental variation (tree basal area and bird community). Environmental heterogeneity was minimally spatially autocorrelated, and there were no obvious geographical barriers to dispersal in the study region. The null hypothesis was that we would see either no phenotypic variation or random variation that was not explained by the tested variables. Half of the cases analyzed showed intraspecific morphological variation. Coloration varied more frequently than morphometrics, and color was better explained by environmental heterogeneity, particularly in males, whereas brightness also varied with geographic distance. Geographic distance explained the only case of variation in morphometrics. Our results indicate that coloration, particularly plumage brightness, is more labile than morphometric traits and that plumage color might be under stronger effects of local adaptation than brightness, which also seems to be under effects of neutral drift and gene flow among populations. Higher frequencies of association between male coloration and the environment suggest a role of non-arbitrary mechanisms of sexual selection on the expression of male phenotypes, whereas arbitrary intersexual selection might explain the randomly distributed variation that is not explained by environmental heterogeneity or geographic distance. We revealed intraspecific phenotypic variation in a spatial extent usually not considered in biogeographic studies in the Amazon and demonstrate that both local adaptation and neutral drift are important to explain intraspecific trait diversification at this geographical scale.     

Recent and rapid speciation with limited morphological disparity in the genus Rattus

Recent and rapid radiations provide rich material to examine the factors that drive speciation. Most recent and rapid radiations that have been well-characterized involve species that exhibit overt ecomorphological differences associated with clear partitioning of ecological niches in sympatry. The most diverse genus of rodents, Rattus (66 species), evolved fairly recently, but without overt ecomorphological divergence among species. We used multilocus molecular phylogenetic data and five fossil calibrations to estimate the tempo of diversification in Rattus, and their radiation on Australia and New Guinea (Sahul, 24 species). Based on our analyses, the genus Rattus originated at a date centered on the Pliocene-Pleistocene boundary (1.84-3.17 Ma) with a subsequent colonization of Sahul in the middle Pleistocene (0.85-1.28 Ma). Given these dates, the per lineage diversification rates in Rattus and Sahulian Rattus are among the highest reported for vertebrates (1.1-1.9 and 1.6-3.0 species per lineage per million years, respectively). Despite their rapid diversification, Rattus display little ecomorphological divergence among species and do not fit clearly into current models of adaptive radiations. Lineage through time plots and ancestral state reconstruction of ecological characters suggest that diversification of Sahulian Rattus was most rapid early on as they expanded into novel ecological conditions. However, rapid lineage accumulation occurred even when morphological disparity within lineages was low suggesting that future studies consider other phenotypes in the diversification of Rattus.     

Speciation with gene flow in the large white-headed gulls: does selection counterbalance introgression?

We investigated the role of selection in generating and maintaining species distinctness in spite of ongoing gene flow, using two zones of secondary contact between large gull species in Europe (Larus argentatus and Larus cachinnans) and North America (Larus glaucescens and Larus occidentalis). We used the pattern of neutral genetic differentiation at nine microsatellite loci (F(ST)) as an indicator of expected changes under neutral processes and compared it with phenotypic differentiation (P(ST)) for a large number of traits (size, plumage melanism and coloration of bare parts). Even assuming very low heritability, interspecific divergence between L. glaucescens and L. occidentalis in plumage melanism and orbital ring colour clearly exceeded neutral differentiation. Similarly, melanism of the central primaries was highly divergent between L. argentatus and L. cachinnans. Such divergence is unlikely to have arisen randomly and is therefore attributed to spatially varying selection. Variation in plumage melanism in both transects agrees with Gloger's rule, which suggests that latitude (and associated sun and humidity gradients) could be the selective pressure shaping differentiation in plumage melanism. We suggest that strong species differentiation in orbital ring colour results from sexual selection. We conclude that these large gull species, along with other recently diverged species that hybridize after coming into secondary contact, may differ only in restricted regions of the genome that are undergoing strong disruptive selection because of their phenotypic effects.     

Molecular investigation of genetic assimilation during the rapid adaptive radiations of East African cichlid fishes

Adaptive radiations are characterized by adaptive diversification intertwined with rapid speciation within a lineage resulting in many ecologically specialized, phenotypically diverse species. It has been proposed that adaptive radiations can originate from ancestral lineages with pronounced phenotypic plasticity in adaptive traits, facilitating ecologically driven phenotypic diversification that is ultimately fixed through genetic assimilation of gene regulatory regions. This study aimed to investigate how phenotypic plasticity is reflected in gene expression patterns in the trophic apparatus of several lineages of East African cichlid fishes, and whether the observed patterns support genetic assimilation. This investigation used a split brood experimental design to compare adaptive plasticity in species from within and outside of adaptive radiations. The plastic response was induced in the crushing pharyngeal jaws through feeding individuals either a hard or soft diet. We find that nonradiating, basal lineages show higher levels of adaptive morphological plasticity than the derived, radiated lineages, suggesting that these differences have become partially genetically fixed during the formation of the adaptive radiations. Two candidate genes that may have undergone genetic assimilation, gif and alas1, were identified, in addition to alterations in the wiring of LPJ patterning networks. Taken together, our results suggest that genetic assimilation may have dampened the inducibility of plasticity related genes during the adaptive radiations of East African cichlids, flattening the reaction norms and canalizing their feeding phenotypes, driving adaptation to progressively more narrow ecological niches.     

High levels of interspecific gene flow in an endemic cichlid fish adaptive radiation from an extreme lake environment

Studying recent adaptive radiations in isolated insular systems avoids complicating causal events and thus may offer clearer insight into mechanisms generating biological diversity. Here, we investigate evolutionary relationships and genomic differentiation within the recent radiation of Alcolapia cichlid fish that exhibit extensive phenotypic diversification, and which are confined to the extreme soda lakes Magadi and Natron in East Africa. We generated an extensive RAD data set of 96 individuals from multiple sampling sites and found evidence for genetic admixture between species within Lake Natron, with the highest levels of admixture between sympatric populations of the most recently diverged species. Despite considerable environmental separation, populations within Lake Natron do not exhibit isolation by distance, indicating panmixia within the lake, although individuals within lineages clustered by population in phylogenomic analysis. Our results indicate exceptionally low genetic differentiation across the radiation despite considerable phenotypic trophic variation, supporting previous findings from smaller data sets; however, with the increased power of densely sampled SNPs, we identify genomic peaks of differentiation (F_ST outliers) between Alcolapia species. While evidence of ongoing gene flow and interspecies hybridization in certain populations suggests that Alcolapia species are incompletely reproductively isolated, the identification of outlier SNPs under diversifying selection indicates the radiation is undergoing adaptive divergence.     

Extensive phenotypic diversification coexists with little genetic divergence and a lack of population structure in the White Wagtail subspecies complex (Motacilla alba)

Geographically clustered phenotypes often demonstrate consistent patterns in molecular markers, particularly mitochondrial DNA (mtDNA) traditionally used in phylogeographic studies. However, distinct evolutionary trajectories among traits and markers can lead to their discordance. First, geographic structure in phenotypic traits and nuclear molecular markers can be co‐aligned but inconsistent with mtDNA (mito‐nuclear discordance). Alternatively, phenotypic variation can have little to do with patterns in neither mtDNA nor nuclear markers. Disentangling between these distinct patterns can provide insight into the role of selection, demography and gene flow in population divergence. Here, we examined a previously reported case of strong inconsistency between geographic structure in mtDNA and plumage traits in a widespread polytypic bird species, the White Wagtail (Motacilla alba). We tested whether this pattern is due to mito‐nuclear discordance or discrepancy between morphological evolution and both nuclear and mtDNA markers. We analysed population differentiation and structure across six out of nine commonly recognized subspecies using 17 microsatellite loci and a combination of microsatellites and plumage indices in a comprehensively sampled region of a contact between two subspecies. We did not find support for the mito‐nuclear discordance hypothesis: nuclear markers indicated a subtle signal of genetic clustering only partially consistent with plumage groups, similar to previous findings that relied on mtDNA. We discuss evolutionary factors that could have shaped the intricate patterns of phenotypic diversification in the White wagtail and the role that repeated selection on plumage ‘hotspots’ and hybridization may have played.     

Ancient hybridization and phenotypic novelty within Lake Malawi's cichlid fish radiation

Does hybridization play a broad innovative role in evolution? Many studies have shown hybrid origins of individual species, particularly in major adaptive radiations, but this may be a consequence, rather than a cause, of the existence of many closely related species. Cases of hybridization in the early stages of major adaptive radiations are comparatively rare. Here, we report phylogenetic evidence for ancient introgression between distinct lineages of the species-rich Lake Malawi haplochromine cichlid fishes. Mitochondrial DNA (mtDNA) sequences indicated surprisingly close relationships between the shallow-water rocky habitat "Mbuna" species and a group of dark-adapted "Deep-Benthic" species specialized for feeding in low-light conditions (dawn/dusk, under overhangs, and deep water). By contrast, analyses of nuclear amplified fragment length polymorphism data demonstrated that these Deep-Benthic cichlids were more closely related to shallow water "Shallow-Benthic" soft-sediment feeders, a group that shares similar head and body morphology. A coalescent-based computer simulation indicated that the mtDNA similarity of rocky habitat Mbuna species and dark-adapted Deep-Benthic species was due to hybridization rather than incomplete lineage sorting. Comparisons of morphology indicated that some Deep-Benthic species possessed novel morphology not present in other Lake Malawi species groups. Thus, these analyses support the hypothesis that ancient hybridization occurred within the Lake Malawi cichlid radiation, that the event occurred before the radiation of a species group adapted to low-light benthic habitats, and that this group went on to dominate the deep-water regions of Lake Malawi. The results of this study contribute to a growing literature consistent with a creative role of hybridization in the evolution of species diversity and adaptive radiations.     

Why do melanin ornaments signal individual quality? Insights from metal element analysis of barn owl feathers

Melanin-based variation in colour patterns is under strong genetic control and not, or weakly, sensitive to the environment and body condition. Current signalling theory predicts that such traits may not signal honestly phenotypic quality because their production does not entail a significant fitness cost. However, recent studies revealed that in several bird species melanin-based traits covary with phenotypic attributes. In a first move to understand whether such covariations have a physiological basis, we quantified concentrations of five chemical elements in two pigmented plumage traits in the barn owl (Tyto alba). This bird shows continuous variation from immaculate to heavily marked with black spots (plumage spottiness) and from dark reddish-brown to white (plumage coloration), two traits that signal various aspects of individual quality. These two traits are sexually dimorphic with females being spottier and darker coloured than males. We found an enhancement in calcium and zinc concentration within black spots compared with the unspotted feather parts. The degree to which birds were spotted was positively correlated with calcium concentration within spots, whereas the unspotted feather parts of darker reddish-brown birds were more concentrated in zinc. This suggests that two different pigments are responsible for plumage spottiness and plumage coloration. We discuss the implications of our results in light of recent experimental field studies showing that female spottiness signals offspring humoral response towards an artificially administrated antigen, parasite resistance and fluctuating asymmetry of wing feathers.An erratum to this article can be found at     

Pigeons in the sun: Thermal constraints of eumelanic plumage in the rock pigeon (Columba livia)

In wild vertebrates, several species exhibit eumelanic color polymorphism with the coexistence of dark and light morphs. The maintenance of such polymorphism suggests the existence of a selective balance between the morphs and a large body of literature has reported the costs and benefits of darker plumage coloration in birds. Among them, it has been suggested that melanin and dark plumage could entail high energetic costs especially under hot and sunny climates. However, to my knowledge, the thermal constraints of sun exposure have rarely been studied in polymorphic species. Here, we tested the impact of eumelanic plumage coloration on plumage and body temperatures, and evaporative cooling behavior in the polymorphic rock pigeon (Columbia livia). We experimentally exposed light and dark pigeons to direct sun radiation for 1 h while a few birds were maintained in the shade as controls. We found that sun exposure was associated with increased plumage temperature, and this effect was greater for darker pigeons. In addition, we found that sun exposure was also associated with higher cloacal temperature but for dark pigeons only. Finally, light and dark pigeons were more likely to show cooling evaporative behavior when exposed to sun and as their cloacal temperature increases. Altogether, these results suggest that darker pigeons may have a lower ability to cope with heat and solar radiations and that dark plumage can be associated with thermal costs in this polymorphic species.     

Evolutionary radiation of "stone plants" in the genus Argyroderma (Aizoaceae): unraveling the effects of landscape, habitat, and flowering time

Recent phylogenetic evidence suggests that the extraordinary diversity of the Cape Floristic Kingdom in South Africa may be the result of widespread evolutionary radiation. Our understanding of the role of adaptive versus neutral processes in these radiations remains largely speculative. In this study we investigated factors involved in the diversification of Argyroderma, a genus within the most spectacular of the Cape radiations, that of the Ruschioid subfamily of the Aizoaceae. We used amplified fragment length polymorphisms and a suite of morphological traits to elucidate patterns of differentiation within and between species of Argyroderma across the range of the genus. We then used a matrix correlation approach to assess the influence of landscape structure, edaphic gradients, and flowering phenology on phenotypic and neutral genetic divergence in the system. We found evidence for strong spatial genetic isolation at all taxonomic levels. In addition, genetic differentiation occurs along a temporal axis, between sympatric species with divergent flowering times. Morphological differentiation, which previous studies suggest is adaptive, occurs along a habitat axis, between populations occupying different edaphic microenvironments. Morphological differentiation is in turn significantly associated with flowering time shifts. Thus we propose that diversification within Argyroderma has occurred through a process of adaptive speciation in allopatry. Spatially isolated populations diverge phenotypically in response to divergent habitat selection, which in turn leads to the evolution of reproductive isolation through divergence of flowering phenologies, perhaps as a correlated response to morphological divergence. Evidence suggests that diversification of the group has proceeded in two phases: the first involving divergence of allopatric taxa on varied microhabitats within a novel habitat type (the quartz gravel plains), and the second involving range expansion of an early flowering phenotype on the most extreme edaphic habitat and subsequent incomplete differentiation of allopatric populations of the early flowering group. These results point to adaptive speciation in allopatry as a likely model for the spectacular diversification of the ice-plant family in the dissected landscapes of the southern African winter rainfall deserts.     

Morphological and plumage colour variation in the Réunion grey white‐eye (Aves: Zosterops borbonicus): assessing the role of selection

The Réunion grey white‐eye (Zosterops borbonicus), a small passerine endemic to the island of Réunion (Mascarene archipelago), constitutes an extraordinary case of phenotypic variation within a bird species, with conspicuous plumage colour differentiation at a microgeographical scale. To understand whether natural selection could explain such variability, we compared patterns of variation in morphological and plumage colour traits within and among populations. To quantify morphological variation, we used measurements obtained by Frank Gill in the 1960s from 239 individuals collected in 60 localities distributed over the entire island of Réunion. To quantify colour variation, we measured the reflectance spectra of plumage patches of 50 males from a subset of Gill's specimens belonging to the five recognized plumage colour variants and used a visual model to project these colours in an avian‐appropriate, tetrachromatic, colour space. We found that variants occupy different regions of the avian colour space and that between‐variant differences for most plumage patches could be discriminated by the birds. Differences in morphology were also detected, but these were, in general, smaller than colour differences. Overall, we found that variation in both plumage colour and morphology among variants is greater than would be expected if genetic drift alone was responsible for phenotypic divergence. As the plumage colour variants correspond to four geographical forms, our results suggest that phenotypic evolution in the Réunion grey white‐eye is at least partly explained by divergent selection in different habitats or regions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 459–473.