[Show abstract][Hide abstract] ABSTRACT: Many arctic-alpine plant genera have undergone speciation during the Quaternary. The bases for these radiations have been ascribed to geographic isolation, abiotic and biotic differences between populations, and/or hybridization and polyploidization. The Cordilleran Campanula L. (Campanulaceae Juss.), a monophyletic clade of mostly endemic arctic-alpine taxa from western North America, experienced a recent and rapid radiation. We set out to unravel the factors that likely influenced speciation in this group. To do so, we integrated environmental, genetic, and morphological datasets, tested biogeographic hypotheses, and analyzed the potential consequences of the various factors on the evolutionary history of the clade. We created paleodistribution models to identify potential Pleistocene refugia for the clade and estimated niche space for individual taxa using geographic and climatic data. Using 11 nuclear loci, we reconstructed a species tree and tested biogeographic hypotheses derived from the paleodistribution models. Finally, we tested 28 morphological characters, including floral, vegetative, and seed characteristics, for their capacity to differentiate taxa. Our results show that the combined effect of Quaternary climatic variation, isolation among differing environments in the mountains in western North America, and biotic factors influencing floral morphology contributed to speciation in this group during the mid-Pleistocene. Furthermore, our biogeographic analyses uncovered asynchronous consequences of interglacial and glacial periods for the timing of refugial isolation within the southern and northwestern mountains, respectively. These findings have broad implications for understanding the processes promoting speciation in arctic-alpine plants and the rise of numerous endemic taxa across the region.
Ecology and Evolution 08/2014; · 1.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: One of the grand goals of historical biogeography is to understand how and why species' population sizes and distributions change over time. Multiple types of data drawn from disparate fields, combined into a single modelling framework, are necessary to document changes in a species's demography and distribution, and to determine the drivers responsible for change. Yet truly integrated approaches are challenging and rarely performed. Here, we discuss a modelling framework that integrates spatio-temporal fossil data, ancient DNA, palaeoclimatological reconstructions, bioclimatic envelope modelling and coalescence models in order to statistically test alternative hypotheses of demographic and potential distributional changes for the iconic American bison (Bison bison). Using different assumptions about the evolution of the bioclimatic niche, we generate hypothetical distributional and demographic histories of the species. We then test these demographic models by comparing the genetic signature predicted by serial coalescence against sequence data derived from subfossils and modern populations. Our results supported demographic models that include both climate and human-associated drivers of population declines. This synthetic approach, integrating palaeoclimatology, bioclimatic envelopes, serial coalescence, spatio-temporal fossil data and heterochronous DNA sequences, improves understanding of species' historical biogeography by allowing consideration of both abiotic and biotic interactions at the population level.
Proceedings of the Royal Society B: Biological Sciences 01/2014; 281(1777):20132782. · 5.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: AimSpecies distribution models (SDMs) are commonly used to forecast climate change impacts. These models, however, are subject to important assumptions and limitations. By integrating two independent but complementary methods, ensemble SDMs and statistical phylogeography, we addressed key assumptions and created robust assessments of climate change impacts on species distributions while improving the conservation value of these projections. LocationNorth American cordillera. Methods
This approach was demonstrated using the arctic‐alpine plant Rhodiola integrifolia (Crassulaceae). SDMs were fitted to current and past climates using eight models, two thresholds and one to three climate data sets. These projections were combined to create a map of stable climate (refugia) since the Last Interglacial (124,000 kya). Five biogeographic hypotheses were developed based on the configuration of refugia and tested using statistical phylogeography. Projection of SDMs into the future was contingent on agreement across approaches; future projections (to 2085) used five climate data sets and two greenhouse gas scenarios. ResultsA multiple‐refugia hypothesis was supported by both methods, confirming the assumption of niche conservatism in R. integrifolia and justifying the projection of SDMs onto future climates. Future projections showed substantial loss of climatically suitable habitat. Southern populations had the greatest losses, although the biogeographic scale of modelling may overpredict extinction risks in areas of topographic complexity. Past and future SDMs were assessed for novel values of climate variables; areas of novel climate were flagged as having higher uncertainty. Main conclusionsIntegrating molecular approaches with spatial analyses of species distributions under global change has great potential to improve conservation decision‐making. Molecular tools can support and improve current methods for understanding the vulnerability of species to climate change and provide additional data upon which to base conservation decisions, such as prioritizing the conservation of areas of high genetic diversity to build evolutionary resiliency within populations.
[Show abstract][Hide abstract] ABSTRACT: Arctic-alpine plants in the genus Saxifraga L. (Saxifragaceae Juss.) provide an excellent system for investigating the process of diversification in northern regions. Yet, sect. Trachyphyllum (Gaud.) Koch, which is comprised of about 8 to 26 species, has still not been explored by molecular systematists even though taxonomists concur that the section needs to be thoroughly re-examined. Our goals were to use chloroplast trnL-F and nuclear ITS DNA sequence data to circumscribe the section phylogenetically, test models of geographically-based population divergence, and assess the utility of morphological characters in estimating evolutionary relationships. To do so, we sequenced both genetic markers for 19 taxa within the section. The phylogenetic inferences of sect. Trachyphyllum using maximum likelihood and Bayesian analyses showed that the section is polyphyletic, with S. aspera L. and S bryoides L. falling outside the main clade. In addition, the analyses supported several taxonomic re-classifications to prior names. We used two approaches to test biogeographic hypotheses: i) a coalescent approach in Mesquite to test the fit of our reconstructed gene trees to geographically-based models of population divergence and ii) a maximum likelihood inference in Lagrange. These tests uncovered strong support for an origin of the clade in the Southern Rocky Mountains of North America followed by dispersal and divergence episodes across refugia. Finally we adopted a stochastic character mapping approach in SIMMAP to investigate the utility of morphological characters in estimating evolutionary relationships among taxa. We found that few morphological characters were phylogenetically informative and many were misleading. Our molecular analyses provide a foundation for the diversity and evolutionary relationships within sect. Trachyphyllum and hypotheses for better understanding the patterns and processes of divergence in this section, other saxifrages, and plants inhabiting the North Pacific Rim.
PLoS ONE 01/2013; 8(7):e69814. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Despite the strength of climatic variability at high latitudes and upper elevations, we still do not fully understand how plants in North America that are distributed between Arctic and alpine areas responded to the environmental changes of the Quaternary. To address this question, we set out to resolve the evolutionary history of the King's Crown, Rhodiola integrifolia using multi-locus population genetic and phylogenetic analyses in combination with ecological niche modeling. Our population genetic analyses of multiple anonymous nuclear loci revealed two major clades within R. integrifolia that diverged from each other ~ 700 kya: one occurring in Beringia to the north (including members of subspecies leedyi and part of subspecies integrifolia), and the other restricted to the Southern Rocky Mountain refugium in the south (including individuals of subspecies neomexicana and part of subspecies integrifolia). Ecological niche models corroborate our hypothesized locations of refugial areas inferred from our phylogeographic analyses and revealed some environmental differences between the regions inhabited by its two subclades. Our study underscores the role of geographic isolation in promoting genetic divergence and the evolution of endemic subspecies in R. integrifolia. Furthermore, our phylogenetic analyses of the plastid spacer region trnL-F demonstrate that among the native North American species, R. integrifolia and R. rhodantha are more closely related to one another than either is to R. rosea. An understanding of these historic processes lies at the heart of making informed management decisions regarding this and other Arctic-alpine species of concern in this increasingly threatened biome.
PLoS ONE 01/2013; 8(11):e79451. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
Species distribution models (SDMs) are commonly used to forecast climate change impacts on species and ecosystems. These models, however, are subject to important assumptions and limitations. By integrating two independent but complementary methods, ensemble SDMs and statistical phylogeography, we were able to address key assumptions and create robust assessments of climate change impacts on species’ distributions while improving the conservation value of these projections.
This approach was demonstrated using Rhodiola integrifolia, an alpine-arctic plant specialist distributed at high elevations and latitudes throughout the North American cordillera. SDMs for R. integrifolia were fit to current and past climates using eight model algorithms, two threshold methods, and between one and three climate data sets (from general circulation models, GCMs). This ensemble of projections was combined using consensus methods to create a map of stable climate (refugial habitat) since the Last Interglacial (124,000 years before present).
Four biogeographic hypotheses were developed based on the configuration of refugial habitat and tested using a statistical phylogeographic approach. Statistical phylogeography evaluates the probability of alternative models of population history given uncertainty about past population parameters. Agreement between SDM projections and phylogeographic analyses provides support for niche conservatism and the ability of the SDMs to accurately project distributions over time. Disagreement can indicate dispersal barriers, a lack of niche conservatism, or errors in the modeling process.
For R. integrifolia the multiple-refugia hypothesis was supported by both methods, validating the assumption of niche conservatism and justifying the projection of the SDMs onto future climates. SDMs were projected onto two greenhouse gas scenarios (A1B and A2) for 2085 using climate data from five GCMs. Projections at 2085 showed substantial losses of climatically suitability habitat for R. integrifolia across its range. Southern populations had the greatest losses, though the biogeographic scale of modeling may overpredict extinction risks in these areas of topographic complexity. Finally, past and future SDM projections were assessed for novel values of climate variables; projections in areas of novel climate were flagged as having higher uncertainty.
Integrating genetic approaches with spatial analyses of species distributions under global change has great potential to improve conservation decision-making. Genetic tools can support and improve current methods for understanding species vulnerability to climate change, and provide additional data upon which to base conservation decisions, such as prioritizing the conservation of areas of high genetic diversity in order to build evolutionary resiliency within populations.
[Show abstract][Hide abstract] ABSTRACT: To assess effects of historical climate change on northern species, we quantified the population history of the arctic ground squirrel (Spermophilus parryii), an arctic-adapted rodent that evolved in Beringia and was strongly influenced by climatic oscillations of the Quaternary. Competing hypotheses for the species' population history were derived from patterns of mitochondrial (mtDNA) structure and a bioclimatic envelope model (BEM). Hypotheses invoked (1) sequential isolation of regional populations beginning with the Arctic, (2) deep isolation only across central Alaska, and (3) widespread panmixia, and were tested using coalescent methods applied to eight nuclear (nDNA) loci. The data rejected strict interpretations of all three hypotheses, but perspectives underlying each encompassed aspects of the species' history. Concordance between mtDNA and nDNA geographic structure revealed three semi-independently evolving phylogroups, whereas signatures of gene flow at nDNA loci were consistent with a historical contact between certain populations as inferred by the BEM. Demographic growth was inferred for all regions despite expectations of postglacial habitat contraction for parts of Beringia. Our results highlight the complementary perspectives on species' histories that multiple lines of evidence provide, and underscore the utility of multilocus data for resolving complex population histories relevant to understanding effects of climate change.
[Show abstract][Hide abstract] ABSTRACT: Recent phylogenetic works have begun to address long-standing questions regarding the systematics of Campanula (Campanulaceae). Yet, aspects of the evolutionary history, particularly in northwestern North America, remain unresolved. Thus, our primary goal in this study was to infer the phylogenetic positions of northwestern Campanula species within the greater Campanuloideae tree. We combined new sequence data from 5 markers (atpB, rbcL, matK, and trnL-F regions of the chloroplast and the nuclear ITS) representing 12 species of Campanula with previously published datasets for worldwide campanuloids, allowing us to include approximately 75% of North American Campanuleae in a phylogenetic analysis of the Campanuloideae. Because all but one of North American Campanula species are nested within a single campanuloid subclade (the Rapunculus clade), we conducted a separate set of analyses focused specifically on this group. Our findings show that i) the campanuloids have colonized North America at least 6 times, 4 of which led to radiations, ii) all but one North American campanuloid are nested within the Rapunculus clade, iii) in northwestern North America, a C. piperi-C. lasiocarpa ancestor gave rise to a monophyletic Cordilleran clade that is sister to a clade containing C. rotundifolia, iv) within the Cordilleran clade, C. parryi var. parryi and C. parryi var. idahoensis exhibit a deep, species-level genetic divergence, and v) C. rotundifolia is genetically diverse across its range and polyphyletic. Potential causes of diversification and endemism in northwestern North America are discussed.
PLoS ONE 01/2011; 6(9):e23559. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background: Evidence strongly suggests that Beringia was a refugium for tundra taxa throughout the Quaternary (the last 2 million years). However, the genetic consequences of the repeated formation and flooding of the Bering Land Bridge remain uncertain.Aims: The goal of this paper was to determine the role that the unique environmental history of Beringia played in the diversification of tundra flora.Methods: I adopted a comparative coalescent approach to test models of divergence for arctic flora within Beringia. The literature was surveyed for phylogeographic studies that sampled broadly across the region and incorporated molecular markers appropriate for coalescent analyses. Of the 13 possible taxa, only two fit these criteria: Saxifraga oppositifolia (Saxifragaceae) and Vaccinium uliginosum (Ericaceae). Observed gene trees were compared with a distribution of trees simulated under neutral coalescence to test models of population divergence. Population models fell within two major categories reflecting the importance of either the Bering Land Bridge or the Bering Sea dispersal barrier on the distribution of genetic diversity in the species.Results: Both species fit ‘bridge’ models, but S. oppositifolia supported a model of eastward migration while V. uliginosum fits a unified Beringia refugium model. The evolutionary implications of these findings are discussed.Conclusions: The limited number of studies emphasises the need for more sequence-based research in the region. This will help resolve the history of the Beringia tundra ecosystem, which has important implications for the diversification of tundra flora, the history of Beringia, and the potential consequences of climate change on the distribution of biological diversity.
[Show abstract][Hide abstract] ABSTRACT: Recent evidence suggests that deep-sea vestimentiferan tube worms acquire their endosymbiotic bacteria from the environment each generation; thus, free-living symbionts should exist. Here, free-living tube worm symbiont phylotypes were detected in vent seawater and in biofilms at multiple deep-sea vent habitats by PCR amplification, DNA sequence analysis, and fluorescence in situ hybridization. These findings support environmental transmission as a means of symbiont acquisition for deep-sea tube worms.
Applied and Environmental Microbiology 07/2008; 74(12):3895-8. · 3.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Organisms at hydrothermal vents inhabit discontinuous chemical 'islands' along mid-ocean ridges, a scenario that may promote genetic divergence among populations. The 2003 discovery of mussels at the Lost City Hydrothermal Field provided a means of evaluating factors that govern the biogeography of symbiotic bacteria in the deep sea. The unusual chemical composition of vent fluids, the remote location, and paucity of characteristic vent macrofauna at the site, raised the question of whether microbial symbioses existed at the extraordinary Lost City. If so, how did symbiotic bacteria therein relate to those hosted by invertebrates at the closest known hydrothermal vents along the Mid-Atlantic Ridge (MAR)? To answer these questions, we performed microscopic and molecular analyses on the bacteria found within the gill tissue of Bathymodiolus mussels (Mytilidae, Bathymodiolinae) that were discovered at the Lost City. Here we show that Lost City mussels harbour chemoautotrophic and methanotrophic endosymbionts simultaneously. Furthermore, populations of the chemoautotrophic symbionts from the Lost City and two sites along the MAR are genetically distinct from each other, which suggests spatial isolation of bacteria in the deep sea. These findings provide new insights into the processes that drive diversification of bacteria and evolution of symbioses at hydrothermal vents.
[Show abstract][Hide abstract] ABSTRACT: Bathymodiolus azoricus and Bathymodiolus puteoserpentis are symbiont-bearing mussels that dominate hydrothermal vent sites along the northern Mid-Atlantic Ridge (MAR). Both species live in symbiosis with two physiologically and phylogenetically distinct Gammaproteobacteria: a sulfur-oxidizing chemoautotroph and a methane-oxidizer. A detailed analysis of mussels collected from four MAR vent sites (Menez Gwen, Lucky Strike, Rainbow, and Logatchev) using comparative 16S rRNA sequence analysis and fluorescence in situ hybridization (FISH) showed that the two mussel species share highly similar to identical symbiont phylotypes. FISH observations of symbiont distribution and relative abundances showed no obvious differences between the two host species. In contrast, distinct differences in relative symbiont abundances were observed between mussels from different sites, indicating that vent chemistry may influence the relative abundance of thiotrophs and methanotrophs in these dual symbioses.
[Show abstract][Hide abstract] ABSTRACT: The Quaternary climate cycles forced species to repeatedly migrate across a continually changing landscape. How these shifts in distribution impacted the evolution of unrelated but ecologically associated taxa has remained elusive due to the stochastic nature of the evolutionary process and variation in species-specific biological characteristics and environmental constraints. To account for the uncertainty in genealogical estimates, we adopted a coalescent approach for testing hypotheses of population divergence in coevolving taxa. We compared genealogies of a specialized herbivorous insect, Parnassius smintheus (Papilionidae), and its host plant, Sedum lanceolatum (Crassulaceae), from the alpine tundra of the Rocky Mountains to null distributions from coalescent simulations to test whether tightly associated taxa shared a common response to the paleoclimatic cycles. Explicit phylogeographic models were generated from geologic and biogeographic data and evaluated over a wide range of divergence times given calibrated mutation rates for both species. Our analyses suggest that the insect and its host plant responded similarly but independently to the climate cycles. By promoting habitat expansion and mixing among alpine populations, glacial periods repeatedly reset the distributions of genetic variation in each species and inhibited continual codivergence among pairs of interacting species.
[Show abstract][Hide abstract] ABSTRACT: The symbioses between invertebrates and chemosynthetic bacteria allow both host and symbiont to colonize and thrive in otherwise inhospitable deep-sea habitats. Given the global distribution of the bathymodioline symbioses, this association is an excellent model for evaluating co-speciation and evolution of symbioses. Thus far, the methanotroph and chemoautotroph endosymbionts of mussels are tightly clustered within two independent clades of gamma Proteobacteria, respectively. Further physiological and genomic studies will elucidate the ecological and evolutionary roles that these bacterial clades play in the symbiosis and chemosynthetic community. Due to the overall abundance of the methanotrophic symbioses at hydrothermal vents and hydrocarbon seeps, they likely play a significant, but as of yet unquantified, role in the biogeochemical cycling of methane. With this in mind, the search for methanotrophic symbioses should not be restricted to these known deep-sea habitats, but rather should be expanded to include methane-rich coastal marine and freshwater environments inhabited by methanotrophs and bivalves. Our current understanding of the bathymodioline symbioses provides a strong foundation for future explorations into the origin, ecology, and evolution of methanotroph symbioses, which are now becoming possible through a combination of classical and advanced molecular techniques.
Progress in molecular and subcellular biology 02/2006; 41:227-49.
[Show abstract][Hide abstract] ABSTRACT: Aim We inferred the phylogeography of the alpine butterfly Colias meadii Edwards (Pieridae) and compared its genetic structure with that of another high elevation, co-distributed butterfly, Parnassius smintheus Doubleday (Papilionidae), to test if the two Rocky Mountain butterflies responded similarly to the palaeoclimatic cycles of the Quaternary.Location Specimens were collected from 18 alpine sites in the Rocky Mountains of North America, from southern Colorado to northern Montana.Methods We sequenced 867 and 789 nucleotides of cytochrome oxidase I from an average of 19 and 20 individuals for C. meadii and P. smintheus, respectively, from each of the same 18 localities. From the sequence data, we calculated measures of genetic diversity within each population (H, θ), genetic divergence among populations (FST), and tested for geographic structure through an analysis of molecular variance (amova). Population estimates were compared against latitude and between species using a variety of statistical tests. Furthermore, nested clade analysis was implemented to infer historic events underlying the geographic distribution of genetic variation in each species. Then, we compared the number of inferred population events between species using a nonparametric Spearman's rank correlation test. Finally, we ran coalescent simulations on each species’ genealogy to test whether the two species of Lepidoptera fit the same model of population divergence.Results Our analyses revealed that: (1) measures of within-population diversity were not correlated with latitude for either species, (2) within-site diversity was not correlated between species, (3) within a species, nearly all populations were genetically isolated, (4) both species exhibited significant and nearly identical partitioning of genetic variation at all hierarchical levels of the amova, including a strong break between populations across the Wyoming Basin, (5) both species experienced similar cycles of expansion and contraction, although fewer were inferred for C. meadii, and (6) data from both species fit a model of three refugia diverging during the Pleistocene.Main conclusions While our findings supported a shared response of the two butterfly species to historic climate change across coarse spatial scales, a common pattern was not evident at finer spatial and temporal scales. The shared demographic history of the two species is consistent with an expanding–contracting archipelago model, suggesting that populations persisted across the geographic range throughout the climate cycles, experiencing isolation on ‘sky islands’ during interglacial periods and becoming connected as they migrated down-slope during cool, wet climates.
[Show abstract][Hide abstract] ABSTRACT: Climate change during the Quaternary played an important role in the differentiation and evolution of plants. A prevailing hypothesis is that alpine and arctic species survived glacial periods in refugia at the periphery of glaciers. Though the Rocky Mountains, south of the southernmost extent of continental ice, served as an important glacial refuge, little is known about how climate cycles influenced populations within this region. We inferred the phylogeography of Sedum lanceolatum (Crassulaceae) within the Rocky Mountain refugium to assess how this high-elevation plant responded to glacial cycles. We sequenced 884 base pairs (bp) of cpDNA intergenic spacers (tRNA-L to tRNA-F and tRNA-S to tRNA-G) for 333 individuals from 18 alpine populations. Our highly variable markers allowed us to infer that populations persisted across the latitudinal range throughout the climate cycles, exhibited significant genetic structure, and experienced cycles of range expansion and fragmentation. Genetic differentiation in S. lanceolatum was most likely a product of short-distance elevational migration in response to climate change, low seed dispersal, and vegetative reproduction. To the extent that Sedum is a good model system, paleoclimatic cycles were probably a major factor preserving genetic variation and promoting divergence in high-elevation flora of the Rocky Mountains.
American Journal of Botany 03/2005; 92(3):477-86. · 2.59 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Climate oscillations of the Quaternary drove the repeated expansion and contraction of ecosystems. Alpine organisms were probably isolated in sky island refugia during warm interglacials, such as now, and expanded their range by migrating down-slope during glacial periods. We used population genetic and phylogenetic approaches to infer how paleoclimatic events influenced the distribution of genetic variation in the predominantly alpine butterfly Parnassius smintheus. We sequenced a 789 bp region of cytochrome oxidase I for 385 individuals from 20 locations throughout the Rocky Mountains, ranging from southern Colorado to northern Montana. Analyses revealed at lease two centers of diversity in the northern and southern Rocky Mountains and strong population structure. Nested clade analysis suggested that the species experienced repeated cycles of population expansion and fragmentation. The estimated ages of these events, assuming a molecular clock, corresponded with paleoclimatic data on habitat expansion and contraction over the past 400,000 years. We propose that alpine butterflies persisted in an archipelago of isolated sky islands during interglacials and that populations expanded and became more connected during cold glacial periods. An archipelago model implies that the effects of genetic drift and selection varied among populations, depending on their latitude, area, and local environment. Alpine organisms are sensitive indicators of climate change and their history can be used to predict how high-elevation ecosystems might respond to further climate warming.
[Show abstract][Hide abstract] ABSTRACT: Abstract Climate oscillations of the Quaternary drove the repeated expansion and contraction of ecosystems. Alpine organisms were probably isolated in sky island refugia during warm interglacials, such as now, and expanded their range by migrating down-slope during glacial periods. We used population genetic and phylogenetic approaches to infer how paleoclimatic events influenced the distribution of genetic variation in the predominantly alpine butterfly Parnassius smintheus. We sequenced a 789 bp region of cytochrome oxidase I for 385 individuals from 20 locations throughout the Rocky Mountains, ranging from southern Colorado to northern Montana. Analyses revealed at lease two centers of diversity in the northern and southern Rocky Mountains and strong population structure. Nested clade analysis suggested that the species experienced repeated cycles of population expansion and fragmentation. The estimated ages of these events, assuming a molecular clock, corresponded with paleoclimatic data on habitat expansion and contraction over the past 400,000 years. We propose that alpine butterflies persisted in an archipelago of isolated sky islands during interglacials and that populations expanded and became more connected during cold glacial periods. An archipelago model implies that the effects of genetic drift and selection varied among populations, depending on their latitude, area, and local environment. Alpine organisms are sensitive indicators of climate change and their history can be used to predict how high-elevation ecosystems might respond to further climate warming.