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Central Problem (central + problem)
Selected AbstractsOur Knowledge of Numbers as Self-Subsistent ObjectsDIALECTICA, Issue 2 2005William Demopoulos A feature of Frege's philosophy of arithmetic that has elicited a great deal of attention in the recent secondary literature is his contention that numbers are ,self-subsistent' objects. The considerable interest in this thesis among the contemporary philosophy of mathematics community stands in marked contrast to Kreisel's folk-lore observation that the central problem in the philosophy of mathematics is not the existence of mathematical objects, but the objectivity of mathematics. Although Frege was undoubtedly concerned with both questions, a goal of the present paper is to argue that his success in securing the objectivity of arithmetic depends on a less contentious commitment to numbers as objects than either he or his critics have supposed. As such, this paper is an articulation and defense of both Frege's analysis of arithmetic and Kreisel's observation. [source] Parasites in food webs: the ultimate missing linksECOLOGY LETTERS, Issue 6 2008Kevin D. Lafferty Abstract Parasitism is the most common consumer strategy among organisms, yet only recently has there been a call for the inclusion of infectious disease agents in food webs. The value of this effort hinges on whether parasites affect food-web properties. Increasing evidence suggests that parasites have the potential to uniquely alter food-web topology in terms of chain length, connectance and robustness. In addition, parasites might affect food-web stability, interaction strength and energy flow. Food-web structure also affects infectious disease dynamics because parasites depend on the ecological networks in which they live. Empirically, incorporating parasites into food webs is straightforward. We may start with existing food webs and add parasites as nodes, or we may try to build food webs around systems for which we already have a good understanding of infectious processes. In the future, perhaps researchers will add parasites while they construct food webs. Less clear is how food-web theory can accommodate parasites. This is a deep and central problem in theoretical biology and applied mathematics. For instance, is representing parasites with complex life cycles as a single node equivalent to representing other species with ontogenetic niche shifts as a single node? Can parasitism fit into fundamental frameworks such as the niche model? Can we integrate infectious disease models into the emerging field of dynamic food-web modelling? Future progress will benefit from interdisciplinary collaborations between ecologists and infectious disease biologists. [source] GEOGRAPHIC VARIATION, FREQUENCY-DEPENDENT SELECTION, AND THE MAINTENANCE OF A FEMALE-LIMITED POLYMORPHISMEVOLUTION, Issue 1 2010Ryan Calsbeek A central problem in evolutionary biology is to understand how spatial and temporal variation in selection maintain genetic variation within and among populations. Brown anole lizards (Anolis sagrei) exhibit a dorsal pattern polymorphism that is expressed only in females, which occur in "diamond,""bar," and intermediate "diamond-bar" morphs. To understand the inheritance of this polymorphism, we conducted a captive breeding study that refuted several single-locus models and supported a two-locus mode of inheritance. To describe geographic variation in morph frequencies, we surveyed 13 populations from two major islands in The Bahamas. Morph frequencies differed substantially between major islands but were highly congruent within each island. Finally, we measured viability selection on each island to test two hypotheses regarding the maintenance of the polymorphism: (1) that spatial variation in selection maintains variation in morph frequencies between islands, and (2) that temporal variation in selection across years maintains variation within islands. Although bar females had relatively lower survival where they were rare, our data do not otherwise suggest that selection varies spatially between islands. However, diamond-bar females were subject to positive frequency-dependent selection across years, and the relative fitness of bar and diamond females alternated across years. We propose that this polymorphism is maintained by temporal variation in selection coupled with the sheltering of alleles via a two-locus inheritance pattern and sex-limited expression. [source] RAPID EVOLUTIONARY ESCAPE BY LARGE POPULATIONS FROM LOCAL FITNESS PEAKS IS LIKELY IN NATUREEVOLUTION, Issue 6 2005Daniel M. Weinreich Abstract Fitness interactions between loci in the genome, or epistasis, can result in mutations that are individually deleterious but jointly beneficial. Such epistasis gives rise to multiple peaks on the genotypic fitness landscape. The problem of evolutionary escape from such local peaks has been a central problem of evolutionary genetics for at least 75 years. Much attention has focused on models of small populations, in which the sequential fixation of valley genotypes carrying individually deleterious mutations operates most quickly owing to genetic drift. However, valley genotypes can also be subject to mutation while transiently segregating, giving rise to copies of the high fitness escape genotype carrying the jointly beneficial mutations. In the absence of genetic recombination, these mutations may then fix simultaneously. The time for this process declines sharply with increasing population size, and it eventually comes to dominate evolutionary behavior. Here we develop an analytic expression for Ncrit, the critical population size that defines the boundary between these regimes, which shows that both are likely to operate in nature. Frequent recombination may disrupt high-fitness escape genotypes produced in populations larger than Ncrit before they reach fixation, defining a third regime whose rate again slows with increasing population size. We develop a novel expression for this critical recombination rate, which shows that in large populations the simultaneous fixation of mutations that are beneficial only jointly is unlikely to be disrupted by genetic recombination if their map distance is on the order of the size of single genes. Thus, counterintuitively, mass selection alone offers a biologically realistic resolution to the problem of evolutionary escape from local fitness peaks in natural populations. [source] Northern Atlantic Oscillation effects on the temporal and spatial dynamics of green spruce aphid populations in the UKJOURNAL OF ANIMAL ECOLOGY, Issue 4 2007SILVERIO SALDAÑA Summary 1The role of climate variability in determining the spatial and temporal patterns of numerical fluctuations is a central problem in ecology. The influence of the North Atlantic Oscillation (NAO) index on the population dynamics and spatial synchrony of the green spruce aphid Elatobium abietinum across the UK was shown. 2Fifteen overlapping time series within the UK were analysed; we used nonparametric models for determining the feedback nonlinear structure and the climatic effects. The spatial synchrony of these populations and the relationship between synchrony and NAO was estimated. 3From the 15 time series across the UK, 11 showed positive and significant NAO effects. In most of the cases the NAO effects were nonlinear showing strong negative effects of low values. The NAO variation improve the explained variance of the first-order feedback models in 14·5%; ranging from 0% to 48%. All data showed strong-nonlinear (concave) feedback structure. In most of the localities the explained variance by the first-order feedback was about 50,60%. 4The spatial synchrony of the per capita growth rates and residuals is high across long distances for those populations affected by NAO. The correlation function predicts a spatial scale of synchrony of about 350,400 km for NAO influenced populations. 5We think that simple population theoretical models describing the link between NAO fluctuations and green spruce aphid dynamics may be fundamental for predicting and simulating the consequences of different climatic scenarios of the future. [source] Elementary Propositions and Essentially Incomplete Knowledge: A Framework for the Interpretation of Quantum MechanicsNOUS, Issue 1 2004William Demopoulos A central problem in the interpretation of non-relativistic quantum mechanics is to relate the conceptual structure of the theory to the classical idea of the state of a physical system. This paper approaches the problem by presenting an analysis of the notion of an elementary physical proposition. The notion is shown to be realized in standard formulations of the theory and to illuminate the significance of proofs of the impossibility of hidden variable extensions. In the interpretation of quantum mechanics that emerges from this analysis, the philosophically distinctive features of the theory derive from the fact that it seeks to represent a reality of which complete knowledge is essentially unattainable. [source] Computational Biology: Toward Deciphering Gene Regulatory Information in Mammalian GenomesBIOMETRICS, Issue 3 2006Hongkai Ji Summary Computational biology is a rapidly evolving area where methodologies from computer science, mathematics, and statistics are applied to address fundamental problems in biology. The study of gene regulatory information is a central problem in current computational biology. This article reviews recent development of statistical methods related to this field. Starting from microarray gene selection, we examine methods for finding transcription factor binding motifs and cis -regulatory modules in coregulated genes, and methods for utilizing information from cross-species comparisons and ChIP-chip experiments. The ultimate understanding of cis -regulatory logic in mammalian genomes may require the integration of information collected from all these steps. [source] Childhood risks and protective factors in social exclusionCHILDREN & SOCIETY, Issue 5 2001John Bynner Combating social exclusion is a dominant theme in the current policy agenda. Yet the term social exclusion is of relatively recent origin. It was promoted originally in France in policy debates surrounding disability (Evans, 2000) and through theoretical developments in sociology and political science about the increasing detachment of certain individuals and groups from the state in late modernity (Beck and others, 1994). A quite different and more long-standing research tradition is to be found in developmental psychology,respectively in the sub-fields of ,developmental psychopathology' (Rutter, 1993) and ,life course theory and lifespan developmental psychology' (Elder and others 1993, 1998a&b; Lerner,1998; Lerner and others, 2000). The two themes come together in the idea of risk: Which children are most vulnerable to adult psychiatric disorders or criminality? Which children are likely to become socially excluded as adults? A dialogue between risk and social exclusion is likely to be fruitful in bringing together large and diverse research literatures combining both explanatory and intervention studies to bear on a central problem of modern society. The purpose of this paper is to begin such a task, but selectively, focusing on the main themes of research, as illuminated by key findings. The paper concludes with a consideration of recent policy initiatives to combat social exclusion, in which the ideas of risk and protection have a central place. Copyright © 2001 John Wiley & Sons, Ltd. [source] The psychological contract: A critical reviewINTERNATIONAL JOURNAL OF MANAGEMENT REVIEWS, Issue 2 2006Niall Cullinane Literature on the psychological contract has blossomed progressively over the last ten years to the extent that it is now firmly located within the lexicon of the Human Resource Management (HRM) discipline. Yet as this review indicates, the theoretical assumptions that seem to pervade the psychological contract literature are not without major deficiencies, which in turn pose serious questions around the continued sustainability of the construct as currently constituted. This paper addresses some of the central problems presently confronting the theoretical side of the psychological contract literature. In seeking to advance knowledge and understanding, this review calls for an alternative approach to studying the psychological contract on the basis of a more critical and discursive literature analysis. From this, the authors unpick the construct of the psychological contract as portrayed in much of the extant literature and argue that, in its present form, it symbolizes an ideologically biased formula designed for a particular managerialist interpretation of contemporary work and employment. [source] Society for Latin American Studies 2004 Plenary Lecture The Return of Cuba to Latin America: The End of Cuban Exceptionalism?BULLETIN OF LATIN AMERICAN RESEARCH, Issue 4 2004Miguel Angel Centeno Comparing Cuban history and contemporary circumstances to those in the rest of the region, this article challenges the idea of Cuban exceptionalism. The last four decades have seen Cuba move away from typical Latin American patterns such as economic and geographical inequalities. The Revolution has not been able, however, to reverse a historical dependence on external financing nor has it resolved racial inequities. While the Revolution did establish an unusually effective political apparatus, it did not use this opportunity to nurture democracy. As the social welfare advances of the revolution erode, Cuba resembles the rest of the continent ever more and reflects the central problems facing the region. [source] | |||||||||||||