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Empirical Observations (empirical + observation)
Selected AbstractsComplexity can enhance stability in competitive systemsECOLOGY LETTERS, Issue 5 2001Ian D. Rozdilsky Empirical observations often indicate that complexity enhances stability, while most theoretical studies, such as May's (1972) classic paper, point to the opposite. Despite the wide generality of these latter theoretical analyses, our examination of the well-known competitive Lotka,Volterra system reveals that increasing complexity (measured in terms of connectance) can enhance species coexistence and persistence in model communities (measured in terms of their feasibility and stability). The high feasibility and stability found for tightly interconnected competitive subsystems might provide an explanation for the clumped structure in food webs. [source] The genic view of the process of speciationJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 6 2001Chung-I Wu The unit of adaptation is usually thought to be a gene or set of interacting genes, rather than the whole genome, and this may be true of species differentiation. Defining species on the basis of reproductive isolation (RI), on the other hand, is a concept best applied to the entire genome. The biological species concept (BSC; Mayr, 1963) stresses the isolation aspect of speciation on the basis of two fundamental genetic assumptions , the number of loci underlying species differentiation is large and the whole genome behaves as a cohesive, or coadapted genetic unit. Under these tenets, the exchange of any part of the genomes between diverging groups is thought to destroy their integrity. Hence, the maintenance of each species' genome cohesiveness by isolating mechanisms has become the central concept of species. In contrast, the Darwinian view of speciation is about differential adaptation to different natural or sexual environments. RI is viewed as an important by product of differential adaptation and complete RI across the whole genome need not be considered as the most central criterion of speciation. The emphasis on natural and sexual selection thus makes the Darwinian view compatible with the modern genic concept of evolution. Genetic and molecular analyses of speciation in the last decade have yielded surprisingly strong support for the neo-Darwinian view of extensive genetic differentiation and epistasis during speciation. However, the extent falls short of what BSC requires in order to achieve whole-genome ,cohesiveness'. Empirical observations suggest that the gene is the unit of species differentiation. Significantly, the genetic architecture underlying RI, the patterns of species hybridization and the molecular signature of speciation genes all appear to support the view that RI is one of the manifestations of differential adaptation, as Darwin (1859, Chap. 8) suggested. The nature of this adaptation may be as much the result of sexual selection as natural selection. In the light of studies since its early days, BSC may now need a major revision by shifting the emphasis from isolation at the level of whole genome to differential adaptation at the genic level. With this revision, BSC would in fact be close to Darwin's original concept of speciation. [source] Investment risk allocation in decentralised electricity markets.OPEC ENERGY REVIEW, Issue 2 2008The need of long-term contracts, vertical integration None of the far-reaching experiments in electricity industry liberalisation was able to ensure the timely and optimal capacity mix development. The theoretical market model features market failures due to the specific volatility of prices, and the difficulty of creating complete markets for hedging. In this paper, we focused on a specific failure, i.e. the impossibility of allocating the various risks borne by the producer onto suppliers and consumers in order to allow capacity development. Promotion of short-term competition by mandating vertical de-integration tends to distort investments in generation by impeding efficient risk allocation. Following Joskow's (2006) line, we developed an empirical analysis of how to secure investments in generation through vertical arrangements between decentralised generators and large purchasers, suppliers or consumers. Empirical observations as risk analysis shows that adopting such arrangements may prove necessary. Various types of long-term contracts between generators and suppliers (fixed-quantity and fixed-price contract, indexed price contract, tolling contract, financial option) appear to offer effective solutions for risk allocation. Vertical integration appears to be another effective way to allocate risk. But it remains an important complementary condition to efficient risk allocation, i.e. that retail competition is sticky or legally limited in order to have a large part of risks borne by consumers on the different market segments. [source] Are People Ashamed of Paying with Food Stamps?JOURNAL OF AGRICULTURAL ECONOMICS, Issue 2 2003Robert Breunig We evaluate the claim that marginal ,welfare stigma'causes a dollar of food to provide less utility if bought with food stamps rather than cash, and that this explains why, in the United States, the marginal propensity to consume food out of food stamps is larger than that out of income. This hypothesis has been advanced to explain the so-called ,cash-out puzzle': the empirical observation that the marginal propensity to consume food out of food stamps is much higher than that out of income, even for households who spend some cash income on food. We develop a theoretical model to identify the restrictions imposed by the hypothesis that the puzzle is indeed caused by such stigma. Using data from San Diego County, we find that two of the three predictions of the marginal stigma model are violated. These results cast serious doubt on the marginal stigma hypothesis. [source] The tale of the sword , swords and swordfighters in Bronze Age EuropeOXFORD JOURNAL OF ARCHAEOLOGY, Issue 4 2002Kristian Kristiansen In this article it is demonstrated through empirical observation that Bronze Age swords were functional and efficient weapons. Their use in real combat is testified by recurring patterns of blade damage and resharpening. Furthermore, ritual depositions of swords with unrepaired scars on the blade demonstrate the prehistoric roots of the Celtic,Germanic,Greek ritual of sacrificing weapons after a victorious fight. [source] Covalent Linkage Mediates Communication between ACP and TE Domains in Modular Polyketide SynthasesCHEMBIOCHEM, Issue 6 2008Lucky Tran Abstract Polyketide natural products such as erythromycin A and epothilone are assembled on multienzyme polyketide synthases (PKSs), which consist of modular sets of protein domains. Within these type I systems, the fidelity of biosynthesis depends on the programmed interaction among the multiple domains within each module, centered around the acyl carrier protein (ACP). A detailed understanding of interdomain communication will therefore be vital for attempts to reprogram these pathways by genetic engineering. We report here that the interaction between a representative ACP domain and its downstream thioesterase (TE) is mediated largely by covalent tethering through a short "linker" region, with only a minor energetic contribution from protein,protein molecular recognition. This finding helps explain in part the empirical observation that TE domains can function out of their normal context in engineered assembly lines, and supports the view that overall PKS architecture may dictate at least a subset of interdomain interactions. [source] SPECIAL SECTION: EVALUATION OF THE WESTERN AUSTRALIAN CANNABIS INFRINGEMENT NOTICE SCHEME,PHASE 1: Community attitudes towards cannabis law and the proposed Cannabis Infringement Notice scheme in Western AustraliaDRUG AND ALCOHOL REVIEW, Issue 4 2005JAMES FETHERSTON Abstract Western Australia (WA) became the fourth Australian jurisdiction to adopt a prohibition with civil penalties scheme for minor cannabis offences when its Cannabis Infringement Notice (CIN) scheme became law on 22 March 2004. Previous criminological research has demonstrated the importance of public attitudes towards the law in determining the effectiveness of legislation. This survey represents the first phase of a pre-post study that attempted to gauge public attitudes towards the legal status of cannabis, the proposed legislative reforms surrounding the drug and their likely effects. A random telephone survey of 809 members of the WA population was conducted prior to the implementation of the new laws with a view to exploring contemporary views of the existing legal status of cannabis, attitudes to the proposed legislative model and respondent perceptions of its likely effects. Despite cannabis being viewed negatively by large numbers of the sample, criminal penalties for minor cannabis offences were viewed as inappropriate and ineffective. Once explained, the proposed civil penalty scheme was viewed as ,a good idea' by 79% of the sample, despite significant differences due to personal experience of cannabis use, political affiliation, religiosity and age of offspring. Most believed that the legislative change would not result in changes to levels of cannabis use (70%) or ease of obtaining cannabis (59%). These data suggest that prior to its implementation the new legislation was highly acceptable to the majority of the community. These baseline data will be compared with data to be collected at the post-change phase of the study to allow empirical observations of attitudinal and behavioural changes occurring in the community. [source] Reconciling differences in trophic control in mid-latitude marine ecosystemsECOLOGY LETTERS, Issue 10 2006Kenneth T. Frank Abstract The dependence of long-term fishery yields on primary productivity, largely based on cross-system comparisons and without reference to the potential dynamic character of this relationship, has long been considered strong evidence for bottom-up control in marine systems. We examined time series of intensive empirical observations from nine heavily exploited regions in the western North Atlantic and find evidence of spatial variance of trophic control. Top-down control dominated in northern areas, the dynamics evolved from bottom-up to top-down in an intermediate region, and bottom-up control governed the southern areas. A simplified, trophic control diagram was developed accounting for top-down and bottom-up forcing within a larger region whose base state dynamics are bottom-up and can accommodate time-varying dynamics. Species diversity and ocean temperature co-varied, being relatively high in southern areas and lower in the north, mirroring the shifting pattern of trophic control. A combination of compensatory population dynamics and accelerated demographic rates in southern areas seems to account for the greater stability of the predator species complex in this region. [source] An Economic Model of Friendship: Homophily, Minorities, and SegregationECONOMETRICA, Issue 4 2009Sergio Currarini We develop a model of friendship formation that sheds light on segregation patterns observed in social and economic networks. Individuals have types and see type-dependent benefits from friendships. We examine the properties of a steady-state equilibrium of a matching process of friendship formation. We use the model to understand three empirical patterns of friendship formation: (i) larger groups tend to form more same-type ties and fewer other-type ties than small groups, (ii) larger groups form more ties per capita, and (iii) all groups are biased towards same-type relative to demographics, with the most extreme bias coming from middle-sized groups. We show how these empirical observations can be generated by biases in preferences and biases in meetings. We also illustrate some welfare implications of the model. [source] The euro and the competitiveness of European firmsECONOMIC POLICY, Issue 57 2009Gianmarco I.P. Ottaviano SUMMARY EMU and competition Much attention has been paid to the impact of a single currency on actual trade volumes. Lower trade costs, however, matter over and beyond their effects on trade flows: as less productive firms are forced out of business by the tougher competitive conditions of international markets, economic integration fosters lower prices and higher average productivity. We assess the quantitative relevance of these effects calibrating a general equilibrium model using country, sector and firm-level empirical observations. The euro turns out to have increased the overall competitiveness of Eurozone firms, and the effects differ along interesting dimensions: they tend to be stronger for countries which are smaller or with better access to foreign markets, and for firms which specialize in sectors where international competition is fiercer and barriers to entry lower. , Gianmarco I.P. Ottaviano, Daria Taglioni and Filippo di Mauro [source] LEAKY PREZYGOTIC ISOLATION AND POROUS GENOMES: RAPID INTROGRESSION OF MATERNALLY INHERITED DNAEVOLUTION, Issue 4 2005Kai M. A. Chan Abstract Accurate phylogenies are crucial for understanding evolutionary processes, especially species diversification. It is commonly assumed that "good" species are sufficiently isolated genetically that gene genealogies represent accurate phylogenies. However, it is increasingly clear that good species may continue to exchange genetic material through hybridization (introgression). Many studies of closely related species reveal introgression of some genes without others, often with more rapid introgression of maternally inherited chloroplast or mitochondrial DNA (cpDNA, mtDNA). We seek a general explanation for this biased introgression using simple models of common reproductive isolating barriers (RIBs). We compare empirically informed models of prezygotic isolation (for pre- and postinsemination mechanisms of both female choice and male competition) with postzygotic isolation and demonstrate that rate of introgression depends critically upon type of RIB and mode of genetic inheritance (maternal versus biparental versus paternal). Our frequency-dependent prezygotic RIBs allow much more rapid introgression of biparentally and maternally inherited genes than do commonly modeled postzygotic RIBs (especially maternally inherited DNA). After considering the specific predictions in the context of empirical observations, we conclude that our model of prezygotic RIBs is a general explanation for biased introgression of maternally inherited genomic components. These findings suggest that we should use extreme caution when interpreting single gene genealogies as species phylogenies, especially for cpDNA and mtDNA. [source] Strategies for preventing defection based on the mean time to defection and their implementations on a self-organizing mapEXPERT SYSTEMS, Issue 5 2005Young Ae Kim Abstract: Customer retention is a critical issue for the survival of any business in today's competitive marketplace. In this paper, we propose a dynamic procedure utilizing self-organizing maps and a Markov process for detecting and preventing customer defection that uses data of past and current customer behavior. The basic concept originates from empirical observations that identified that a customer has a tendency to change behavior (i.e. trim-out usage volumes) before eventual withdrawal and defection. Our explanatory model predicts when potential defectors are likely to withdraw. Two strategies are suggested to respond to the question of where to lead potential defectors for the next stage, based on anticipating when the potential defector will leave. Our model predicts potential defectors with little deterioration of prediction accuracy compared with that of the multilayer perceptron neural network and decision trees. Moreover, it performs reasonably well in a controlled experiment using an online game. [source] The role of metabolic memory in the ATP paradox and energy homeostasisFEBS JOURNAL, Issue 21 2008Juan C. Aledo In yeast, a sudden transition from glucose limitation to glucose excess leads to a new steady state at increased metabolic fluxes with a sustained decrease in the ATP concentration. Although this behaviour has been rationalized as an adaptive metabolic strategy, the mechanism behind it remains unclear. Nevertheless, it is thought that, on glucose addition, a metabolite derived from glycolysis may up-regulate ATP-consuming reactions. The adenine nucleotides themselves have been ruled out as the signals that mediate this regulation. This is mainly because, in that case, it would be expected that the new steady state at increased fluxes would be accompanied by an increased stationary ATP concentration. In this study, we present a core model consisting of a monocyclic interconvertible enzyme system. Using a supply,demand approach, we demonstrate that this system can account for the empirical observations without involving metabolites other than the adenine nucleotides as effectors. Moreover, memory is an emerging property of such a system, which may allow the cell to sense both the current energy status and the direction of the changes. [source] Schooling and migration of large pelagic fishes relative to environmental cuesFISHERIES OCEANOGRAPHY, Issue 2 2000Robert Humston A kinesis model driven by high-resolution sea surface temperature maps is used to simulate Atlantic bluefin tuna movements in the Gulf of Maine during summer months. Simulations showed that individuals concentrated in areas of thermal preference. Results are compared to empirical distribution maps of bluefin tuna schools determined from aerial overflights of the stock during the same time periods. Simulations and empirical observations showed similar bluefin tuna distributions along fronts, although interannual variations in temperature ranges occupied suggest that additional foraging factors are involved. Performance of the model is further tested by simulating the relatively large-scale annual north,south migrations of bluefin tuna that followed a preferred thermal regime. Despite the model's relatively simple structure, results suggest that kinesis is an effective mechanism for describing movements of large pelagic fish in the expansive ocean environment. [source] Genetic transduction in freshwater ecosystemsFRESHWATER BIOLOGY, Issue 6 2008OLADELE A. OGUNSEITAN Summary 1. Lateral genetic exchange is a profound consequence of the co-existence of viruses (bacteriophages) and bacteria in freshwater ecosystems. Transduction is distinct from other mechanisms of genetic exchange because it is driven by potentially lethal agents external to the donor and recipient cells. Therefore, transduction is reputed to be a major driving force behind the diversity in natural populations and communities of bacteria. 2. Both generalized transduction (where every segment of the donor's genome has equal chance of being transferred to a recipient cell) and specialized transduction (where certain donor gene sequences are transferred at higher frequencies than others based on their proximity to the integration site of the transducing bacteriophage genome) have been demonstrated for various freshwater bacteria. However, these genetic exchange events occur at frequencies that vary widely, from 10,2 to 10,10 transductants per recipient, depending on the influence of various physical, chemical and biotic environmental factors on the outcome of phage,host encounters. Methodological constraints limit the interpretation of results from early studies of transduction in freshwaters because those studies introduced exogenous organisms in microcosms and excluded, to different extents, naturally occurring environmental conditions and their variability. 3. To assist the design and extrapolation of empirical observations, mathematical models including application of Group Theory are useful to estimate boundaries of the impact of transduction in generating and maintaining microbial diversity in freshwater. These theoretical excursions generate hypotheses and questions that can only be answered through refinement of current empirical estimates of transduction frequency, polarity of gene mobilization, bacteriophage host ranges, and the influence of gradients in environmental parameters that characterize freshwater ecosystems. [source] Dynamic models allowing for flexibility in complex life histories accurately predict timing of metamorphosis and antipredator strategies of preyFUNCTIONAL ECOLOGY, Issue 6 2009Andrew D. Higginson Summary 1.,The development of antipredator defences in the larval stage of animals with complex life cycles is likely to be affected by costs associated with creating and maintaining such defences because of their impact on the timing of maturation or metamorphosis. 2.,Various theoretical treatments have suggested that investment in defence should both increase or decrease with increasing resource availability, but a recent model predicts investment in defences should be highest at intermediate resource level and predator density. 3.,Previous models of investment in defence and timing of metamorphosis provide a poor match to empirical data. Here we develop a dynamic state-dependent model of investment in behavioural and morphological defences that enables us to allow flexibility in investment in defences over development, the timing of metamorphosis and the size of the organism at metamorphosis that were absent from previous theory. 4.,We show that the inclusion of this flexibility results in different predictions to those of the fixed investment approach used previously, especially when we allow metamorphosis to occur at the optimal time and state for the organism. 5.,Under these more flexible conditions, we predict that morphological defences should be insensitive to resource level whilst behavioural defences should either increase or decrease with increasing resources depending on the predation risk and the magnitude of the fitness benefits of large size at metamorphosis. 6.,Our work provides a formal framework in which we might progress in the study of how the use of antipredator defences is affected by their costs. Most of the predictions of our model in are in good accord with empirical results, and can be understood in terms of the underlying biological assumptions. The reasons why simpler models failed to match empirical observations can be explained, and our predictions that are a poor match help to target the circumstances which warrant future study. [source] A CITY IN MOTION: TIME-SPACE ACTIVITY AND MOBILITY PATTERNS OF SUBURBAN INHABITANTS AND THE STRUCTURATION OF THE SPATIAL ORGANIZATION OF THE PRAGUE METROPOLITAN AREAGEOGRAFISKA ANNALER SERIES B: HUMAN GEOGRAPHY, Issue 2 2007Jakub Novák ABSTRACT. This contribution attempts to reveal the relations between new suburban areas and other parts of the Prague metropolitan area by investigating the time-space activity and mobility patterns of the inhabitants of newly built suburban districts. The focus on some aspects of the everyday life of people in new suburbs helps us to identify the impact of suburbanization on the changing geography of the metropolitan region and to better understand how the spatial organization of the Prague metropolitan area is produced, reproduced and transformed. We use several interrelated concepts, which serve the theoretical foundation of our work, namely time geography, structuration theory and the post-communist city. The empirical data utilized are primarily based on 262 diaries completed by eighty-eight individuals from thirty-eight households, accompanied by household questionnaires and interviews with the heads of households. The research confirmed the implicit, generally unspoken view that new suburbs in the Prague metropolitan region are heavily dependent on the core of the metropolitan area for the provision of jobs and services. However, newly built suburban shopping facilities to some extent disrupt this pattern, keeping some daily activities of inhabitants within the suburban zone. In addition to empirical observations, the key purpose of this contribution has been to discuss and apply time geography concepts and methods to the research of urban restructuring, and to understand the structuration of metropolitan spatial organization. [source] Virtual team collaboration: building shared meaning, resolving breakdowns and creating translucenceINFORMATION SYSTEMS JOURNAL, Issue 3 2009Pernille Bjørn Abstract Managing international teams with geographically distributed participants is a complex task. The risk of communication breakdowns increases due to cultural and organizational differences grounded in the geographical distribution of the participants. Such breakdowns indicate general misunderstandings and a lack of shared meaning between participants. In this paper, we address the complexity of building shared meaning. We examine the communication breakdowns that occurred in two globally distributed virtual teams by providing an analytical distinction of the organizational context as the foundation for building shared meaning at three levels. Also we investigate communication breakdowns that can be attributed to differences in lifeworld structures, organizational structures, and work process structures within a virtual team. We find that all communication breakdowns are manifested and experienced by the participants at the work process level; however, resolving breakdowns may require critical reflection at other levels. Where previous research argues that face-to-face interaction is an important variable for virtual team performance, our empirical observations reveal that communication breakdowns related to a lack of shared meaning at the lifeworld level often becomes more salient when the participants are co-located than when geographically distributed. Last, we argue that creating translucence in communication structures is essential for building shared meanings at all three levels. [source] Shear-induced degradation of plasmid DNAJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 7 2002C. S. Lengsfeld Abstract The majority of gene therapy clinical trials use plasmid DNA that is susceptible to shear-induced degradation. Many processing steps in the extraction, purification, and preparation of plasmid-based therapeutics can impart significant shear stress that can fracture the phosphodiester backbone of polynucleotides, and reduce biological activity. Much of the mechanistic work on shear degradation of DNA was conducted over 30 years ago, and we rely heavily on this early work in an attempt to explain the empirical observations of more recent investigations concerning the aerosolization of plasmids. Unfortunately, the sporadic reports of shear degradation in the literature use different experimental systems, making it difficult to quantitatively compare results and reach definitive mechanistic conclusions. In this review, we describe the forces imparted to DNA during shear stress, and use published data to quantitatively evaluate their relative effects. In addition, we discuss the effects of molecular weight, strain rate, particle size, flexibility, ionic strength, gas,liquid interfaces, and turbulence on the fluid flow degradation of supercoiled plasmid DNA. Finally, we speculate on computational methods that might allow degradation rates in different experimental systems to be predicted. © 2002 Wiley-Liss Inc. and the American Pharmaceutical Association J Pharm Sci 91:1581,1589, 2002 [source] The role of social anxiety in volunteeringNONPROFIT MANAGEMENT & LEADERSHIP, Issue 1 2007Femida Handy The volunteer management literature suggests that the most effective means of recruitment is personal asking. However, agencies that apply this method do not report the expected success in volunteer recruitment. Often they face the volunteer recruitment fallacy: those people assumed to be interested in volunteering do not necessarily volunteer. Based on the literature of shyness or social anxiety and on empirical observations, this article suggests that social anxiety often deters volunteering by new recruits. We hypothesize that people with greater levels of social anxiety will be less likely to volunteer. Furthermore, we hypothesize that people with high social anxiety will prefer to give monetary support to worthy causes rather than volunteer their time, and if they do choose to volunteer, they will do so alongside friends. Our hypotheses are supported based on the findings from a large-scale nonrandom sample in North America. We suggest how to avoid the volunteer recruitment fallacy by creating a personal environment in which high-social-anxiety recruits feel safe and accepted. By removing the fear of being negatively judged by strangers as they enter the agency and creating a more personal approach, new recruits may have a higher probability of becoming long-term and consistent volunteers. [source] The rational design of biological complexity: A deceptive metaphorPROTEINS: STRUCTURE, FUNCTION AND BIOINFORMATICS, Issue 6 2007Marc H. V. Van Regenmortel Dr. Abstract Biologists often claim that they follow a rational design strategy when their research is based on molecular knowledge of biological systems. This claim implies that their knowledge of the innumerable causal connections present in biological systems is sufficient to allow them to deduce and predict the outcome of their experimental interventions. The design metaphor is shown to originate in human intentionality and in the anthropomorphic fallacy of interpreting objects, events, and the behavior of all living organisms in terms of goals and purposes. Instead of presenting rational design as an effective research strategy, it would be preferable to acknowledge that advances in biomedicine are nearly always derived from empirical observations based on trial and error experimentation. The claim that rational design is an effective research strategy was tested in the case of current attempts to develop synthetic vaccines, in particular against human immunodeficiency virus. It was concluded that in this field of biomedicine, trial and error experimentation is more likely to succeed than a rational design approach. Current developments in systems biology may give us eventually a better understanding of the immune system and this may enable us in the future to develop improved vaccines. [source] | ||||||||||||||||||||||