			Home About us Contact

Strong Barrier (strong + barrier)

Distribution by Scientific Domains

Life Sciences	75%

Selected Abstracts

ESTIMATING INCUMBENCY EFFECTS IN U.S. STATE LEGISLATURES: A QUASI-EXPERIMENTAL STUDY

ECONOMICS & POLITICS, Issue 2 2010
YOGESH UPPAL
This paper estimates the incumbency effects in elections to the House of Representatives of 45 states in the United States using a quasi-experimental research method, regression discontinuity design (RDD). This design isolates the causal effect of incumbency from other contemporaneous factors, such as candidate quality, by comparing incumbents and non-incumbents in close contests. I find that incumbents in state legislative elections have a significant advantage, and this advantage serves as a strong barrier to re-entry of challengers who had previously been defeated. However, the incumbency advantage estimated using the RDD is much smaller than are the estimates using existing methods, implying a significant selection bias in the latter. [source]

History and evolution of the arctic flora: in the footsteps of Eric Hultén

MOLECULAR ECOLOGY, Issue 2 2003
Richard J. Abbott
Abstract A major contribution to our initial understanding of the origin, history and biogeography of the present-day arctic flora was made by Eric Hultén in his landmark book Outline of the History of Arctic and Boreal Biota during the Quarternary Period, published in 1937. Here we review recent molecular and fossil evidence that has tested some of Hultén's proposals. There is now excellent fossil, molecular and phytogeographical evidence to support Hultén's proposal that Beringia was a major northern refugium for arctic plants throughout the Quaternary. In contrast, most molecular evidence fails to support his proposal that contemporary east and west Atlantic populations of circumarctic and amphi-Atlantic species have been separated throughout the Quaternary. In fact, populations of these species from opposite sides of the Atlantic are normally genetically very similar, thus the North Atlantic does not appear to have been a strong barrier to their dispersal during the Quaternary. Hultén made no detailed proposals on mechanisms of speciation in the Arctic; however, molecular studies have confirmed that many arctic plants are allopolyploid, and some of them most probably originated during the Holocene. Recurrent formation of polyploids from differentiated diploid or more low-ploid populations provides one explanation for the intriguing taxonomic complexity of the arctic flora, also noted by Hultén. In addition, population fragmentation during glacial periods may have lead to the formation of new sibling species at the diploid level. Despite the progress made since Hultén wrote his book, there remain large gaps in our knowledge of the history of the arctic flora, especially about the origins of the founding stocks of this flora which first appeared in the Arctic at the end of the Pliocene (approximately 3 Ma). Comprehensive analyses of the molecular phylogeography of arctic taxa and their relatives together with detailed fossil studies are required to fill these gaps. [source]

Waterlogging tolerance in the tribe Triticeae: the adventitious roots of Critesion marinum have a relatively high porosity and a barrier to radial oxygen loss

PLANT CELL & ENVIRONMENT, Issue 6 2001
M. P. Mcdonald
Abstract Nine species from the tribe Triticeae , three crop, three pasture and three ,wild' wetland species , were evaluated for tolerance to growth in stagnant deoxygenated nutrient solution and also for traits that enhance longitudinal O2 movement within the roots. Critesion marinum (syn. Hordeum marinum) was the only species evaluated that had a strong barrier to radial O2 loss (ROL) in the basal regions of its adventitious roots. Barriers to ROL have previously been documented in roots of several wetland species, although not in any close relatives of dryland crop species. Moreover, the porosity in adventitious roots of C. marinum was relatively high: 14% and 25% in plants grown in aerated and stagnant solutions, respectively. The porosity of C. marinum roots in the aerated solution was 1·8,5·4-fold greater, and in the stagnant solution 1·2,2·8-fold greater, than in the eight other species when grown under the same conditions. These traits presumably contributed to C. marinum having a 1·4,3 times greater adventitious root length than the other species when grown in deoxygenated stagnant nutrient solution or in waterlogged soil. The length of the adventitious roots and ROL profiles of C. marinum grown in waterlogged soil were comparable to those of the extremely waterlogging-tolerant species Echinochloa crus-galli L. (P. Beauv.). The superior tolerance of C. marinum, as compared to Hordeum vulgare (the closest cultivated relative), was confirmed in pots of soil waterlogged for 21 d; H. vulgare suffered severe reductions in shoot and adventitious root dry mass (81% and 67%, respectively), whereas C. marinum shoot mass was only reduced by 38% and adventitious root mass was not affected. [source]

Distinctiveness in the face of gene flow: hybridization between specialist and generalist gartersnakes

MOLECULAR ECOLOGY, Issue 18 2008
BENJAMIN M. FITZPATRICK
Abstract Patterns of divergence and polymorphism across hybrid zones can provide important clues as to their origin and maintenance. Unimodal hybrid zones or hybrid swarms are composed predominantly of recombinant individuals whose genomes are patchworks of alleles derived from each parental lineage. In contrast, bimodal hybrid zones contain few identifiable hybrids; most individuals fall within distinct genetic clusters. Distinguishing between hybrid swarms and bimodal hybrid zones can be important for taxonomic and conservation decisions regarding the status and value of hybrid populations. In addition, the causes of bimodality are important in understanding the generation and maintenance of biological diversity. For example, are distinct clusters mostly reproductively isolated and co-adapted gene complexes, or can distinctiveness be maintained by a few ,genomic islands' despite rampant gene flow across much of the genome? Here we focus on three patterns of distinctiveness in the face of gene flow between gartersnake taxa in the Great Lakes region of North America. Bimodality, the persistence of distinct clusters of genotypes, requires strong barriers to gene flow and supports recognition of distinct specialist (Thamnophis butleri) and generalist (Thamnophis radix) taxa. Concordance of DNA-based clusters with morphometrics supports the hypothesis that trophic morphology is a key component of divergence. Finally, disparity in the level of differentiation across molecular markers (amplified fragment length polymorphisms) indicates that distinctiveness is maintained by strong selection on a few traits despite high gene flow currently or in the recent past. [source]