341 research outputs found
Genetic Assimilation and Canalisation in the Baldwin Effect
The Baldwin Effect indicates that individually learned behaviours acquired during an organism’s lifetime can influence the evolutionary path taken by a population, without any direct Lamarckian transfer of traits from phenotype to genotype. Several computational studies modelling this effect have included complications that restrict its applicability. Here we present a simplified model that is used to reveal the essential mechanisms and highlight several conceptual issues that have not been clearly defined in prior literature. In particular, we suggest that canalisation and genetic assimilation, often conflated in previous studies, are separate concepts and the former is actually not required for non-heritable phenotypic variation to guide genetic variation. Additionally, learning, often considered to be essential for the Baldwin Effect, can be replaced with a more general phenotypic plasticity model. These simplifications potentially permit the Baldwin Effect to operate in much more general circumstances
Natural disturbances and the physiognomy of pine savannas : A phenomenological model
Abstract. Question: The decline of the Pinus palustris ecosystems has resulted from anthropogenic influences, such as conversion to pine plantation forestry, agriculture and land development, all of which are closely related to increases in human populations. Other effects, however, have arisen from alterations in disturbance regimes that maintain the structure and function of these ecosystems. How have alterations of the disturbance regime altered the physiognomy of ‘old-growth’ stands, and what are the implications for ecosystem conservation and restoration?
Methods: In contrast to models that emphasize close interactions among the vertically complex strata, we develop a conceptual phenomenological model for the physiognomic structure of Pinus palustris stands. We relate two natural disturbances (tropical storms and fire) that affect different stages of the life cycle to different aspects of the physiognomic structure. We then compare overstorey stand structure and ground cover composition of two old-growth longleaf stands near the extremes of different composite disturbance regimes: the Wade Tract (frequent hurricanes and fire) and the Boyd Tract (infrequent hurricanes and long-term fire exclusion).
Results: We predict that tropical storms and fires have different effects on stand physiognomy. Tropical storms are periodic, and sometimes intense, whereas fires are more frequent and less intense. Hurricanes directly influence the overstorey via wind-caused damage and mortality, and indirectly influence the herb layer by altering the spatial distribution of shading and litter accumulation. Fire exerts direct effects on juvenile stages and indirect effects on the herb layer via fine fuel consumption and selective mortality of potential competitors of P. palustris juveniles. These differences in effects of disturbances can result in widely different physiognomies for P. palustris stands. Finally, some global climate change scenarios have suggested that changes may occur in tropical storm and fire regimes, altering frequency and severity. Such changes may greatly affect pine stands, and ultimately entire pine savanna ecosystems.
Conclusions: Our phenomenological model of disturbance regimes in Pinus palustris old-growth produces very different physiognomies for different disturbances regimes that reflect natural process and human management actions. This model can be used to derive restoration strategies for pine savannas that are linked to reinstitution of important ecological processes rather than specific physiognomic states
Fano Resonances in Electronic Transport through a Single Electron Transistor
We have observed asymmetric Fano resonances in the conductance of a single
electron transistor resulting from interference between a resonant and a
nonresonant path through the system. The resonant component shows all the
features typical of quantum dots, but the origin of the non-resonant path is
unclear. A unique feature of this experimental system, compared to others that
show Fano line shapes, is that changing the voltages on various gates allows
one to alter the interference between the two paths.Comment: 8 pages, 6 figures. Submitted to PR
A radium assay technique using hydrous titanium oxide adsorbent for the Sudbury Neutrino Observatory
As photodisintegration of deuterons mimics the disintegration of deuterons by
neutrinos, the accurate measurement of the radioactivity from thorium and
uranium decay chains in the heavy water in the Sudbury Neutrino Observatory
(SNO) is essential for the determination of the total solar neutrino flux. A
radium assay technique of the required sensitivity is described that uses
hydrous titanium oxide adsorbent on a filtration membrane together with a
beta-alpha delayed coincidence counting system. For a 200 tonne assay the
detection limit for 232Th is a concentration of 3 x 10^(-16) g Th/g water and
for 238U of 3 x 10^(-16) g U/g water. Results of assays of both the heavy and
light water carried out during the first two years of data collection of SNO
are presented.Comment: 12 pages, 4 figure
Planck scale effects in neutrino physics
We study the phenomenology and cosmology of the Majoron (flavon) models of
three active and one inert neutrino paying special attention to the possible
(almost) conserved generalization of the Zeldovich-Konopinski-Mahmoud lepton
charge. Using Planck scale physics effects which provide the breaking of the
lepton charge, we show how in this picture one can incorporate the solutions to
some of the central issues in neutrino physics such as the solar and
atmospheric neutrino puzzles, dark matter and a 17 keV neutrino. These
gravitational effects induce tiny Majorana mass terms for neutrinos and
considerable masses for flavons. The cosmological demand for the sufficiently
fast decay of flavons implies a lower limit on the electron neutrino mass in
the range of 0.1-1 eV.Comment: 24 pages, 1 figure (not included but available upon request), LaTex,
IC/92/196, SISSA-140/92/EP, LMU-09/9
Quasi BPS Wilson loops, localization of loop equation by homology and exact beta function in the large N limit of SU(N) Yang-Mills theory
We localize the loop equation of large-N YM theory in the ASD variables on a
critical equation for an effective action by means of homological methods as
opposed to the cohomological localization of equivariantly closed forms in
local field theory. Our localization occurs for some special simple quasi BPS
Wilson loops, that have no perimeter divergence and no cusp anomaly for
backtracking cusps, in a partial Eguchi-Kawai reduction from four to two
dimensions of the non-commutative theory in the limit of infinite
non-commutativity and in a lattice regularization in which the ASD integration
variables live at the points of the lattice, thus implying an embedding of
parabolic Higgs bundles in the YM functional integral. We find that the beta
function of the effective action is saturated by the non-commutative ASD
vortices of the EK reduction. An exact canonical beta function of NSVZ type
that reproduces the universal first and second perturbative coefficients
follows by the localization on vortices. Finally we argue that a scheme can be
found in which the canonical coupling coincides with the physical charge
between static quark sources in the large-N limit and we compare our
theoretical calculation with some numerical lattice result.Comment: 52 pages, latex; a change of notation in the vortices equations; a
footnote adde
The Mathematical Universe
I explore physics implications of the External Reality Hypothesis (ERH) that
there exists an external physical reality completely independent of us humans.
I argue that with a sufficiently broad definition of mathematics, it implies
the Mathematical Universe Hypothesis (MUH) that our physical world is an
abstract mathematical structure. I discuss various implications of the ERH and
MUH, ranging from standard physics topics like symmetries, irreducible
representations, units, free parameters, randomness and initial conditions to
broader issues like consciousness, parallel universes and Godel incompleteness.
I hypothesize that only computable and decidable (in Godel's sense) structures
exist, which alleviates the cosmological measure problem and help explain why
our physical laws appear so simple. I also comment on the intimate relation
between mathematical structures, computations, simulations and physical
systems.Comment: Replaced to match accepted Found. Phys. version, 31 pages, 5 figs;
more details at http://space.mit.edu/home/tegmark/toe.htm
Spallation reactions. A successful interplay between modeling and applications
The spallation reactions are a type of nuclear reaction which occur in space
by interaction of the cosmic rays with interstellar bodies. The first
spallation reactions induced with an accelerator took place in 1947 at the
Berkeley cyclotron (University of California) with 200 MeV deuterons and 400
MeV alpha beams. They highlighted the multiple emission of neutrons and charged
particles and the production of a large number of residual nuclei far different
from the target nuclei. The same year R. Serber describes the reaction in two
steps: a first and fast one with high-energy particle emission leading to an
excited remnant nucleus, and a second one, much slower, the de-excitation of
the remnant. In 2010 IAEA organized a worskhop to present the results of the
most widely used spallation codes within a benchmark of spallation models. If
one of the goals was to understand the deficiencies, if any, in each code, one
remarkable outcome points out the overall high-quality level of some models and
so the great improvements achieved since Serber. Particle transport codes can
then rely on such spallation models to treat the reactions between a light
particle and an atomic nucleus with energies spanning from few tens of MeV up
to some GeV. An overview of the spallation reactions modeling is presented in
order to point out the incomparable contribution of models based on basic
physics to numerous applications where such reactions occur. Validations or
benchmarks, which are necessary steps in the improvement process, are also
addressed, as well as the potential future domains of development. Spallation
reactions modeling is a representative case of continuous studies aiming at
understanding a reaction mechanism and which end up in a powerful tool.Comment: 59 pages, 54 figures, Revie
The PHENIX Experiment at RHIC
The physics emphases of the PHENIX collaboration and the design and current
status of the PHENIX detector are discussed. The plan of the collaboration for
making the most effective use of the available luminosity in the first years of
RHIC operation is also presented.Comment: 5 pages, 1 figure. Further details of the PHENIX physics program
available at http://www.rhic.bnl.gov/phenix
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Community psychiatric nurses and the care co-ordinator role: squeezed to provide ‘limited nursing’.
Background: The Care Programme Approach (CPA) is the key policy underpinning community-focused mental health services but has been unevenly implemented and is associated with increased inpatient bed use. The care co-ordinator role is central to the CPA and is most often held by Community Psychiatric Nurses (CPNs), but there has been little research into how this role is conducted or how it impacts on the work of CPNs and their ability to meet the needs of service users.
Aim: The study aimed to identify and illuminate the factors that either facilitated or constrained the ability of CPNs, in their role as care co-ordinators, to meet service users’ and carers’ needs.
Methods: A multiple case study of seven sectorised community mental health teams was employed over two years using predominantly qualitative methods of participant observation, semi-structured interviews and document review.
Findings: Additional duties and responsibilities specifically associated with the care co-ordinator role and multidisciplinary working, combined with heavy workloads, combined to produce ‘limited nursing’, whereby CPNs are unable to provide evidence-based psychosocial interventions that are recognised to reduce relapse amongst people with severe mental illness.
Conclusions: The role of the CPA care co-ordinator was not designed to support the provision of psychosocial interventions. Consequently, CPNs in the co-ordinator role faced with competing demands are unable to provide the range of structured, evidence-based interventions required. This may partially account for the increased inpatient bed use associated with the CPA
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