Clustering of contact zones, hybrid zones, and phylogeographic breaks in North America

A recent test for the existence of suture zones in North America, based on hybrid zones studied since 1970, found support for only two of the 13 suture zones identified by Remington in 1968 (Swenson and Howard 2004). One limitation of that recent study was the relatively small number of hybrid zones available for mapping. In this study, we search for evidence of clustering of contact zones between closely related taxa using data not only from hybrid zones but from species range maps of trees, birds, and mammals and from the position of phylogeographic breaks within species. Digital geographic range maps and a geographic information system approach allowed for accurate and rapid mapping of distributional data. Areas of contact between closely related species and phylogeographic breaks within species clustered into areas characterized by common physiographic features or predicted by previously hypothesized glacial refugia. The results underscore the general importance of geographic barriers to dispersal (mountain chains) and climate change (periods of cooling alternating with periods of warming, which lead to the contraction and expansion of species ranges) in species evolution.     

Pleistocene climatic fluctuations drive isolation and secondary contact in the red diamond rattlesnake (Crotalus ruber) in Baja California

Aim

Many studies have investigated the phylogeographic history of species on the Baja California Peninsula, and they often show one or more genetic breaks that are spatially concordant among many taxa. These phylogeographic breaks are commonly attributed to vicariance as a result of geological or climatic changes, followed by secondary contact when barriers are no longer present. We use restriction‐site associated DNA sequence data and a phylogeographic model selection approach to explicitly test the secondary contact hypothesis in the red diamond rattlesnake, Crotalus ruber.

Location

Baja California and Southern California.

Methods

We used phylogenetic and population clustering approaches to identify population structure. We then used coalescent methods to simultaneously estimate population parameters and test the fit of phylogeographic models to the data. We used ecological niche models to infer suitable habitat for C. ruber at the Last Glacial Maximum (LGM).

Results

Crotalus ruber is composed of distinct northern and southern populations with a boundary near the town of Loreto in Baja California Sur. A model of isolation followed by secondary contact provides the best fit to the data, with both divergence and contact occurring in the Pleistocene. We also identify a genomic signature of northern range expansion in the northern population, consistent with LGM niche models showing that the northern‐most portion of the range of C. ruber was not suitable habitat during the LGM.

Main conclusions

We provide the first explicitly model‐based test of the secondary contact model in Baja California and show that populations of C. ruber were isolated before coming back into contact near Loreto, a region that shows phylogeographic breaks for other taxa. Given the timing of divergence and contact, we suggest that climatic fluctuations have driven the observed phylogeographic structure observed in C. ruber and that they may have driven similar patterns in other taxa.     

Phylogeographic breaks in low-dispersal species: the emergence of concordance across gene trees

Computer simulations were used to investigate population conditions under which phylogeographic breaks in gene genealogies can be interpreted with confidence to infer the existence and location of historical barriers to gene flow in continuously distributed, low-dispersal species. We generated collections of haplotypic gene trees under a variety of demographic scenarios and analyzed them with regard to salient genealogical breaks in their spatial patterns. In the first part of the analysis, we estimated the frequency in which the spatial location of the deepest phylogeographic break between successive pairs of populations along a linear habitat coincided with a spatial physical barrier to dispersal. Results confirm previous reports that individual gene trees can show ‘haphazard’ phylogeographic discontinuities even in the absence of historical barriers to gene flow. In the second part of the analysis, we assessed the probability that pairs of gene genealogies from a set of population samples agree upon the location of a geographical barrier. Our findings extend earlier reports by demonstrating that spatially concordant phylogeographic breaks across independent neutral loci normally emerge only in the presence of longstanding historical barriers to gene flow. Genealogical concordance across multiple loci thus becomes a deciding criterion by which to distinguish between stochastic and deterministic causation in accounting for phylogeographic discontinuities in continuously distributed species.     

Concordant genetic breaks, identified by combining clustering and tessellation methods, in two co-distributed alpine plant species

Natural genetic breaks may indicate limitations to gene flow or the presence of contact zones of previously isolated populations. Molecular evidence suggests that genetic breaks have aggregated in distinct geographical areas. We propose a new application of well-established statistical methods for analysing multilocus genetic data to identify intraspecific genetic breaks. The methodological approach combines Bayesian clustering with a spatially explicit maximum-difference algorithm to visualize and quantify breaks between clusters. We used amplified fragment length polymorphism data of two co-distributed, silicicolous alpine plant species, Geum montanum and Geum reptans , exhaustively sampled on a consistent, regular grid over their entire range of the European Alps and the Carpathians. We found a distinct and highly similar allocation of genetic breaks in both species. The pattern of breaks did not conform to scenarios of glacial refugial survival, alternatively in peripheral refugia or on nunataks, as expected because of the species' ecologies (late-successional low alpine vs. early-successional high alpine). Our findings rather substantiated the postulate of a general pattern of genetic breaks in alpine plants and corresponded well with biogeographical regions formerly described. Our approach could serve as a tool not only for landscape genetics and comparative phylogeography, but also for floristics or faunistics to compare biogeographic with phylogeographic breaks.     

Phylogeographically concordant chloroplast DNA divergence in sympatric Nothofagus s.s. How deep can it be?

? Here, we performed phylogenetic analyses and estimated the divergence times on mostly sympatric populations of five species within subgenus Nothofagus. We aimed to investigate whether phylogenetic relationships by nuclear internal transcribed spacer (ITS) and phylogeographic patterns by chloroplast DNA (cpDNA) mirror an ancient evolutionary history that was not erased by glacial eras. Extant species are restricted to Patagonia and share a pollen type that was formerly widespread in all southern land masses. Weak reproductive barriers exist among them. ? Fifteen cpDNA haplotypes resulted from the analysis of three noncoding regions on 330 individuals with a total alignment of 1794 bp. Nuclear ITS data consisted of 822 bp. We found a deep cpDNA divergence dated 32 Ma at mid-latitudes of Patagonia that predates the phylogenetic divergence of extant taxa. Other more recent breaks by cpDNA occurred towards the north. ? Complex paleogeographic features explain the genetic discontinuities. Long-lasting paleobasins and marine ingressions have impeded transoceanic dispersal during range expansion towards lower latitudes under cooler trends since the Oligocene. ? Cycles of hybridization-introgression among extant and extinct taxa have resulted in widespread chloroplast capture events. Our data suggest that Nothofagus biogeography will be resolved only if thorough phylogeographic analyses and molecular dating methods are applied using distinct genetic markers.     

Avian distribution patterns in the Guiana Shield: implications for the delimitation of Amazonian areas of endemism

Aim To develop a methodology for defining the boundaries of the Guianan area of endemism using complementary approaches that include GIS tools, multivariate statistics and analyses of physical barriers in the distribution patterns of an entire endemic avifauna. As a case study, I used the distribution patterns of lowland terra firme forest birds. Location Guiana Shield, northern South America. Methods I identified Guianan endemics using the ornithological literature, subsequently gathering distributional data for these taxa using mainly museum collections and my own fieldwork in the region. I used these distributional data to map the spatial patterns of endemicity in the region and to compare distributions across taxa. I employed community composition data from 34 localities from throughout the Guiana Shield to identify spatial patterns of clustering using an ordination analysis (non‐metric multidimensional scaling), and to recognize the region’s main biogeographical barriers for birds using Monmonier’s algorithm. Results At least 88 avian taxa are restricted to the terra firme forests of the Guianan area of endemism, which is roughly delimited by the Amazon, Negro and Branco rivers. These large rivers, however, are not the only boundaries. I identified seven additional barriers, including medium‐sized rivers, non‐forested areas and mountains, which also contribute to delimiting the area of endemism. Within the endemic avifauna, I identified three distinct distribution patterns. The ordination analysis shows the presence of two distinct avifaunas within the Guiana Shield. Main conclusions Although the proposed boundaries of the Guianan area of endemism are consistent with previously postulated configurations, this study reveals a more complex delimitation than formerly recognized, highlighting the importance of several landscape features besides large rivers, and the existence of three distinct distributional patterns within one endemic avifauna. The Branco/Negro interfluvium, often included within this area of endemism, actually represents a transition zone between two distinct avifaunas. The longstanding view of the Amazon Basin as a mosaic of parapatric areas of endemism delimited by major rivers appears to be an oversimplification, at least for the Guiana Shield. This finding suggests the need for more rigorous approaches to re‐evaluate the traditional boundaries of such areas.     

Phylogeographic concordance in the southeastern United States: the flatwoods salamander, Ambystoma cingulatum, as a test case

Well-supported, congruent phylogeographic and biogeographic patterns permit the development of a priori phylogeographic and distributional predictions. In the southeastern Coastal Plain of the United States, the common discovery of east-west disjunctions (phylogeographic breaks and species' distributional boundaries) suggests that similar disjunctions should occur in codistributed taxa. Despite the near ubiquity of these disjunctions, the most recent morphological analyses of the flatwoods salamander, Ambystoma cingulatum, indicate that none occur in this low-vagility, Coastal Plain endemic. We conducted molecular and morphological analyses to test whether the flatwoods salamander is an exception to this common biogeographic pattern. Assessing geographic variation in this species is also an important management tool for this threatened, declining amphibian. We demonstrate that flatwoods salamanders, as predicted by comparisons to codistributed taxa, are polytypic with a major disjunction at the Apalachicola River. This drainage is a common site for east-west phylogeographic breaks, probably because repeated marine embayments during the Pliocene and Pleistocene interglacials generated barriers to gene flow. Based on mitochondrial DNA, morphology, and allozymes, we recognize two species of flatwoods salamanders -- Ambystoma cingulatum to the east of the Apalachicola drainage and Ambystoma bishopi to the west. Given this increased diversity, the conservation status of these two taxa may warrant re-evaluation. More generally, these results emphasize that in the absence of taxon-specific data, established comparative patterns can provide strong expectations for designing management units for unstudied species of conservation concern.     

Phylogeographic breaks without geographic barriers to gene flow

Abstract.— The spatial distribution of genetic markers can be useful both in estimating patterns of gene flow and in reconstructing biogeographic history, particularly when gene genealogies can be estimated. Genealogies based on nonrecombining genetic units such as mitochondrial and chloroplast DNA often consist of geographically separated clades that come into contact in narrow regions. Such phylogeographic breaks are usually assumed to be the result of long-term barriers to gene flow. Here I show that deep phylogeographic breaks can form within a continuously distributed species even when there are no barriers to gene flow. The likelihood of observing phylogeographic breaks increases as the average individual dispersal distance and population size decrease. Those molecular markers that are most likely to show evidence of real geographic barriers are also most likely to show phylogeographic breaks that formed without any barrier to gene flow. These results might provide an explanation as to why some species, such as the greenish warblers ( Phylloscopus trochiloides ), have phylogeographic breaks in mitochondrial or chloroplast DNA that do not coincide with sudden changes in other traits.     

Phylogeography of a stream-dwelling frog (Pseudacris cadaverina) in southern California

Recent phylogeographic studies of animal taxa in California have revealed common geographic patterns of evolutionary divergence and genetic diversity that are generally attributable to landscape influences. However, there remains a paucity of knowledge on the evolution of freshwater taxa in southern California. Here, we investigate phylogeographic patterns in a stream-dwelling frog (Pseudacris cadaverina). Two hundred and twenty-one individuals were collected from 46 populations across the species’ range in southern California. Using 1100 bp of sequence data from cytochrome b and tRNA-Glu, we conducted phylogenetic analyses, analysis of molecular variance, and nested clade phylogeographic analysis to gain insight into the factors contributing to the distribution of genetic diversity in P. cadaverina. We tested for evidence of two putative phylogeographic breaks and tested hypotheses that genetic diversity in this species is partitioned into (1) major watersheds, (2) mountain ranges, and (3) coastal and desert regions. Our results suggest that the eastern Transverse Ranges are the center of origin for extant P. cadaverina lineages and that the observed genetic structure in this species was established during the Pleistocene Epoch. There is strong support for three major haplotype groups and a Transverse Range break in P. cadaverina that is concordant with breaks found in numerous other taxa. The distribution of genetic diversity in P. cadaverina is due in large part to the separation of populations into different major watersheds and mountain ranges. Gene flow appears to be generally limited among disjunct populations throughout the region and some desert populations have been isolated by historical habitat fragmentation.     

Idiosyncratic responses to climate‐driven forest fragmentation and marine incursions in reed frogs from Central Africa and the Gulf of Guinea Islands

Organismal traits interact with environmental variation to mediate how species respond to shared landscapes. Thus, differences in traits related to dispersal ability or physiological tolerance may result in phylogeographic discordance among co‐distributed taxa, even when they are responding to common barriers. We quantified climatic suitability and stability, and phylogeographic divergence within three reed frog species complexes across the Guineo‐Congolian forests and Gulf of Guinea archipelago of Central Africa to investigate how they responded to a shared climatic and geological history. Our species‐specific estimates of climatic suitability through time are consistent with temporal and spatial heterogeneity in diversification among the species complexes, indicating that differences in ecological breadth may partly explain these idiosyncratic patterns. Likewise, we demonstrated that fluctuating sea levels periodically exposed a land bridge connecting Bioko Island with the mainland Guineo‐Congolian forest and that habitats across the exposed land bridge likely enabled dispersal in some species, but not in others. We did not find evidence that rivers are biogeographic barriers across any of the species complexes. Despite marked differences in the geographic extent of stable climates and temporal estimates of divergence among the species complexes, we recovered a shared pattern of intermittent climatic suitability with recent population connectivity and demographic expansion across the Congo Basin. This pattern supports the hypothesis that genetic exchange across the Congo Basin during humid periods, followed by vicariance during arid periods, has shaped regional diversity. Finally, we identified many distinct lineages among our focal taxa, some of which may reflect incipient or unrecognized species.     

Impact of ocean barriers, topography, and glaciation on the phylogeography of the catfish Trichomycterus areolatus (Teleostei: Trichomycteridae) in Chile

We examined the role of several earth history events on the phylogeographic distribution of the catfish Trichomycterus areolatus in Chile using the cytochrome b gene. We explored three biogeographic hypotheses: that sea level changes have resulted in the isolation of populations by drainages; that glaciation has impacted genetic diversity; and that ichthyological subprovince boundaries correspond to phylogeographic breaks in our focal species. We found seven well-supported clades within T. areolatus with high levels of genetic divergence. The strongest signal in our data was for an important role of sea level changes structuring populations. Five of the seven clades mapped cleanly to the geographic landscape and breaks corresponded closely to areas of narrowest continental shelf. In addition, few haplotypes were shared between rivers within clades, suggesting that only limited local movement of individuals has occurred. There was no relationship between the levels of genetic diversity and the proportion of individual drainages covered by glaciers during the last glacial maximum. Two phylogeographic breaks within T. areolatus did match the two previously identified faunal boundaries, but we found three additional breaks, which suggests that faunal breaks have only limited utility in explaining phylogeographic patterns. These results imply that the narrow continental shelf coupled with sea level changes had a strong influence on the obligate freshwater fishes in Chile.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 876–892.     

Lines in the land: a review of evidence for eastern Australia's major biogeographical barriers to closed forest taxa

The influence of climatic changes occurring since the late Miocene on Australia's eastern mesic ecosystems has received significant attention over the past 20 years. In particular, the impact of the dramatic shift from widespread rainforest habitat to a much drier landscape in which closed forest refugia were dissected by open woodland/savannah ecosystems has long been a focal point in Australian ecology and biogeography. Several specific regions along the eastern coast have been identified previously as potentially representing major biogeographical disjunctions for closed forest taxa. Initially, evidence stemmed from recognition of common zones where avian species/subspecies distributions and/or floral communities were consistently separated, but the body of work has since grown significantly with the rise of molecular phylogeographic tools and there is now a significant literature base that discusses the drivers, processes and effects of these hypothesised major biogeographical junctions (termed barriers). Here, we review the literature concerning eight major barriers argued to have influenced closed forest taxa; namely, the Laura Basin, Black Mountain Corridor, Burdekin Gap, Saint Lawrence Gap, Brisbane Valley Barrier, Hunter Valley Barrier, Southern Transition Zone and East Gippsland Barrier. We synthesise reported phylogeographical patterns and the inferred timing of influence with current climatic, vegetation and geological characteristics for each barrier to provide insights into regional evolution and seek to elicit common trends. All eight putative biogeographical barriers are characterised currently by lowland zones of drier, warmer, more open woodland and savannah habitat, with adjacent closed forest habitats isolated to upland cool, wet refugia. Molecular divergence estimates suggest two pulses of divergence, one in the early Miocene (~20–15 Mya) and a later one from the Pliocene–Pleistocene (~6–0.04 Mya). We conclude with a prospectus for future research on the eastern Australian closed forests and highlight critical issues for ongoing studies of biogeographical barriers worldwide.     

Complex evolutionary history of a Neotropical lowland forest bird (Lepidothrix coronata) and its implications for historical hypotheses of the origin of Neotropical avian diversity

Here we apply a combination of phylogeographic and historical demographic analyses to the study of mtDNA sequence variation within the Blue-crowned Manakin (Lepidothrix coronata), a widespread Neotropical bird. A high degree of phylogeographic structure allowed us to demonstrate that several vicariant events, including Andean uplift, the formation of riverine barriers, and climatically induced vegetational shifts, as well as a non-vicariant process, range expansion, have all acted, at varying spatial and temporal scales, to influence genetic structure within L. coronata, suggesting that current historical hypotheses of the origin of Neotropical avian diversity that focus on single vicariant mechanisms may be overly simplistic. Our data also support an origin (>2 mybp) that is substantially older than the late Pleistocene for the genetic structure within this species and indicate that phylogeographic patterns within the species are not concordant with plumage-based subspecific taxonomy. These data add to a growing body of evidence suggesting that the origin of several Neotropical avian species may have occurred in the mid-Pliocene, thus, geological arguments surrounding putative Pleistocene vicariant events, while interesting in their own right, may have little relevance to Neotropical avian diversification at the species level.     

Landscape attributes governing local transmission of an endemic zoonosis: Rabies virus in domestic dogs

Landscape heterogeneity plays an important role in disease spread and persistence, but quantifying landscape influences and their scale dependence is challenging. Studies have focused on how environmental features or global transport networks influence pathogen invasion and spread, but their influence on local transmission dynamics that underpin the persistence of endemic diseases remains unexplored. Bayesian phylogeographic frameworks that incorporate spatial heterogeneities are promising tools for analysing linked epidemiological, environmental and genetic data. Here, we extend these methodological approaches to decipher the relative contribution and scale‐dependent effects of landscape influences on the transmission of endemic rabies virus in Serengeti district, Tanzania (area ~4,900 km²). Utilizing detailed epidemiological data and 152 complete viral genomes collected between 2004 and 2013, we show that the localized presence of dogs but not their density is the most important determinant of diffusion, implying that culling will be ineffective for rabies control. Rivers and roads acted as barriers and facilitators to viral spread, respectively, and vaccination impeded diffusion despite variable annual coverage. Notably, we found that landscape effects were scale‐dependent: rivers were barriers and roads facilitators on larger scales, whereas the distribution of dogs was important for rabies dispersal across multiple scales. This nuanced understanding of the spatial processes that underpin rabies transmission can be exploited for targeted control at the scale where it will have the greatest impact. Moreover, this research demonstrates how current phylogeographic frameworks can be adapted to improve our understanding of endemic disease dynamics at different spatial scales.     

Identifying biases at different spatial and temporal scales of diversification: a case study in the Neotropical parrotlet genus Forpus

The temporal origins of the extraordinary biodiversity of the Neotropical region are highly debated. Recent empirical work has found support for alternative models on the tempo of speciation in Neotropical species further fuelling the debate. However, relationships within many Neotropical lineages are poorly understood, and it is unclear how this uncertainty impacts inferences on the evolution of taxa in the region. We examined the robustness of diversification patterns in the avian genus Forpus by testing whether the use of different units of biodiversity (i.e. biological species and statistically inferred species) impacted diversification rates and inferences regarding important biogeographic breaks in the genus. We found that the best‐fit model of diversification for the biological species data set was a declining rate of diversification; whereas a model of constant diversification was the best‐fit model for statistically inferred species or subspecies. Moreover, the relative importance of different landscape features in delimiting genetic structure across the landscape varied across data sets with differing units of biodiversity. Patterns based on divergence times among biological species indicated old speciation events across major geographic and river barriers. In contrast, data sets more inclusive of the diversity in Forpus illustrate the role of both old divergence across major landscape features and more recent divergences that are possibly attributed to Pleistocene climatic changes. Overall, these results indicate that conflicting models on the temporal origins of Neotropical birds may be attributable to sampling biases.     

Large rivers do not always act as species barriers for Lepilemur sp.

Sportive lemurs constitute a highly diverse endemic lemur family (24 species) for which many biogeographic boundaries are not yet clarified. Based on recent phylogeographic models, this study aims to determine the importance of two large rivers (the Antainambalana and Rantanabe) in northeastern Madagascar as species barriers for Lepilemur seali. The Antainambalana River was previously assumed to act as the southern border of its distribution. A total of 1,038 bp of the mtDNA of four individuals stemming from two adjacent inter-river systems south of the Antainambalana River was sequenced and compared to sequences of 22 described Lepilemur species. The phylogenetic reconstruction did not find support for either of the two rivers as species barrier for Lepilemur, as all captured individuals clustered closely with and therefore belonged to L. seali. However, a previously published sequence of an individual from a site south of our study sites belongs to a separate species. The southern boundary of L. seali must therefore be one of two large rivers further south of our study sites. The results suggest that L. seali may possess a relatively large altitudinal range that enabled this species to migrate around the headwaters of the Antainambalana and Rantanabe Rivers. Previous phylogeographic models need to be refined in order to incorporate these findings, and more species-specific altitudinal range data are urgently needed in order to fully understand the biogeographic patterns of lemurs on Madagascar.     

Phylogeographic concordance factors quantify phylogeographic congruence among co‐distributed species in the Sarracenia alata pitcher plant system

Comparative phylogeographic investigations have identified congruent phylogeographic breaks in co‐distributed species in nearly every region of the world. The qualitative assessments of phylogeographic patterns traditionally used to identify such breaks, however, are limited because they rely on identifying monophyletic groups across species and do not account for coalescent stochasticity. Only long‐standing phylogeographic breaks are likely to be obvious; many species could have had a concerted response to more recent landscape events, yet possess subtle signs of phylogeographic congruence because ancestral polymorphism has not completely sorted. Here, we introduce Phylogeographic Concordance Factors (PCFs), a novel method for quantifying phylogeographic congruence across species. We apply this method to the Sarracenia alata pitcher plant system, a carnivorous plant with a diverse array of commensal organisms. We explore whether a group of ecologically associated arthropods have co‐diversified with the host pitcher plant, and identify if there is a positive correlation between ecological interaction and PCFs. Results demonstrate that multiple arthropods share congruent phylogeographic breaks with S. alata, and provide evidence that the level of ecological association can be used to predict the degree of similarity in the phylogeographic pattern. This study outlines an approach for quantifying phylogeographic congruence, a central concept in biogeographic research.     

Complex phylogeography and historical hybridization between sister taxa of freshwater sculpin (Cottus)

Species ranges that span different geographic landscapes frequently contain cryptic species‐ or population‐level structure. Identifying these possible diversification factors can often be accomplished under a comparative phylogeographic framework. However, comparisons suffer if previous studies are limited to a particular group or habitat type. In California, a complex landscape has led to several phylogeographic breaks, primarily in terrestrial species. However, two sister taxa of freshwater fish, riffle sculpin (Cottus gulosus) and Pit sculpin (Cottus pitensis), display ranges based on morphological identifications that do not coincide with these breaks. Using a comprehensive sampling and nuclear, mitochondrial and microsatellite markers, we hypothesized that proposed species ranges are erroneous based on potential hybridization/gene flow between species. Results identified a phylogeographic signature consistent with this hypothesis, with breaks at the Coast Range Mountains and Sacramento/San Joaquin River confluence. Coastal locations of C. gulosus represent a unique lineage, and ‘true’ C. gulosus were limited to the San Joaquin basin, both regions under strong anthropogenic influence and potential conservation targets. C. pitensis limits extended historically throughout the Sacramento/Pit River basin but currently are restricted to the Pit River. Interestingly, locations in the Sacramento River contained low levels of ancestral hybridization and gene flow from C. gulosus but now appear to be a distinct population. The remaining population structure was strongly correlated with Sierra Nevada presence (high) or absence (low). This study stresses the importance of testing phylogeographic breaks across multiple taxa/habitats before conservation decisions are made, but also the potential impact of different geographic landscapes on evolutionary diversification.     

Fine-scale genetic breaks driven by historical range dynamics and ongoing density-barrier effects in the estuarine seaweed Fucus ceranoides L

ABSTRACT: BACKGROUND: Factors promoting the emergence of sharp phylogeographic breaks include restricted dispersal, habitat discontinuity, physical barriers, disruptive selection, mating incompatibility, genetic surfing and secondary contact. Disentangling the role of each in any particular system can be difficult, especially when species are evenly distributed across transition zones and dispersal barriers are not evident. The estuarine seaweed Fucus ceranoides provides a good example of highly differentiated populations along its most persistent distributional range at the present rear edge of the species distribution, in NW Iberia. Intrinsic dispersal restrictions are obvious in this species, but have not prevented F. ceranoides from vastly expanding its range northwards following the last glaciation, implying that additional factors are responsible for the lack of connectivity between neighbouring southern populations. In this study we analyze 22 consecutive populations of F. ceranoides along NW Iberia to investigate the processes generating and maintaining the observed high levels of regional genetic divergence. RESULTS: Variation at seven microsatellite loci and at mtDNA spacer sequences was concordant in revealing that Iberian F. ceranoides is composed of three divergent genetic clusters displaying nearly disjunct geographical distributions. Structure and AFC analyses detected two populations with an admixed nuclear background. Haplotypic diversity was high in the W sector and very low in the N sector. Within each genetic cluster, population structure was also pervasive, although shallower. CONCLUSIONS: The deep divergence between sectors coupled with the lack of support for a role of oceanographic barriers in defining the location of breaks suggested 1) that the parapatric genetic sectors result from the regional reassembly of formerly vicariant sub-populations, and 2) that the genetic discontinuities at secondary contact zones (and elsewhere) are maintained despite normal migration rates. We conclude that colonization and immigration, as sources of gene-flow, have very different genetic effects. Migration between established populations is effectively too low to prevent their differentiation by drift or to smooth historical differences inherited from the colonization process. F. ceranoides, but possibly low-dispersal species in general, appear to be unified to a large extent by historical, non-equilibrium processes of extinction and colonization, rather than by contemporary patterns of gene flow.     

Using coalescent simulations to test the impact of quaternary climate cycles on divergence in an alpine plant-insect association

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.