Phase Transitions Occurring in Models of Neighborhood Racial Segregation

This thesis is organized as two chapters whose contents are closely related yet quite distinct. The first chapter presents a paper Role of \u27Vision\u27 in Neighborhood Racial Segregation: A Variant of the Schelling Segregation Model, authored by myself and Dr. Jaggi, which has been accepted for publication by the journal Urban Studies and is currently in press (as of April 2003). This chapter introduces the well-known Schelling model of neighborhood segregation, outlines the sociopolitical motivation for our work, and presents the key results that we believe are of interest to social scientists. Chapter two, which ought to be of greater interest to the physics community, presents the results of our investigations into the parallels between the Schelling model and critical phenomena. Our primary extension of the Schelling model was to include social agents who can authentically \u27see\u27 their neighbors up to a distance R, called \u27vision\u27. By exploring the consequences of systematically varying R, we have developed an understanding of how vision interacts with racial preferences and minority concentrations and leads to novel, complex segregation behavior. We have discovered three regimes: an unstable regime, where societies invariably segregate; a stable regime, where integrated societies remain stable; and an intermediate regime where a complex behavior is observed. Since the primary audience of Urban Studies consists of sociologists and economists, we have not elaborated in the first chapter upon the phase transition which was strongly suggested by the complex behavior in the intermediate regime. The purpose of chapter two then, is to elucidate these additional physically interesting aspects of our model. Melting is a textbook example of first order (discontinuous) phase transitions. These are marked by two central features: a sharp temperature at which the transition occurs, and the coexistence of the two phases at that melting point. One can study the first-order phase transition that ice undergoes when melting into water by observing the ice while continuously raising its temperature. However, if you were only able to view the system at certain discrete temperatures, you would only see a either a piece of ice or a puddle of water during each observation. Thus in order to study the potential phase transition occurring in our model, we must be able to control the governing parameters continuously. However, in our original \u27discrete\u27 model, R measures how far an agent sees from its own home as an integer number of houses. Since we can only assign discrete values to R, it is meaningless to speak of a phase transition occurring as a function of this variable. To overcome the limitations of our first model, we introduce a continuous model in chapter two where the range of vision (denoted R2 for notational clarity) can be varied continuously. This model uses a utility function that assigns greater weight to neighbors nearer an evaluating agent. The function used to model this decrease in utility contribution with distance is an exponentially decaying curve. We control the steepness of this curve (and thereby control the agents\u27 vision) using R2. Since R2 can be set to equal any positive real number, we can indeed study the possible phase transition in our simulations\u27 behavior as the function of a continuous variable. Additionally, the continuous model demonstrates the robustness of the sociologically relevant conclusions drawn in chapter one. Our continuous model, a generalization of a model developed by Wasserman and Yohe (2001), is in fact more realistic than our first model. In particular, we were pleased to discover the same three behavioral regimes and all associated trends in both our discrete model and our continuous model. This confirms that our original results were robust and not merely algorithmic artifacts related to the specific treatment of vision used in our discrete model

Modeling Pathogenic Mutations of Human Twinkle in Drosophila Suggests an Apoptosis Role in Response to Mitochondrial Defects

The human gene C10orf2 encodes the mitochondrial replicative DNA helicase Twinkle, mutations of which are responsible for a significant fraction of cases of autosomal dominant progressive external ophthalmoplegia (adPEO), a human mitochondrial disease caused by defects in intergenomic communication. We report the analysis of orthologous mutations in the Drosophila melanogaster mitochondrial DNA (mtDNA) helicase gene, d-mtDNA helicase. Increased expression of wild type d-mtDNA helicase using the UAS-GAL4 system leads to an increase in mtDNA copy number throughout adult life without any noteworthy phenotype, whereas overexpression of d-mtDNA helicase containing the K388A mutation in the helicase active site results in a severe depletion of mtDNA and a lethal phenotype. Overexpression of two d-mtDNA helicase variants equivalent to two human adPEO mutations shows differential effects. The A442P mutation exhibits a dominant negative effect similar to that of the active site mutant. In contrast, overexpression of d-mtDNA helicase containing the W441C mutation results in a slight decrease in mtDNA copy number during the third instar larval stage, and a moderate decrease in life span in the adult population. Overexpression of d-mtDNA helicase containing either the K388A or A442P mutations causes a mitochondrial oxidative phosphorylation (OXPHOS) defect that significantly reduces cell proliferation. The mitochondrial impairment caused by these mutations promotes apoptosis, arguing that mitochondria regulate programmed cell death in Drosophila. Our study of d-mtDNA helicase overexpression provides a tractable Drosophila model for understanding the cellular and molecular effects of human adPEO mutations

Sr, Nd, Pb and Os Isotope Systematics of CAMP Tholeiites from Eastern North America (ENA): Evidence of a Subduction-enriched Mantle Source

The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, with an areal extent exceeding 107 km2. Here we document the geochemical characteristics of CAMP basalts from Triassic-Jurassic basins in northeastern USA and Nova Scotia (Canada). The CAMP rocks occur as lava flows, sills and dykes. All of our analysed samples show chemical characteristics typical of CAMP basalts with low titanium content, which include enrichment in the most incompatible elements and negative Nb anomalies. All the basalts also show enriched Sr-Nd-Pb initial (t = 201 Ma) isotopic compositions (206Pb/204Pbini. = 18·155-18·691, 207Pb/204Pbini. = 15·616-15·668, 208Pb/204Pbini. = 38·160-38·616, 143Nd/144Ndini. = 0·512169-0·512499). On the basis of stratigraphy, rare earth element (REE) chemistry and Sr-Nd-Pb isotope composition, three chemical groups are defined. The Hook Mountain group, with the lowest La/Yb ratios, initial 206Pb/204Pbini. >18·5 and 143Nd/144Ndini. > 0·51238, comprises all the lastest and upper stratigraphic units. The Preakness group, with intermediate La/Yb ratios, 206Pb/204Pbini. > 18·5 and 0·51233 > 143Nd/144Ndini. > 0·51225, comprises the intermediate units. The Orange Mountain group has the highest La/Yb ratios and 143Nd/144Ndini. < 0·51235 and involves all the earliest and stratigraphically lowest units, including the entire North Mountain basalts from Nova Scotia. In this last group, three sub-groups may be distinguished: the Rapidan sill, which has 206Pb/204Pbini. higher than 18·5, the Shelburne sub-group, which has 143Nd/144Ndini. < 0·51225, and the remaining Orange Mt samples. With the exception of one sample, the Eastern North America (ENA) CAMP basalts display initial 187Os/188Os ratios in the range of mantle-derived magmas (<0·15). Simple modelling shows that the composition of the ENA CAMP basalts cannot plausibly be explained solely by crustal contamination of oceanic island basalt (OIB), mid-ocean ridge basalt (MORB) or oceanic plateau basalt (OPB) magmas. Mixing of such magma compositions with sub-continental lithospheric mantle (SCLM)-derived melts followed by crustal contamination, by either assimilation-fractional crystallization (AFC) or assimilation through turbulent ascent (ATA) processes is somewhat more successful. However, this latter scenario does not reproduce the REE and isotopic composition of the ENA CAMP in a fully satisfactory manner. Alternatively, we propose a model in which asthenospheric mantle overlying a subducted slab (i.e. mantle wedge) was enriched during Cambrian to Devonian subduction by sedimentary material, isotopically equivalent to Proterozoic-Lower Paleozoic crustal rocks. Subsequently, after subduction ceased, the isotopic composition of this mantle evolved by radioactive decay for another 170 Myr until the CAMP magmatic event. Varying amounts and compositions of the incorporated sedimentary component coupled with radiogenic ingrowth over time can account for the main geochemical characteristics of the ENA CAMP (enriched incompatible element patterns, negative Nb anomalies, enriched Sr-Nd-Pb isotopic composition) and the differences between the three chemical group

Significant Surface-Water Connectivity of Geographically Isolated Wetlands

We evaluated the current literature, coupled with our collective research expertise, on surface-water connectivity of wetlands considered to be geographically isolated (sensu Tiner Wetlands 23:494–516, 2003a) to critically assess the scientific foundation of grouping wetlands based on the singular condition of being surrounded by uplands. The most recent research on wetlands considered to be geographically isolated shows the difficulties in grouping an ecological resource that does not reliably indicate lack of surface water connectivity in order to meet legal, regulatory, or scientific needs. Additionally, the practice of identifying geographically isolated wetlands based on distance from a stream can result in gross overestimates of the number of wetlands lacking ecologically important surface-water connections. Our findings do not support use of the overly simplistic label of geographically isolated wetlands . Wetlands surrounded by uplands vary in function and surface water connections based on wetland landscape setting, context, climate, and geographic region and should be evaluated as such. We found that the geographically isolated grouping does not reflect our understanding of the hydrologic variability of these wetlands and hence does not benefit conservation of the Nation’s diverse wetland resources. Therefore, we strongly discourage use of categorizations that provide overly simplistic views of surface water connectivity of wetlands fully embedded in upland landscapes

Treatment Toxicity: Radiation

Radiation exposures, both intentional and unintentional, have influence on normal tissue function. Short-term and long-term injuries can occur to all cell systems of both limited and rapid self-renewal potential. Radiation effects can last a lifetime for a patient and can produce complications for all organs and systems. Often invisible at the time of exposure, the fingerprints for cell damage can appear at any timepoint after. Health-care providers will need comprehensive knowledge and understanding of the acute and late effects of radiation exposure and how these interrelate with immediate and long-term care