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Search Results: 1 - 10 of 194536 matches for " Barry D. Hughes "
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Theoretical size distribution of fossil taxa: analysis of a null model
William J Reed, Barry D Hughes
Theoretical Biology and Medical Modelling , 2007, DOI: 10.1186/1742-4682-4-12
Abstract: New species arise through speciations occurring independently and at random at a fixed probability rate, while extinctions either occur independently and at random (background extinctions) or cataclysmically. In addition new genera are assumed to arise through speciations of a very radical nature, again assumed to occur independently and at random at a fixed probability rate.The size distributions of the pioneering genus (following a cataclysm) and of derived genera are determined. Also the distribution of the number of genera is considered along with a comparison of the probability of a monospecific genus with that of a monogeneric family.Mathematical modelling of the evolution of lineages goes back at least to Yule[1] who developed the eponymous Yule process (homogeneous pure birth process) in which speciations occur independently and at random. Yule's model did not include extinctions per se, because he believed that they resulted only from cataclysmic events. This issue was discussed at greater length by Raup[2], who distinguished between background and episodic extinctions. Raup started from a homomogeneous birth-and-death process model (in which background extinctions occur, like speciations, independently and at random) for which he presented mathematical results, and described more complex models of extinction including episodic extinctions and a mixture of episodic and background extinctions. However he gave no mathematical results for these models. Stoyan[3] considered a time in-homogeneous birth-and death process, in which speciation and background extinction rates varied with time, based on the idea that younger paraclades have higher speciation rates, while older ones have higher background extinction rates.There has been considerable discussion (e.g. Raup[2]; Patzkowsky[4]; Przeworski and Wall[5]) about the suitability of the null birth-and-death process model (with constant birth and death rates) as a macroevolutionary model of species diversification
A Problem in Paleobiology
Barry D. Hughes,William J. Reed
Physics , 2002,
Abstract: We present a stochastic model for the size of a taxon in paleobiology, in which we allow for the evolution of new taxon members, and both individual and catastrophic extinction events. The model uses ideas from the theory of birth and death processes. Some general properties of the model are developed, and a fuller discussion is given for specific distributions of the time between catastrophic extinction events. Long tails in the taxon size distribution arise naturally from the model.
Building a Morphogen Gradient without Diffusion in a Growing Tissue
Rebecca H. Chisholm,Barry D. Hughes,Kerry A. Landman
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0012857
Abstract: In many developmental systems, spatial pattern arises from morphogen gradients, which provide positional information for cells to determine their fate. Typically, diffusion is thought to be the mechanism responsible for building a morphogen gradient. An alternative mechanism is investigated here. Using mathematical modeling, we demonstrate how a non-diffusive morphogen concentration gradient can develop in axially growing tissue systems, where growth is due to cell proliferation only. Two distinct cases are considered: in the first, all cell proliferation occurs in a localized zone where active transcription of a morphogen-producing gene occurs, and in the second, cell proliferation is uniformly distributed throughout the tissue, occurring in both the active transcription zone and beyond. A cell containing morphogen mRNA produces the morphogen protein, hence any gradient in mRNA transcripts translates into a corresponding morphogen protein gradient. Proliferation-driven growth gives rise to both advection (the transport term) and dilution (a reaction term). These two key mechanisms determine the resultant mRNA transcript distribution. Using the full range of uniform initial conditions, we show that advection and dilution due to cell proliferation are, in general, sufficient for morphogen gradient formation for both types of axially growing systems. In particular, mRNA transcript degradation is not necessary for gradient formation; it is only necessary with localized proliferation for one special value of the initial concentration. Furthermore, the morphogen concentration decreases with distance away from the transcription zone, except in the case of localized proliferation with the initial concentration sufficiently large, when the concentration can either increase with distance from the transcription zone or sustain a local minimum. In both localized and uniformly distributed proliferation, in order for a concentration gradient to form across the whole domain, transcription must occur in a zone equal to the initial domain size; otherwise, it will only form across part of the tissue.
Exploring Future Impacts of Environmental Constraints on Human Development
Barry B. Hughes,Mohammod T. Irfan,Jonathan D. Moyer,Dale S. Rothman,José R. Solórzano
Sustainability , 2012, DOI: 10.3390/su4050958
Abstract: Environmental constraints have always had, and will always have, important consequences for human development. They have sometimes contributed to, or even caused, the reversal of such development. The possibility that such constraints, including climate change, will grow significantly this century raises the concern that the very significant advances in human development across most of the world in recent decades will slow or even reverse. We use the International Futures (IFs) integrated forecasting system to explore three scenarios: a Base Case scenario, an Environmental Challenge scenario, and an Environmental Disaster scenario. Our purpose is to consider the impact of different aspects and levels of environmental constraint on the course of future human development. Using the Human Development Index (HDI) and its separate components as our key measures of development, we find that environmental constraints could indeed greatly slow progress and even, in disastrous conditions, begin to reverse it. Least developed countries are most vulnerable in relative terms, while middle-income countries can suffer the greatest absolute impact of constraints, and more developed countries are most resilient. Education’s advance is the aspect of development tapped by the HDI that is most likely to continue even in the face of tightening environmental constraints, and that is one reason why human development shows great momentum even in the face of environmental?challenges.
A Computational Model for Collective Cellular Motion in Three Dimensions: General Framework and Case Study for Cell Pair Dynamics
Federico Frascoli, Barry D. Hughes, Muhammad H. Zaman, Kerry A. Landman
PLOS ONE , 2013, DOI: 10.1371/journal.pone.0059249
Abstract: Cell migration in healthy and diseased systems is a combination of single and collective cell motion. While single cell motion has received considerable attention, our understanding of collective cell motion remains elusive. A new computational framework for the migration of groups of cells in three dimensions is presented, which focuses on the forces acting at the microscopic scale and the interactions between cells and their extracellular matrix (ECM) environment. Cell-cell adhesion, resistance due to the ECM and the factors regulating the propulsion of each cell through the matrix are considered. In particular, our approach emphasizes the role of receptors that mediate cell-cell and cell-matrix interactions, and examines how variation in their properties induces changes in cellular motion. As an important case study, we analyze two interacting cells. Our results show that the dynamics of cell pairs depends on the magnitude and the stochastic nature of the forces. Stronger intercellular stability is generally promoted by surface receptors that move. We also demonstrate that matrix resistance, cellular stiffness and intensity of adhesion contribute to migration behaviors in different ways, with memory effects present that can alter pair motility. If adhesion weakens with time, our findings show that cell pair break-up depends strongly on the way cells interact with the matrix. Finally, the motility for cells in a larger cluster (size 50 cells) is examined to illustrate the full capabilities of the model and to stress the role of cellular pairs in complex cellular structures. Overall, our framework shows how properties of cells and their environment influence the stability and motility of cellular assemblies. This is an important step in the advancement of the understanding of collective motility, and can contribute to knowledge of complex biological processes involving migration, aggregation and detachment of cells in healthy and diseased systems.
Effect of Density Gradients on Haptic Discrimination
质地密度逐渐变化对触觉识别的作用

Wang Jin,Barry Hughes,
王进
,Barry Hughes

心理学报 , 2005,
Abstract: This study was designed to contribute to research on the relative roles and contributions of afference,proprioception,and efference to touch-based(or haptic) texture perception. We sought to determine whether theoretical accounts of roughness constancy,in active and passive perception and across broad velocity ranges,would be supported when spatial density gradients were explored.We created textures with sinusoidal spatial gradients that participants explored with sinusoidal velocity directions: positive gradients resulted in densities that increased in the region where exploration velocities were largest while negative gradients counteracted the velocity increases and decreases with decreases and increases in spatial density,respectively.While these gradients could all be detected reasonably well,we found a slight shift in discriminability in favour of positive gradients and found that surfaces with no gradient at all were more likely to be judged to have a positive gradient.This finding runs counter to the roughness constancy models.Counterintuitively,we found that providing the participants with feedback as to their accuracy had a negative impact on gradients.The spatial gradients were of different magnitudes and in different their accuracy and confidence and we found no evidence of perceptual learning in the task.We consider the data relative to existing models and accounts of haptic texture perception.
Projections of global health outcomes from 2005 to 2060 using the International Futures integrated forecasting model
Hughes,Barry B; Kuhn,Randall; Peterson,Cecilia M; Rothman,Dale S; Solórzano,José R; Mathers,Colin D; Dickson,Janet R;
Bulletin of the World Health Organization , 2011, DOI: 10.2471/BLT.10.083766
Abstract: objective: to develop an integrated health forecasting model as part of the international futures (ifs) modelling system. methods: the ifs model begins with the historical relationships between economic and social development and cause-specific mortality used by the global burden of disease project but builds forecasts from endogenous projections of these drivers by incorporating forward linkages from health outcomes back to inputs like population and economic growth. the hybrid ifs system adds alternative structural formulations for causes not well served by regression models and accounts for changes in proximate health risk factors. forecasts are made to 2100 but findings are reported to 2060. findings: the base model projects that deaths from communicable diseases (cds) will decline by 50%, whereas deaths from both non-communicable diseases (ncds) and injuries will more than double. considerable cross-national convergence in life expectancy will occur. climate-induced fluctuations in agricultural yield will cause little excess childhood mortality from cds, although other climate-health pathways were not explored. an optimistic scenario will produce 39 million fewer deaths in 2060 than a pessimistic one. our forward linkage model suggests that an optimistic scenario would result in a 20% per cent increase in gross domestic product (gdp) per capita, despite one billion additional people. southern asia would experience the greatest relative mortality reduction and the largest resulting benefit in per capita gdp. conclusion: long-term, integrated health forecasting helps us understand the links between health and other markers of human progress and offers powerful insight into key points of leverage for future improvements.
Access to expensive cancer drugs in Canada: a comparison of Quebec, Ontario and British-Columbia
Hughes D
Pratiques et Organisation des Soins , 2012,
Abstract: Summary Aim: Among the new cancer drugs, some are extremely expensive and provide few benefits relative to their cost. There is a variation in the coverage of these drugs between provincial public plans. This exploratory comparative study has two main goals. First, to identify differences in coverage which are the results of cost-benefit decisions. Second, to identify structural elements that promote public coverage of cancer drugs that is accessible, efficient and equitable. Methods: We analyzed and compared the reimbursement decisions of Quebec, Ontario and British-Columbia. We also analyzed and compared the evaluation agencies and public drug plans in these three provinces. We examined the eligibility criteria and financial contribution requested. Results: The coverage of cancer drugs is more extensive in Ontario and British-Columbia. However, in terms of the evaluation of cancer drugs, the British-Columbia agency is perating in “silo”, lacks distance from oncology specialists and needs more transparency. In terms of drug insurance plans, the basic premium in Quebec distributes a share of the cost to all insured. Ontario has the advantage of modulating the contributions on the basis of income. British-Columbia provides free access to cancer drugs but since other groups of patients are requested to pay a significant financial contribution, this can be seen as unfair. Conclusion: British-Columbia is presented as the champion of access to cancer drugs. However, we believe that the conditions under which decisions are made in that province unduly promote access at the expense of efficiency and equity. Prat Organ Soins. 2012;43(1):9-18
Utilizing Dimensional Analysis with Observed Data to Determine the Significance of Hydrodynamic Solutions in Coastal Hydrology  [PDF]
Eric D. Swain, Jeremy D. Decker, Joseph D. Hughes
Computational Water, Energy, and Environmental Engineering (CWEEE) , 2014, DOI: 10.4236/cweee.2014.32008
Abstract:

In this paper, the authors present an analysis of the magnitude of the temporal and spatial acceleration (inertial) terms in the surface-water flow equations and determine the conditions under which these inertial terms have sufficient magnitude to be required in the computations. Data from two South Florida field sites are examined and the relative magnitudes of temporal acceleration, spatial acceleration, and the gravity and friction terms are compared. Parameters are derived by using dimensionless numbers and applied to quantify the significance of the hydrodynamic effects. The time series of the ratio of the inertial and gravity terms from field sites are presented and compared with both a simplified indicator parameter and a more complex parameter called the Hydrodynamic Significance Number (HSN). Two test-case models were developed by using the SWIFT2D hydrodynamic simulator to examine flow behavior with and without the inertial terms and compute the HSN. The first model represented one of the previously-mentioned field sites during gate operations of a structure-managed coastal canal. The second model was a synthetic test case illustrating the drainage of water down a sloped surface from an initial stage while under constant flow. The analyses indicate that the times of substantial hydrodynamic effects are sporadic but significant. The simplified indicator parameter correlates much better with the hydrodynamic effect magnitude for a constant width channel such as Miami Canal than at the non-uniform North River. Higher HSN values indicate flow situations where the inertial terms are large and need to be taken into account.

Hydrological education and training needs in sub-Saharan Africa: requirements, constraints and progress
D. A. Hughes
Hydrology and Earth System Sciences (HESS) & Discussions (HESSD) , 2012,
Abstract: This paper represents a perspective on the education and training needs related to hydrology and water resources science within the sub-Saharan Africa region and discusses the requirements of the region, some of the relatively recent developments and initiatives and some of the constraints that exist and remain difficult to surmount. The requirements include the development of academic research capacity and technical skill for both the private and public sector at a variety of levels. Some of the constraints that exist include a lack of adequate funding, lack of follow-up after short training courses, lack of institutional support to continue training, and competition for major water resources development projects from organizations outside the region. One of the main conclusions is that to sustain both educational and practical expertise in hydrology and water resources science within the region there is a need to build a "critical mass" of local expertise. Part of this could be achieved by increasing networking within the region and promoting the sharing of information, tools and expertise. There is also a need to promote institutional support.
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