Land Bridge (land + bridge)

Distribution by Scientific Domains


Selected Abstracts


RAPID ADAPTIVE DIVERGENCE IN NEW WORLD ACHILLEA, AN AUTOPOLYPLOID COMPLEX OF ECOLOGICAL RACES

EVOLUTION, Issue 3 2008
Justin Ramsey
Adaptive evolution is often associated with speciation. In plants, however, ecotypic differentiation is common within widespread species, suggesting that climatic and edaphic specialization can outpace cladogenesis and the evolution of postzygotic reproductive isolation. We used cpDNA sequence (5 noncoding regions, 3.5 kb) and amplified fragment length polymorphisms (AFLPs: 4 primer pairs, 1013 loci) to evaluate the history of ecological differentiation in the North American Achillea millefolium, an autopolyploid complex of "ecological races" exhibiting morphological, physiological, and life-history adaptations to diverse environments. Phylogenetic analyses reveal North American A. millefolium to be a monophyletic group distinct from its European and Asian relatives. Based on patterns of sequence divergence, as well as fossil and paleoecological data, colonization of North America appears to have occurred via the Bering Land Bridge during the Pleistocene (1.8 MYA to 11,500 years ago). Population genetic analyses indicate negligible structure within North American A. millefolium associated with varietal identity, geographic distribution, or ploidy level. North American populations, moreover, exhibit the signature of demographic expansion. These results affirm the "ecotype" concept of the North American Achillea advocated by classical research and demonstrate the rapid rate of ecological differentiation that sometimes occurs in plants. [source]


THE HISTORY OF A NEARCTIC COLONIZATION: MOLECULAR PHYLOGENETICS AND BIOGEOGRAPHY OF THE NEARCTIC TOADS (BUFO)

EVOLUTION, Issue 11 2004
Gregory B. Pauly
Abstract Previous hypotheses of phylogenetic relationships among Nearctic toads (Bufonidae) and their congeners suggest contradictory biogeographic histories. These hypotheses argue that the Nearctic Bufo are: (1) a polyphyletic assemblage resulting from multiple colonizations from Africa; (2) a paraphyletic assemblage resulting from a single colonization event from South America with subsequent dispersal into Eurasia; or (3) a monophyletic group derived from the Neotropics. We obtained approximately 2.5 kb of mitochondrial DNA sequence data for the 12S, 16S, and intervening valine tRNA gene from 82 individuals representing 56 species and used parametric bootstrapping to test hypotheses of the biogeographic history of the Nearctic Bufo. We find that the Nearctic species of Bufo are monophyletic and nested within a large clade of New World Bufo to the exclusion of Eurasian and African taxa. This suggests that Nearctic Bufo result from a single colonization from the Neotropics. More generally, we demonstrate the utility of parametric bootstrapping for testing alternative biogeographic hypotheses. Through parametric bootstrapping, we refute several previously published biogeographic hypotheses regarding Bufo. These previous studies may have been influenced by homoplasy in osteological characters. Given the Neotropical origin for Nearctic Bufo, we examine current distributional patterns to assess whether the Nearctic-Neotropical boundary is a broad transition zone or a narrow boundary. We also survey fossil and paleogeographic evidence to examine potential Tertiary and Cretaceous dispersal routes, including the Paleocene Isthmian Link, the Antillean and Aves Ridges, and the current Central American Land Bridge, that may have allowed colonization of the Nearctic. [source]


Floristic turnover in Iceland from 15 to 6 Ma , extracting biogeographical signals from fossil floral assemblages

JOURNAL OF BIOGEOGRAPHY, Issue 9 2007
Fri­geir GrÝmsson
Abstract Aim, This study aims to document the floristic changes that occurred in Iceland between 15 and 6 Ma and to establish the dispersal mechanisms for the plant taxa encountered. Using changing patterns of dispersal, two factors controlling floristic changes are tested. Possible factors are (1) climate change, and (2) the changing biogeography of Iceland over the time interval studied; that is, the presence or absence of a Miocene North Atlantic Land Bridge. Location, The North Atlantic. Methods, Species lists of fossil plants from Iceland in the time period 15 to 6 Ma were compiled using published data and new data. Closest living analogues were used to establish dispersal properties for the fossil taxa. Dispersal mechanisms of fossil plants were then used to reconstruct how Iceland was colonized during various periods. Results, Miocene floras of Iceland (15,6 Ma) show relatively high floristic turnover from the oldest floras towards the youngest; and few taxa from the oldest floras persist in the younger floras. The frequencies of the various dispersal mechanisms seen in the 15-Ma floras are quite different from those recorded in the 6-Ma floras, and there is a gradual change in the prevailing mode of dispersal from short-distance anemochory and dyschory to long-distance anemochory. Two mechanisms can be used to explain changing floral composition: (1) climate change, and (2) the interaction between the dispersal mechanisms of plants and the increasing isolation of proto-Iceland during the Miocene. Main conclusions, Dispersal mechanisms can be used to extract palaeogeographic signals from fossil floras. The composition of floras and dispersal mechanisms indicate that Iceland was connected both to Greenland and to Europe in the early Middle Miocene, allowing transcontinental migration. The change in prevalence of dispersal modes from 15 to 6 Ma appears to reflect the break-up of a land bridge and the increasing isolation of Iceland after 12 Ma. Concurrent gradual cooling and isolation caused changes in species composition. Specifically, the widening of the North Atlantic Ocean prevented taxa with limited dispersal capability from colonizing Iceland, while climate cooling led to the extinction of thermophilous taxa. [source]


A comparison of taiga flora in north-eastern Russia and Alaska/Yukon

JOURNAL OF BIOGEOGRAPHY, Issue 7 2003
David K. Swanson
Abstract Aim, To understand the similarities and differences between the taiga floras of far north-eastern Asia and north-western North America in the light of their Tertiary and Quaternary histories. Does the taiga flora follow the tundra pattern (Asian,American commonality of species as a result of continuity through the Quaternary), the temperate forest pattern (distinct species because of late Tertiary disjunction), a combination of these two patterns, or some pattern unique to the taiga? Location, The taiga regions of interior Alaska and the Yukon in North America (the ,Alaskan taiga'), and the Kolyma and eastern Indigirka River basins in Russia (the ,Kolyma taiga'). The study areas include both forested and unforested habitats below elevational treeline. The two regions have similar climate and topography and were linked via the Bering Land Bridge in the Tertiary and for several extended periods during Quaternary cold periods. Methods, Systematic comparison of the vascular floras of the two regions from published sources; and review of palaeoecological literature for the region. Results, Of the 796 species found in the study areas, 27% occur only in the Alaskan taiga, 35% occur only in the Kolyma taiga, and 38% occur in both the regions. The following subsets of species show a high proportion of species in common between the study areas (subsets are not mutually exclusive): plants that occur on the tundra and the taiga, non-flowering plants, abundant taiga understory plants, and wetland and aquatic plants. A lower proportion of shared plants was noted for warm, south-facing steppe communities. No tree species are common to both areas. Main conclusions, The Bering Strait region in the Quaternary has acted as a biogeographical filter for taiga plants. Significant divergence between northeast Asia and northwest North America has developed among the more southerly ranging fraction of the flora (e.g. trees), while the more cosmopolitan and the most cold-adapted elements of the taiga flora are common to both areas. Many plants in the former group have been disjunct between Asia and North America for millions of years, while many plants in the latter group have probably maintained continuity between the study areas via the Bering Land Bridge through much of the late Tertiary and Quaternary periods. Repeated extirpation of the less cold-adapted species from both study areas during Pleistocene cold periods has probably enhanced floristic differences between the two regions. [source]


Evidence for an ice-free Wrangel Island, northeast Siberia during the Last Glacial Maximum

BOREAS, Issue 3 2005
LYN GUALTIERI
10Be and 26Al surface exposure ages from 22 tors and bedrock samples from Wrangel Island, northeast Siberia, indicate that the East Siberian and Chukchi shelves were ice-free during the Last Glacial Maximum (LGM). The paucity of glacial landforms and deposits, the absence of erratics and the presence of radiocarbon dates on plant and mammal fossils that span the LGM suggest that Wrangel Island also remained free of extensive glacial ice during the LGM. The lack of moisture due to the continental climate on the emergent Bering Land Bridge is the most likely reason for limited ice in this part of the Arctic. Alternative interpretations regarding the age and origin of ,glaciogenic' bedforms on the Chukchi shelf should be considered. [source]


The Great American Biotic Interchange revisited

ECOGRAPHY, Issue 2 2010
Sarah Cody
The "Great American Biotic Interchange" (GABI) is regarded as a defining event in the biogeography of the Americas. It is hypothesized to have occurred when the Isthmus of Panama closed ca three million years ago (Ma), ending the isolation of South America and permitting the mixing of its biota with that of North America. This view of the GABI is based largely upon the animal fossil record, but recent molecular biogeographic studies of plants that show repeated instances of long-distance dispersal over major oceanic barriers suggest that perhaps the land bridge provided by the isthmus may have been less necessary for plant migration. Here we show that plants have significantly earlier divergence time estimates than animals for historical migration events across the Isthmus of Panama region. This difference in timing indicates that plants had a greater propensity for dispersal over the isthmus before its closure compared with animals. The GABI was therefore asynchronous for plants and animals, which has fundamental implications for the historical assembly of tropical biomes in the most species-rich forests on the planet. [source]


The profound influence of the Late Pliocene Panamanian uplift on the exchange, diversification, and distribution of New World birds

ECOGRAPHY, Issue 2 2010
Brian Tilston Smith
Separated throughout most of the Cenozoic era, North and South America were joined during the mid-Pliocene when the uplift of Panama formed a land bridge between these two continents. The fossil record indicates that this connection allowed an unprecedented degree of inter-continental exchange to occur between unique, previously isolated biotic assemblages, a phenomenon now recognized as the "Great American Biotic Interchange". However, a relatively poor avian fossil record has prevented our understanding the role of the land bridge in shaping New World avian communities. To address the question of avian participation in the GABI, we compiled 64 avian phylogenetic studies and applied a relaxed molecular clock to estimate the timing of trans-isthmus diversification events. Here, we show that a significant pulse of avian interchange occurred in concert with the isthmus uplift. The avian exchange was temporally consistent with the well understood mammalian interchange, despite the presumed greater vagility of birds. Birds inhabiting a variety of habitats and elevational zones responded to the newly available corridor. Within the tropics, exchange was equal in both directions although between extratropical and tropical regions it was not. Avian lineages with Nearctic origins have repeatedly invaded the tropics and radiated throughout South America; whereas, lineages with South American tropical origins remain largely restricted to the confines of the Neotropical region. This previously unrecognized pattern of asymmetric niche conservatism may represent an important and underappreciated contributor to the latitude diversity gradient. [source]


Floristic turnover in Iceland from 15 to 6 Ma , extracting biogeographical signals from fossil floral assemblages

JOURNAL OF BIOGEOGRAPHY, Issue 9 2007
Fri­geir GrÝmsson
Abstract Aim, This study aims to document the floristic changes that occurred in Iceland between 15 and 6 Ma and to establish the dispersal mechanisms for the plant taxa encountered. Using changing patterns of dispersal, two factors controlling floristic changes are tested. Possible factors are (1) climate change, and (2) the changing biogeography of Iceland over the time interval studied; that is, the presence or absence of a Miocene North Atlantic Land Bridge. Location, The North Atlantic. Methods, Species lists of fossil plants from Iceland in the time period 15 to 6 Ma were compiled using published data and new data. Closest living analogues were used to establish dispersal properties for the fossil taxa. Dispersal mechanisms of fossil plants were then used to reconstruct how Iceland was colonized during various periods. Results, Miocene floras of Iceland (15,6 Ma) show relatively high floristic turnover from the oldest floras towards the youngest; and few taxa from the oldest floras persist in the younger floras. The frequencies of the various dispersal mechanisms seen in the 15-Ma floras are quite different from those recorded in the 6-Ma floras, and there is a gradual change in the prevailing mode of dispersal from short-distance anemochory and dyschory to long-distance anemochory. Two mechanisms can be used to explain changing floral composition: (1) climate change, and (2) the interaction between the dispersal mechanisms of plants and the increasing isolation of proto-Iceland during the Miocene. Main conclusions, Dispersal mechanisms can be used to extract palaeogeographic signals from fossil floras. The composition of floras and dispersal mechanisms indicate that Iceland was connected both to Greenland and to Europe in the early Middle Miocene, allowing transcontinental migration. The change in prevalence of dispersal modes from 15 to 6 Ma appears to reflect the break-up of a land bridge and the increasing isolation of Iceland after 12 Ma. Concurrent gradual cooling and isolation caused changes in species composition. Specifically, the widening of the North Atlantic Ocean prevented taxa with limited dispersal capability from colonizing Iceland, while climate cooling led to the extinction of thermophilous taxa. [source]


Absence of post-Miocene Red Sea land bridges: biogeographic implications

JOURNAL OF BIOGEOGRAPHY, Issue 6 2006
Carlos A. Fernandes
Abstract In a large number of studies concerned with species movements between Africa and Eurasia, including the migrations of hominids out of Africa, a frequently-cited dispersal route is across a hypothetical land bridge in the southern Red Sea, which is suggested to have emerged during glacial sea-level lowstands. This paper, however, unequivocally demonstrates that palaeoceanographic and palaeoecological data are incompatible with the existence of Red Sea land bridges since the Miocene. The case is made by presenting the first quantitative history of water depth above the Red Sea sill for the last 470,000 years, a time period that includes the four most recent glacial,interglacial cycles, and by discussing the predictable consequences of any land bridge formation on the Red Sea sedimentary and microfossil records. The absence of post-Miocene Red Sea land bridges has extensive implications for biogeographic models in the Afro-Arabian region. Genetic, morphometric and palaeontological patterns reported in the literature cannot be related to dispersals over a land bridge, or in the case of marine organisms, separation of the Red Sea from the Indian Ocean by a land bridge. If such patterns in terrestrial species are only congruent with a southern Red Sea dispersal route, then they need to be considered in terms of sweepstake rafting, anthropogenic introduction, or in the particular case of the Out-of-Africa migration by modern humans, seafaring. The constraints imposed by our palaeoenvironmental record on biogeographic reconstructions within and around the Red Sea will hopefully encourage both the review of previous works and the preference for multidisciplinary approaches in future studies. [source]


Phylogenetics and biogeography of eastern Asian,North American disjunct genus Pachysandra (Buxaceae) inferred from nucleotide sequences

JOURNAL OF SYSTEMATICS EVOLUTION, Issue 3 2009
Zhihua JIAO
Abstract Pachysandra is an eastern Asian,North American disjunct genus with three species, two in eastern Asia (Pachysandra axillaris and Pachysandra terminalis) and one in eastern North America (Pachysandra procumbens). Although morphological and cytological studies suggest a close affinity of P. procumbens with P. axillaris, molecular data from nuclear and chloroplast DNA regions have provided conflicting signals. In this study, we tested previous phylogenetic hypotheses using sequences of nuclear ribosomal DNA internal transcribed spacers and chloroplast ndhF gene from multiple individuals of each of the three species. We also estimated the time of divergence between eastern Asia and eastern North America. Our results support the morphological and cytological conclusion that P. procumbens is more closely related to P. axillaris than to P. terminalis. The estimated time of divergence of P. axillaris and P. procumbens was 14.6▒5.5 mya, consistent with estimates from many other eastern Asian,North American disjunct genera. The migration of Pachysandra populations from eastern Asia to North America might have occurred by way of the North Atlantic land bridge. [source]


Sea-level changes and palaeo-ranges: reconstruction of ancient shorelines and river drainages and the phylogeography of the Australian land crayfish Engaeus sericatus Clark (Decapoda: Parastacidae)

MOLECULAR ECOLOGY, Issue 24 2008
MARK B. SCHULTZ
Abstract Historical sea levels have been influential in shaping the phylogeography of freshwater-limited taxa via palaeodrainage and palaeoshoreline connections. In this study, we demonstrate an approach to phylogeographic analysis incorporating historical sea-level information in a nested clade phylogeographic analysis (NCPA) framework, using burrowing freshwater crayfish as the model organism. Our study area focuses on the Bass Strait region of southeastern Australia, which is marine region encompassing a shallow seabed that has emerged as a land bridge during glacial cycles connecting mainland Australia and Tasmania. Bathymetric data were analysed using Geographical Information Systems (GIS) to delineate a palaeodrainage model when the palaeocoastline was 150 m below present-day sea level. Such sea levels occurred at least twice in the past 500 000 years, perhaps more often or of larger magnitude within the last 10 million years, linking Victoria and Tasmania. Inter-locality distance measures confined to the palaeodrainage network were incorporated into an NCPA of crayfish (Engaeus sericatus Clark 1936) mitochondrial 16S rDNA haplotypes. The results were then compared to NCPAs using present-day river drainages and traditional great-circle distance measures. NCPA inferences were cross-examined using frequentist and Bayesian procedures in the context of geomorphological and historical sea-level data. We found distribution of present-day genetic variation in E. sericatus to be partly explained not only by connectivity through palaeodrainages but also via present-day drainages or overland (great circle) routes. We recommend that future studies consider all three of these distance measures, especially for studies of coastally distributed species. [source]


Phylogeography of the olive sea snake, Aipysurus laevis (Hydrophiinae) indicates Pleistocene range expansion around northern Australia but low contemporary gene flow

MOLECULAR ECOLOGY, Issue 16 2007
V. LUKOSCHEK
Abstract Pleistocene sea-level fluctuations profoundly changed landmass configurations around northern Australia. The cyclic emergence of the Torres Strait land bridge and concomitant shifts in the distribution of shallow-water marine habitats repeatedly sundered east and west coast populations. These biogeographical perturbations invoke three possible scenarios regarding the directions of interglacial range expansion: west to east, east to west, or bidirectional. We evaluated these scenarios for the olive sea snake, Aipysurus laevis, by exploring its genetic structure around northern Australia based on 354 individuals from 14 locations in three regions (Western Australia, WA; Gulf of Carpentaria, GoC; Great Barrier Reef, GBR). A 726-bp fragment of the mitochondrial DNA ND4 region revealed 41 variable sites and 38 haplotypes, with no shared haplotypes among the three regions. Population genetic structure was strong overall, ,ST = 0.78, P < 0.001, and coalescent analyses revealed no migration between regions. Genetic diversity was low in the GBR and GoC and the genetic signatures of these regions indicated range or population expansions consistent with their recent marine transgressions around 7000 years ago. By contrast, genetic diversity on most WA reefs was higher and there were no signals of recent expansion events on these reefs. Phylogenetic analyses indicated that GBR and GoC haplotypes were derived from WA haplotypes; however, statistical parsimony suggested that recent range expansion in the GBR-GoC probably occurred from east coast populations, possibly in the Coral Sea. Levels of contemporary female-mediated gene flow varied within regions and reflected potential connectivity among populations afforded by the different regional habitat types. [source]


Chloroplast DNA phylogeography reveals colonization history of a Neotropical tree, Cedrela odorata L., in Mesoamerica

MOLECULAR ECOLOGY, Issue 6 2003
S. Cavers
Abstract Spanish Cedar (Cedrela odorata L.) is a globally important timber species which has been severely exploited in Mesoamerica for over 200 years. Using polymerase chain reaction,restriction fragment length polymorphisms, its chloroplast (cp) DNA phylogeography was studied in Mesoamerica with samples from 29 populations in six countries. Five haplotypes were characterized, phylogenetically grouped into three lineages (Northern, Central and Southern). Spatial analysis of ordered genetic distance confirmed deviation from a pattern of isolation by distance. The geographically proximate Northern and Central cpDNA lineages were genetically the most differentiated, with the Southern lineage appearing between them on a minimum spanning tree. However, populations possessing Southern lineage haplotypes occupy distinct moist habitats, in contrast to populations possessing Northern and Central lineage haplotypes which occupy drier and more seasonal habitats. Given the known colonization of the proto-Mesoamerican peninsula by South American flora and fauna prior to the formation of the Isthmus of Panama, it seems most likely that the observed population structure in C. odorata results from repeated colonization of Mesoamerica from South American source populations. Such a model would imply an ancient, pre-Isthmian colonization of a dry-adapted type (possessing the Northern lineage or a prototype thereof), with a secondary colonization via the land bridge. Following this, a more recent (possibly post-Pleistocene) expansion of moist-adapted types possessing the Southern lineage from the south fits the known vegetation history of the region. [source]


GEOCHEMICAL ANALYSIS AND SOCIOCULTURAL COMPLEXITY: A CASE STUDY FROM EARLY IRON AGE MEGIDDO (ISRAEL)*

ARCHAEOMETRY, Issue 4 2005
T. P. HARRISON
Few archaeological sites can claim a more celebrated position than Megiddo, the Armageddon of biblical revelation. Guardian to a strategic pass on the ancient land bridge that traverses the region, it has long been known that Megiddo played a prominent role in the emergence of the Iron Age nation-states of biblical fame. Given its pivotal location, Megiddo provides an ideal opportunity to examine the experience of a community that found itself at the centre of these developments. The archaeological and textual evidence indicates a community that enjoyed extensive contact with an array of culturally distinct sociopolitical groups emerging in its hinterland. To further explore the nature and extent of this interaction, an assemblage of 86 ceramic sherds was analysed by instrumental neutron activation analysis (INAA). This paper presents the results of this analysis, together with an evaluation of potential geochemical and archaeological interconnections. Based on this comparative analysis, implications are drawn regarding Megiddo's role in the changing cultural and political landscape of this formative period in the history of the region. [source]


Absence of post-Miocene Red Sea land bridges: biogeographic implications

JOURNAL OF BIOGEOGRAPHY, Issue 6 2006
Carlos A. Fernandes
Abstract In a large number of studies concerned with species movements between Africa and Eurasia, including the migrations of hominids out of Africa, a frequently-cited dispersal route is across a hypothetical land bridge in the southern Red Sea, which is suggested to have emerged during glacial sea-level lowstands. This paper, however, unequivocally demonstrates that palaeoceanographic and palaeoecological data are incompatible with the existence of Red Sea land bridges since the Miocene. The case is made by presenting the first quantitative history of water depth above the Red Sea sill for the last 470,000 years, a time period that includes the four most recent glacial,interglacial cycles, and by discussing the predictable consequences of any land bridge formation on the Red Sea sedimentary and microfossil records. The absence of post-Miocene Red Sea land bridges has extensive implications for biogeographic models in the Afro-Arabian region. Genetic, morphometric and palaeontological patterns reported in the literature cannot be related to dispersals over a land bridge, or in the case of marine organisms, separation of the Red Sea from the Indian Ocean by a land bridge. If such patterns in terrestrial species are only congruent with a southern Red Sea dispersal route, then they need to be considered in terms of sweepstake rafting, anthropogenic introduction, or in the particular case of the Out-of-Africa migration by modern humans, seafaring. The constraints imposed by our palaeoenvironmental record on biogeographic reconstructions within and around the Red Sea will hopefully encourage both the review of previous works and the preference for multidisciplinary approaches in future studies. [source]


The human colonisation of Europe: where are we?,,

JOURNAL OF QUATERNARY SCIENCE, Issue 5 2006
Wil Roebroeks
Abstract This paper focuses on the earlier parts of the human colonisation of Europe and its wider setting and addresses the two basic tasks of archaeologists working in this field: (1) to identify the spatio-temporal patterns of human presence and absence, i.e. getting the pattern ,right'; (2) to explain these patterns. Archaeologists have invested mostly in the first task, while the second one takes us to the field of biogeography. Study of biogeographical limits of hominins necessitates integration of many aspects of a species, e.g. diet, life history and social organisation, and the way environmental factors shape these. Palaeoanthropologists need to combine these with establishing data on the chronology of hominin presence, on palaeoenvironment and climatic oscillations, on emergence and disappearance of land bridges, and so on. They further have to acknowledge the fact that only very small parts of the former ranges of the species have been sampled ,adequately'. The paper explores some of the key issues at stake in dealing with the human colonisation of Europe. Copyright ę 2006 John Wiley & Sons, Ltd. [source]


Phylogeography of a northeast Asian spruce, Picea jezoensis, inferred from genetic variation observed in organelle DNA markers

MOLECULAR ECOLOGY, Issue 16 2007
M. AIZAWA
Abstract Range-wide genetic variation of the widespread cold-temperate spruce Picea jezoensis was studied throughout northeast Asia using maternally inherited mitochondrial DNA and paternally inherited chloroplast DNA markers. This study assessed 33 natural populations including three varieties of the species in Japan, Russia, China, and South Korea. We depicted sharp suture zones in straits around Japan in the geographical distribution pattern of mitochondrial haplotypes (GST = 0.901; NST = 0.934). In contrast, we detected possible extensive pollen flow without seed flow across the straits around Japan during the past population history in the distribution pattern of chloroplast haplotypes (GST = 0.233; NST = 0.333). The analysis of isolation by distance of the species implied that by acting as a barrier for the movement of seeds and pollen, the sharp suture zones contributed considerably to the level of genetic differentiation between populations. Constructed networks of mitochondrial haplotypes allowed inference of the phylogeographical history of the species. We deduced that the disjunction with Kamchatka populations reflects range expansion and contraction to the north of the current distribution. Within Japan, we detected phylogeographically different types of P. jezoensis between Hokkaido and Honshu islands; P. jezoensis in Honshu Island may have colonized this region from the Asian continent via the Korean peninsula and the species in Hokkaido Island is likely to have spread from the Asian continent via Sakhalin through land bridges. Japanese endemism of mitochondrial haplotypes in Hokkaido and Honshu islands might have been promoted by separation of these islands from each other and from the Asian continent by the straits during the late Quaternary. [source]


Permafrost Characteristics of the Qinghai-Tibet Plateau and Methods of Roadbed Construction of Railway

ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 5 2008
NIU Fujun
Abstract Permafrost along the Qinghai-Tibet railway is featured by abundant ground ice and high ground temperature. Under the influence of climate warming and engineering activities, the permafrost is under degradation process. The main difficulty in railway roadbed construction is how to prevent thawing settlement caused by degradation of permafrost Therefore the proactively cooling methods based on controlling solar radiation, heat conductivity and heat convection were adopted instead of the traditional passive methods, which is simply increasing thermal resistance. The cooling methods used in the Qinghai-Tibet railway construction include sunshine-shielding roadbeds, crushed rock based roadbeds, roadbeds with rock revetments, duct-ventilated roadbeds, thermosyphon installed roadbeds and land bridges. The field monitored data show that the cooling methods are effective in protecting the underlying permafrost, the permafrost table was uplifted under the embankments and therefore the roadbed stability was guaranteed. [source]