Hawaiian Silversword Research Articles

Directionally biased habitat shifts and biogeographically informative cytonuclear discordance in the Hawaiian silversword alliance (Compositae).

Expanded phylogenetic analyses of the Hawaiian silversword alliance (Argyroxiphium, Dubautia, Wilkesia; Compositae) were undertaken to assess evolutionary and biogeographic informativeness of cytonuclear discordance and any biases in evolutionary directionality of ecological transitions within this prominent example of adaptive radiation. Samples spanning the geographic and ecological distributions of all recognized taxa were included in phylogenetic and biogeographic analyses of nuclear ribosomal DNA (nrDNA) and cpDNA sequences. Bayesian model testing approaches were used to model ecological evolution and the evolution of nuclear chromosomal arrangements while accounting for phylogenetic uncertainty. Cytonuclear discordance detected previously appears to reflect chloroplast capture, at least in part, with nrDNA trees being largely congruent with nuclear chromosomal structural data and fine-scale taxonomy. Comparison of biogeographic histories estimated from the posterior distributions of nrDNA and cpDNA trees, including inferred chloroplast-capture events, provides additional resolution of dispersal history, including a back-dispersal to Maui Nui from Hawai'i. A newly resolved major nrDNA clade of endemic Kaua'i taxa that mostly were described as new-to-science since the 1980s strengthens the earlier hypothesis that diversification on Kaua'i has not waned since the island began to decline in area through subsidence and erosion. Bias in habitat shifts was estimated, with transitions from dry-to-mesic or -wet and from wet-to-mesic or -bog habitats dominating diversification of the silversword alliance from a dry-adapted tarweed ancestor. The habitat-transition biases estimated here may indicate more limited pathways of ecological evolution than proposed previously for an adaptive radiation involving such major ecological shifts.

The Next Generation of Adaptive Radiation Studies in Plants

Adaptive radiation is an evolutionary process that has been promulgated in some clades as an explanation for species richness and disparity in morphological forms across ecological gradients. Studies designed to elucidate the mechanisms and causes of adaptive radiation have largely focused on animal systems, but plant clades have tremendous potential to answer elusive questions regarding adaptive radiations. The goals of this review are to (1) produce a synthetic understanding of adaptive radiations through studies that have investigated plants systems, (2) critically reflect on contemporary studies to highlight how approaches have been successful as well as limiting, and (3) outline gaps in our understanding of adaptive radiations while emphasizing that plants have ideal characteristics to answer future questions. Thirty-five adaptive radiation clades are highlighted, of which several are supported with multiple lines of evidence, such as the Hawaiian silverswords, Hawaiian lobeliads, and columbines. Plant adaptive radiation examples are commonly insular, diversified in the Miocene or Pliocene, are associated with dispersal-mediated ecological opportunities, are polyploids, and have experienced hybridization. From those studies, a general model of plant insular adaptive radiation is proposed. The limitations of the current reliance on phylogenetic comparative approaches to detect adaptive radiations are considered, and an integrative approach that includes phylogenetics, genomics, and evolutionary ecology is advocated. The review concludes with a call for additional studies that are needed before we are to fully understand adaptive radiations, and they include the following: (1) how do biological interactions influence adaptive radiations, (2) what role does environmental change play in generating ecological opportunity, (3) how does genetic evolution drive adaptive radiation, (4) do models adequately explain the adaptive radiation process, (5) what is the role of hybridization, and (6) why do some groups not undergo adaptive radiation after ecological opportunity?

Island woodiness underpins accelerated disparification in plant radiations.

Summary The evolution of secondary (insular) woodiness and the rapid disparification of plant growth forms associated with island radiations show intriguing parallels between oceanic islands and tropical alpine sky islands. However, the evolutionary significance of these phenomena remains poorly understood and the focus of debate.We explore the evolutionary dynamics of species diversification and trait disparification across evolutionary radiations in contrasting island systems compared with their nonisland relatives. We estimate rates of species diversification, growth form evolution and phenotypic space saturation for the classical oceanic island plant radiations – the Hawaiian silverswords and Macaronesian Echium – and the well‐studied sky island radiations of Lupinus and Hypericum in the Andes.We show that secondary woodiness is associated with dispersal to islands and with accelerated rates of species diversification, accelerated disparification of plant growth forms and occupancy of greater phenotypic trait space for island clades than their nonisland relatives, on both oceanic and sky islands.We conclude that secondary woodiness is a prerequisite that could act as a key innovation, manifest as the potential to occupy greater trait space, for plant radiations on island systems in general, further emphasizing the importance of combinations of clade‐specific traits and ecological opportunities in driving adaptive radiations.

Host defense triggers rapid adaptive radiation in experimentally evolving parasites.

Adaptive radiation occurs when the members of a single lineage evolve different adaptive forms in response to selection imposed by competitors or predators. Iconic examples include Darwin's finches, Caribbean anoles, and Hawaiian silverswords, all of which live on islands. Although adaptive radiation is thought to be an important generator of biodiversity, most studies concern groups that have already diversified. Here, we take the opposite approach. We experimentally triggered diversification in the descendants of a single population of host‐specific parasites confined to different host “islands.” We show rapid adaptive divergence of experimentally evolving feather lice in response to preening, which is a bird's main defense against ectoparasites. We demonstrate that host defense exerts strong phenotypic selection for crypsis in lice transferred to different colored rock pigeons (Columba livia). During four years of experimental evolution (∼60 generations), the lice evolved heritable differences in color. Strikingly, the observed color differences spanned the range of phenotypes found among congeneric lice adapted to other species of birds. To our knowledge, this is the first real‐time demonstration that microevolution is fast enough to simulate millions of years of macroevolutionary change. Our results further indicate that host‐mediated selection triggers rapid divergence in the adaptive radiation of parasites, which are among the most diverse organisms on Earth.

Retracing the Hawaiian silversword radiation despite phylogenetic, biogeographic, and paleogeographic uncertainty.

The Hawaiian silversword alliance (Asteraceae) is an iconic adaptive radiation. However, like many island plant lineages, no fossils have been assigned to the clade. As a result, the clade's age and diversification rate are not known precisely, making it difficult to test biogeographic hypotheses about the radiation. In lieu of fossils, paleogeographically structured biogeographic processes may inform species divergence times; for example, an island must first exist for a clade to radiate upon it. We date the silversword clade and test biogeographic hypotheses about its radiation across the Hawaiian Archipelago by modeling interactions between species relationships, molecular evolution, biogeographic scenarios, divergence times, and island origination times using the Bayesian phylogenetic framework, RevBayes. The ancestor of living silverswords most likely colonized the modern Hawaiian Islands once from the mainland approximately 5.1 Ma, with the most recent common ancestor of extant silversword lineages first appearing approximately 3.5 Ma. Applying an event-based test of the progression rule of island biogeography, we found strong evidence that the dispersal process favors old-to-young directionality, but strong evidence for diversification continuing unabated into later phases of island ontogeny, particularly for Kaua'i. This work serves as a general example for how diversification studies benefit from incorporating biogeographic and paleogeographic components.

Silversword and lobeliad reintroduction linked to landscape restoration on Mauna Loa and Kīlauea, and its implications for plant adaptive radiation in Hawaiʻi

The endemic Hawaiian silversword and lobeliad lineages are two of the world's premier examples of plant adaptive radiation. The lineages also exemplify the severity of the threats confronting the Hawaiian flora, especially the threats posed by alien species. We have implemented collaborative reintroduction efforts with the endangered Kaʻū silversword (Argyroxiphium kauense) and Pele lobeliad (Clermontia peleana) in Hawaiʻi Volcanoes National Park. The efforts with the Kaʻū silversword have involved rediscovery, helicopter assisted rescue of diminutive remnant founders, managed breeding, and outplanting at two sites in the Park of >21,000 seedlings deriving from 169 founders. Facilitated and natural achene dispersal following significant flowering at the two sites has resulted in the establishment of >3400 seedlings as of summer 2016. The efforts with the Pele lobeliad have involved rediscovery, air-layering of remnant founders, managed breeding, and outplanting at two sites in the Park of >1000 seedlings deriving from six of the seven known founders, with the first flowering at one of the sites occurring in summer 2016. We have linked the reintroduction efforts to landscape restoration at large scales in the Park and in adjacent State and private lands, thereby increasing the opportunities for substantial population growth and expansion of the Kaʻū silversword and Pele lobeliad in the future. Additionally, we have extended the reintroduction efforts, including the link to landscape restoration, to encompass all other endangered silversword and lobeliad taxa occurring historically on the eastern slopes of Mauna Loa or on Kīlauea.

Variation and macroevolution in leaf functional traits in the Hawaiian silversword alliance (Asteraceae)

Summary The Hawaiian silversword alliance is a spectacular example of plant adaptive radiation. The lineage includes 33 species in three endemic genera (Argyroxiphium, Dubautia and Wilkesia) that occupy almost all major habitats of the Hawaiian archipelago. Here, we quantitatively explore functional diversification in the lineage by linking measurements of multiple leaf functional traits with climate niche and phylogenetic data. We show that leaf functional trait variation (i) spans much of the global angiosperm range, (ii) is best explained by a white‐noise evolutionary model and (iii) is integrated in ways consistent with both the global leaf economics spectrum and the predictions of leaf venation network theory. Our results highlight the importance of functional diversification and integration in rapidly evolving plant lineages. They also provide compelling additional support for the view that the Hawaiian silversword alliance is one of the world's premier examples of adaptive radiation in plants.

Ecological Role of Hybridization in Adaptive Radiations: A Case Study in theDubautia arborea–Dubautia ciliolata(Asteraceae) Complex

Premise of research. Hybridization is a mechanism frequently invoked to account for the spectacular radiations observed in oceanic islands, but, surprisingly, there is little empirical support for its ecological role in island plant radiations. Theory predicts that hybridization should provide individuals with the phenotypic novelty required for habitat shifts, thus promoting conditions for subsequent speciation. In this article, we studied the first stages of this process using two hybridizing species of Dubautia (Asteraceae), the most diversified genus of the Hawaiian silversword radiation.Methodology. Phenotypic and habitat differentiation were investigated in two recently derived species (Dubautia arborea and Dubautia ciliolata) and a hybrid zone. Morphological trait expression and physiological trait expression under field and common garden conditions and microclimatic variation were analyzed to address the patterns of differentiation among parental species and hybrids.Pivotal results. Our analyses showed that parental species occupied contrasting habitats and represented the extremes of phenotypic variation. Conversely, hybrids displayed novel phenotypes outside parental ranges, generating a continuum of intermediate phenotypes in the study system. We also found a strong relationship between morphological and physiological variation and plant performance, which suggests that broad phenotypic variation in hybrids could be favored by the environmental heterogeneity of the hybrid zone.Conclusions. The initial expectations for the ecological role of hybridization in adaptive radiations are confirmed in this Dubautia system, in which hybrids of closely related species display novel morphological and physiological variation associated with the colonization of a new habitat. To understand the evolutionary processes creating an unusual degree of variation within particular lineages, molecular studies revealing cases of hybridization in island radiations should be complemented with ecological studies.

Gene Gap Investigation of Chalcone Synthase in California Tarweeds and the Hawaiian Silverswords

Complex organisms such as plants have many different metabolic pathways that influence substrate specific reactions and when changed can lead to speciation events. The anthocyanin biosynthetic pathway (ABP), which is a part of the Phenylpropanoid pathway, controls many different mechanisms that regulate temperature, UV protection, microbial invasion, fruit, seed and flower coloring. In the ABP, Chalcone synthase (CHS) is an enzyme that is responsible for catalyzing the first condensation reaction and essential for other genes further down the pathway to function. The model system we are using is the Hawaiian Silverswords, which are of great interest due to the fact that they are a classic model of adaptive radiation and have large phenotypic but little genotypic differences. In previous studies copies of CHS in Dubautia scabra, Dubautia linearis, and Wilkesia gymnoxiphium in the Hawaiian Silverswords as well as Madia elegans, Madia gracilis, and Holocarpha macradenia, which are apart of the California tarweeds (Silversword Progenitor), have been partially sequenced, analyzed and identified as CHS. The gaps that remain include exon 1 and 2 for Wilkesia gymnoxiphium, and Dubautia linearis as well as the intron region which will help in distinguishing specific gene copies within each genome. We accomplish this by using polymerase chain reaction, cloning, sequencing, and bioinformatics.

Evolutionary diversification and geographical isolation in Dubautia laxa (Asteraceae), a widespread member of the Hawaiian silversword alliance

The Hawaiian silversword alliance (Asteraceae) is one the best examples of a plant adaptive radiation, exhibiting extensive morphological and ecological diversity. No research within this group has addressed the role of geographical isolation, independent of ecological adaptation, in contributing to taxonomic diversity. The aims of this study were to examine genetic differentiation among subspecies of Dubautia laxa (Asteraceae) to determine if allopatric or sympatric populations and subspecies form distinct genetic clusters to understand better the role of geography in diversification within the alliance. Dubautia laxa is a widespread member of the Hawaiian silversword alliance, occurring on four of the five major islands of the Hawaiian archipelago, with four subspecies recognized on the basis of morphological, ecological and geographical variation. Nuclear microsatellites and plastid DNA sequence data were examined. Data were analysed using maximum-likelihood and Bayesian phylogenetic methodologies to identify unique evolutionary lineages. Plastid DNA sequence data resolved two highly divergent lineages, recognized as the Laxa and Hirsuta groups, that are more similar to other members of the Hawaiian silversword alliance than they are to each other. The Laxa group is basal to the young island species of Dubautia, whereas the Hirsuta group forms a clade with the old island lineages of Dubautia and with Argyroxiphium. The divergence between the plastid groups is supported by Bayesian microsatellite clustering analyses, but the degree of nuclear differentiation is not as great. Clear genetic differentiation is only observed between allopatric populations, both within and among islands. These results indicate that geographical separation has aided diversification in D. laxa, whereas ecologically associated morphological differences are not associated with neutral genetic differentiation. This suggests that, despite the stunning ecological adaptation observed, geography has also played an important role in the Hawaiian silversword alliance plant adaptive radiation.

Pollen foraging behaviour of solitary Hawaiian bees revealed through molecular pollen analysis

Obtaining quantitative information concerning pollinator behaviour has become a primary objective of pollination studies, but methodological limitations hinder progress towards this goal. Here, we use molecular genetic methods in an ecological context to demonstrate that endemic Hawaiian Hylaeus bees (Hymenoptera: Colletidae) selectively collect pollen from native plant species in Haleakala and Hawaii Volcanoes National Parks. We identified pollen DNA from the crops (internal storage organs) of 21 Hylaeus specimens stored in ethanol for up to 3 years. Genetic analyses reveal high fidelity in pollen foraging despite the availability of pollen from multiple plant species present at each study site. At high elevations in Haleakala, pollen was available from more than 12 species of flowering plants, but Hawaiian silversword (Argyroxiphium sandwicense subsp. macrocephalum) comprised 86% of all pollen samples removed from bee crops. At lower elevations in both parks, we only detected pukiawe (Leptecophylla (Styphelia) tameiameiae) pollen in Hylaeus crops despite the presence of other plant species in flower during our study. Furthermore, 100% of Hylaeus crops from which we successfully identified pollen contained native plant pollen. The molecular approaches developed in this study provide species-level information about floral visitation of Hawaiian Hylaeus that does not require specialized palynological expertise needed for high-throughput visual pollen identification. Building upon this approach, future studies can thus develop appropriate and customized criteria for assessing mixed pollen loads from a broader range of sources and from other global regions.

Dubautia carrii and D. hanaulaensis, New Species of the Hawaiian Silversword Alliance (Compositae, Madiinae) from Moloka‘i and Maui

Abstract Dubautia carrii B. G. Baldwin & Friar and D. hanaulaensis B. G. Baldwin are new species in Dubautia Gaudich. sect. Railliardia (Gaudich.) G. D. Carr (Compositae, Madiinae) of the Hawaiian silversword alliance that were previously treated within the circumscription of D. linearis (Gaudich.) D. D. Keck subsp. opposita (Sherff) G. D. Carr. The two species are allopatric, with D. carrii in mesic to wet shrubland and forest on eastern Moloka‘i and D. hanaulaensis in mesic shrubland and forest on southern West Maui. Although closely resembling D. linearis subsp. linearis and D. linearis subsp. hillebrandii (H. Mann) G. D. Carr in habit and capitulescence characteristics, D. carrii and D. hanaulaensis differ from both subspecies of D. linearis in the sense adopted here by having consistently opposite (rather than variable) phyllotaxy and toothed (rather than entire) leaf apices, and by occurring in mesic or wet (rather than mostly dry) habitats. Dubautia carrii and D. hanaulaensis are highly similar mor...

NATURAL HISTORY, ECOLOGY, AND EVOLUTION OF A REMARKABLE ADAPTIVE RADIATION

Review of: Losos, J. B. 2009. Lizards in an Evolutionary Tree: Ecology and Adaptive Radiation of Anoles. University of California Press, Berkeley, CA. ISBN: 9780520255913. Price. $75.00 What is an adaptive radiation? In its simplest form, it is the explosive diversification of a monophyletic lineage in response to the acquisition of a key innovation or colonization of new niches. What examples come to the mind of most students of evolutionary biology? Presumably, we all have our favorite example, but I would venture that most top five lists would include one or more of: Darwin’s finches on the Galapagos islands, African rift valley cichlids, Hawaiian honeycreepers, Hawaiian silverswords, and Greater Antillean Anolis lizards. These radiations are textbook classics because of the impressive diversity of form and function in their descendant lineages and/or because they have been studied thoroughly over relatively long periods of time. With the extremely well researched and entertainingly written Lizards in an Evolutionary Tree: Ecology and Adaptive Radiation of Anoles, Losos (2009) makes a rather convincing case that the latter group, along with their Lesser Antillean and mainland counterparts, may represent the ideal marriage of diversity to explore and study system tractability. To be fair, Losos does not directly make this bold claim; rather it is an impression one easily gets after reading the book. Chapter 1 begins with a brief introduction to the field of evolutionary biology, especially as it pertains to inferring historical events from fossil and phylogenetic evidence. In it, Losos makes a cogent defense of inferential approaches, aimed at those who take the narrow-minded view (again, these are my words here) that only experimental work constitutes (good) science. At the end of the chapter, Losos provides some insight into his introduction to the field and what motivates his continued interest in anoles. It is here that we first encounter a thread that will weave through the entire book, which is: although much is known about anoles, even more striking is how much remains undiscovered. Unlike some researchers who appear reluctant to bring other researchers into the fold (or, even, actively discourage others from working on “their” system), Losos (p. 10) extends an open invitation: “There’s plenty of room in the Anolis world for more researchers, and I will consider this book a success if it helps to produce a new generation of anole biologists.” Hear, hear! Throughout the book, Losos ends each chapter with a “Future Directions” section, sometimes brief, other times fairly substantial. Not wanting to steal his thunder, I largely do not touch on their contents, but I will say that they alone make this book a must read for new researchers in the field. With the scene set, the book really gets going in subsequent chapters. In Chapters 2–4 (the first of four informal sections), Losos introduces the diversity and distribution of anoles. Chapter 2 aims to define what anoles are, in terms of general phenotype, where they are found, and a short discussion of reproduction isolation and species concepts as they pertain to anole species. Having known the most about the ecomorphs (a term now widely used in ecology and evolution, it was first applied by the renowned Anolis biologist, Ernest Williams, in this context [see Footnote 72 in

Conservatism and diversification of plant functional traits: Evolutionary rates versus phylogenetic signal

The concepts of niche conservatism and adaptive radiation have played central roles in the study of evolution and ecological diversification. With respect to phenotypic evolution, the two processes may be seen as opposite ends of a spectrum; however, there is no straightforward method for the comparative analysis of trait evolution that will identify these contrasting scenarios. Analysis of the rate of phenotypic evolution plays an important role in this context and merits increased attention. In this article, independent contrasts are used to estimate rates of evolution for continuous traits under a Brownian motion model of evolution. A unit for the rate of phenotypic diversification is introduced: the felsen, in honor of J. Felsenstein, is defined as an increase of one unit per million years in the variance among sister taxa of ln-transformed trait values. The use of a standardized unit of measurement facilitates comparisons among clades and traits. Rates of diversification of three functional traits (plant height, leaf size, and seed size) were estimated for four to six woody plant clades (Acer, Aesculus, Ceanothus, Arbutoideae, Hawaiian lobeliads, and the silversword alliance) for which calibrated phylogenies were available. For height and leaf size, rates were two to approximately 300 times greater in the Hawaiian silversword alliance than in the other clades considered. These results highlight the value of direct estimates of rates of trait evolution for comparative analysis of adaptive radiation, niche conservatism, and trait diversification.

Hybrid Origin and Genomic Mosaicism of Dubautia scabra (Hawaiian Silversword Alliance; Asteraceae, Madiinae)

Incongruence among different estimates of species relationships in plants, from different molecules, cytogenetic data, biogeographic data, morphological/anatomical data or other sources, has been used frequently as an indication of introgression, hybrid species origin, or chloroplast (cp) capture. In plants, these incongruences are most often seen between data derived from the nuclear vs. the cp genomes and the nuclear markers used for comparison usually have been from the nuclear ribosomal (nr) internal transcribed spacer region (ITS). The amount of genomic material shared between introgressing species can be highly variable. In some of these cases, other nuclear genomic regions have moved between species without leaving a signature on the nrITS. An example of well-supported phylogenetic incongruence is the placement of Dubautia scabra (DC.) D. D. Keck in the Hawaiian silversword alliance (HSA); evolutionary hypotheses for D. scabra based on molecular as opposed to cytogenetic data are strongly discordant. In this paper, we test these two conflicting phylogenetic hypotheses regarding the evolutionary relationships of Dubautia scabra using evidence from six low-copy nuclear genes, as well as multiple chloroplast noncoding regions and nrITS. The nrITS region is also examined for the presence of multiple copy types. Incongruence between inferred relationships based on nuclear chromosomal arrangements and molecular phylogenetic data from chloroplast DNA and nrITS is resolved in favor of a hypothesis of ancient hybridization rather than cytogenetic homoplasy involving dysploidy. Most single-copy nuclear genes track histories of D. scabra compatible with cytogenetic data whereas chloroplast and nrITS data track a common, different history that appears to reflect hybridization with a chromosomally distinct lineage that also occurs on Maui Nui and Hawai'i (the Big Island).

Steady Plio-Pleistocene diversification and a 2-million-year sympatry threshold in a New Zealand cicada radiation

Estimation of diversification rates in evolutionary radiations requires a complete accounting of cryptic species diversity. The rapidly evolving songs of acoustically signaling insects make them good model organisms for such studies. This paper examines the timing of diversification of a large (30 taxon) group of New Zealand cicadas (genus Kikihia Dugdale). We use Bayesian relaxed-clock methods and phylogenetic trees based on nuclear and mitochondrial DNA data, and we apply alternative combinations of evolutionary rate priors and geological calibrations. The extant Kikihia taxa began to diversify near the Miocene/Pliocene boundary around the time of increased mountain-building, and both the mitochondrial and nuclear-gene trees confirm early splits of lineages currently represented by lowland forest-dwelling taxa. Most lineages originated in the Pleistocene, and sustained diversification occurred rapidly at over 0.5 lineages/my, a rate comparable to that of the Hawaiian silverswords. Diversification rate tests suggest an increase in the early to mid-Pliocene, followed by constant diversification from the Late Pliocene onward. No descendants of the many Pleistocene-age splits have evolved the ability to coexist in sympatry, and, where they do come into contact, hybrid zones have been documented based on acoustic and DNA evidence. In contrast, lineages separated in time by approximately 2 Myr often overlap in distribution with no evidence of hybridization. This suggests that at least 2 Myr has been required to achieve the level of divergence required for reproductive isolation.

Diversity and divergence patterns in regulatory genes suggest differential gene flow in recently derived species of the Hawaiian silversword alliance adaptive radiation (Asteraceae)

The impact of gene flow and population size fluctuations in shaping genetic variation during adaptive radiation, at both the genome-wide and gene-specific levels, is very poorly understood. To examine how historical population size and gene flow patterns within and between loci have influenced lineage divergence in the Hawaiian silversword alliance, we have investigated the nucleotide sequence diversity and divergence patterns of four floral regulatory genes (ASAP1-A, ASAP1-B, ASAP3-A, ASAP3-B) and a structural gene (ASCAB9). Levels and patterns of molecular divergence across these five nuclear loci were estimated between two recently derived species (Dubautia ciliolata and Dubautia arborea) which are presumed to be sibling species. This multilocus analysis of genetic variation, haplotype divergence and historical demography indicates that population expansion and differential gene flow occurred subsequent to the divergence of these two lineages. Moreover, contrasting patterns of allele- sharing for regulatory loci vs. a structural locus between these two sibling species indicate alternative histories of genetic variation and partitioning among loci where alleles of the floral regulatory loci are shared primarily from D. arborea to D. ciliolata and alleles of the structural locus are shared in both directions. Taken together, these results suggest that adaptively radiating species can exhibit contrasting allele migration rates among loci such that allele movement at specific loci may supersede genetic divergence caused by drift and that lineage divergence during adaptive radiation can be associated with population expansion.

Influences of gene flow on adaptive speciation in the Dubautia arborea–D. ciliolata complex

Mechanisms of reproductive isolation during plant speciation are often unclear because distinct species often experience high levels of gene flow and hybridization. Adaptive radiations such as the Hawaiian silversword alliance (HSA) provide unique opportunities to study the interactions of selection, gene flow and isolating mechanisms during the speciation process. We examined patterns of phenotypic and genetic differentiation in Dubautia arborea and Dubautia ciliolata, two parapatric HSA taxa that show marked morphological divergence but evidence of weak molecular differentiation, in order to estimate genome-wide differentiation and gene flow patterns. We scored 166 amplified fragment length polymorphism markers in a set of 89 plants from two populations each of D. arborea and D. ciliolata and phenotypically D. arborea-like and D. ciliolata-like plants from a natural hybrid zone. Analyses of population subdivision showed low levels of differentiation between the two species (F(ST) = 0.089) and evidence that the phenotypically parental hybrid zone plants were largely of parental species rather than of hybrid origin. A Bayesian analysis of population ancestry identified a number of plants with admixed D. arborea and D. ciliolata ancestry, even in nonhybrid-zone populations. These results suggest that genome-wide low levels of differentiation between D. arborea and D. ciliolata are in part due to gene flow, and favour models of genic speciation and collective evolution in which gene flow has different effects on selected loci vs. nonselected genomic regions. We discuss ecological and climatic factors that may have shaped patterns of differentiation in this species complex.

Gene flow inDubautia arboreaandD. ciliolata: the roles of ecology and isolation by distance in maintaining species boundaries despite ongoing hybridization

The relative roles of gene flow and natural selection in maintaining species differentiation have been a subject of debate for some time. The traditional view is that gene flow constrains adaptive divergence and maintains species cohesiveness. Alternatively, ecological speciation posits that the reverse is true: that adaptive ecological differentiation constrains gene flow. In this study, we examine gene flow and population differentiation among populations of two species of the Hawaiian silversword alliance, Dubautia arborea and D. ciliolata. We compare divergence in putatively neutral microsatellite markers with divergence in leaf morphometric traits, which may be selectively important or physiologically linked to selectively important traits. Gene flow between populations was found to be significant in only one of the two species, D. arborea. Leaf morphometric differentiation between species was significant, though not among populations within species. No evidence of effective genetic introgression was observed between apparently 'pure' populations of these species. Gene flow as measured by microsatellites was not correlated with geographic distance between populations, but was correlated with the linear placement of the widest part of the leaf. Because these two species are interfertile, as demonstrated by the presence of active hybrid zone, the lack of genetic introgression and the maintenance of species boundaries may be associated with natural selection on differential habitat.

Adaptive radiation of shrubby tarweeds (Deinandra) in the California Islands parallels diversification of the Hawaiian silversword alliance (Compositae–Madiinae)

Phylogenetic analyses of nuclear rDNA transcribed spacers and cytogenetic studies of interspecific hybrids reported here uphold Carlquist's hypothesis (1965, Island Biology) that shrubby tarweeds (Deinandra) of Guadalupe Island, Mexico, are products of in situ radiation in the California Islands, where evidence of plant diversification has been equivocal. Based on the rDNA findings, the Guadalupe Island endemics (D. frutescens, D. greeneana subsp. greeneana, and D. palmeri) constitute a clade that arose since the late Pliocene, well after the origin of Guadalupe Island and diversification of annual, mainland Californian lineages of Deinandra. High interfertility and normal meiosis in F(1) hybrids between the three endemics contrast with reduced interfertility (to complete intersterility) and meiotic irregularities in F(1) hybrids between other, mostly mainland species of Deinandra. Cloned rDNA sequences provided no convincing evidence of introgression among the Guadalupe Island deinandras; morphological, phenological, and/or habitat differences among those taxa indicate ecological barriers to gene flow and a probable role for ecological divergence in diversification. Biosystematic and molecular phylogenetic data for shrubby tarweeds of Guadalupe Island and another secondarily woody, oceanic-island tarweed lineage, the Hawaiian silversword alliance, reveal strikingly similar evolutionary histories. Both groups violate Baker's Rule by stemming from self-incompatible ancestors in western North America, and each has undergone within-island diversification without evolution of strong sterility barriers among lineages. Evolutionary parallels between these Hawaiian and California Island lineages of Madiinae, first suggested by Carlquist, may reflect characteristics of tarweeds that facilitate insular colonization and adaptive radiation.