357 research outputs found
Interview with Don Hurst
An interview with Don Hurst regarding his experiences in a one-room school house.https://scholars.fhsu.edu/ors/1064/thumbnail.jp
Characterisation of a phoP P1vir transduction defect and the implication for the regulation of the extracytoplasmic stress response in Escherichia coli
The sensor kinase PhoQ and its cognate response regulator PhoP constitute a two-component system, which is primarily responsible for sensing and responding to Mg2+ starvation in Escherichia coli. Additionally, there is growing evidence of regulatory links between PhoPQ and constituents of the outer membrane. Furthermore, it has been shown that PhoPQ is regulated negatively by MicA, an sRNA controlled by sigmaE. Encoded by rpoE, sigmaE is an alternative sigma factor that is activated in response to extracytoplasmic stress, specifically misfolded outer membrane proteins. Surprisingly, it was not possible to generate ?phoP mutants, using P1vir transduction under standard conditions and kanamycin as the selective agent. Furthermore, a statistical analysis of these results indicates they cannot be explained by chance alone. The results show that PhoP is required for sigmaE activity in an RseA-independent manner, thereby suggesting that PhoP is a chief regulator of sigmaE activity. It is likely that diminished sigmaE activity in a phoP mutant, extracytoplasmic stress and OM deformation, caused by the reagents used in P1vir transduction, are responsible for the inability to transduce the phoP allele. Finally, evidence has been found relating to a second mechanism through which PhoP directly represses rpoE expression, thereby introducing further complexity into the regulator relationship that exists between sigmaE and PhoP
The role of evolutionary time, diversification rates and dispersal in determining the global diversity of a large radiation of passerine birds
Aim: Variation in species diversity among different geographic areas may result from differences in speciation and extinction rates, immigration and time for diversification. An area with high species diversity may be the result of a high net diversification rate, multiple immigration events from adjacent regions,anda long time available for the accumulation of species (know as the "time-for-speciation effect"). Here, we examine the relative importance of the three aforementionedprocesses in shaping the geographic diversity patterns of a large radiation of passerine birds.
Location: Global
Time period: Early Miocene to present
Major taxa studied: Babblers (Aves: Passeriformes)
Methods: Using a comprehensive phylogeny of extant species (~90% sampled) and distributions of the world's babblers, we reconstructed their biogeographic history and analysed the diversification dynamics. We examined how species richness correlates with the timing of regional colonization, the number of immigration events and the rate of speciation within all 13 geographic distribution regions.
Results: We found thatbabblers likely originated in the Sino-Himalayan Mountains (SHM) in the early Miocene, suggesting a long time for diversification and species accumulation within the SHM. Regression analyses showed the regional diversity of babblers can be well explained by the timing of the first colonization within of these areas, while differences in rates of speciation or immigration have far weaker effects. Nonetheless, the rapid speciation of Zosteropsduring the Pleistocene has accounted for the increased diversification and accumulation of species in the oceanic islands.
Main conclusions: Our results suggest that the global diversity patterns of babblers have predominantly been shaped by the time-for-speciation effect. Our findings also support an origin centred in tropical and subtropical parts of the SHM, with a cradle of recent diversification in the oceanic islands of the Indo-Pacific region, which provides new insights into the generation of global biodiversity hotspots.A near-complete phylogeny of babblers has been reconstructed in BEAST 1.8.4 based on 12 gene loci
Critical and Near-Critical Branching Processes
Scale-free dynamics in physical and biological systems can arise from a
variety of causes. Here, we explore a branching process which leads to such
dynamics. We find conditions for the appearance of power laws and study
quantitatively what happens to these power laws when such conditions are
violated. From a branching process model, we predict the behavior of two
systems which seem to exhibit near scale-free behavior--rank-frequency
distributions of number of subtaxa in biology, and abundance distributions of
genotypes in an artificial life system. In the light of these, we discuss
distributions of avalanche sizes in the Bak-Tang-Wiesenfeld sandpile model.Comment: 9 pages LaTex with 10 PS figures. v.1 of this paper contains results
from non-critical sandpile simulations that were excised from the published
versio
A self-adjusted Monte Carlo simulation as model of financial markets with central regulation
Properties of the self-adjusted Monte Carlo algorithm applied to 2d Ising
ferromagnet are studied numerically. The endogenous feedback form expressed in
terms of the instant running averages is suggested in order to generate a
biased random walk of the temperature that converges to criticality without an
external tuning. The robustness of a stationary regime with respect to partial
accessibility of the information is demonstrated. Several statistical and
scaling aspects have been identified which allow to establish an alternative
spin lattice model of the financial market. It turns out that our model alike
model suggested by S. Bornholdt, Int. J. Mod. Phys. C {\bf 12} (2001) 667, may
be described by L\'evy-type stationary distribution of feedback variations with
unique exponent . However, the differences reflected by
Hurst exponents suggest that resemblances between the studied models seem to be
nontrivial.Comment: 19 pages, 9 figures, 30 reference
Piecewise Linear Models for the Quasiperiodic Transition to Chaos
We formulate and study analytically and computationally two families of
piecewise linear degree one circle maps. These families offer the rare
advantage of being non-trivial but essentially solvable models for the
phenomenon of mode-locking and the quasi-periodic transition to chaos. For
instance, for these families, we obtain complete solutions to several questions
still largely unanswered for families of smooth circle maps. Our main results
describe (1) the sets of maps in these families having some prescribed rotation
interval; (2) the boundaries between zero and positive topological entropy and
between zero length and non-zero length rotation interval; and (3) the
structure and bifurcations of the attractors in one of these families. We
discuss the interpretation of these maps as low-order spline approximations to
the classic ``sine-circle'' map and examine more generally the implications of
our results for the case of smooth circle maps. We also mention a possible
connection to recent experiments on models of a driven Josephson junction.Comment: 75 pages, plain TeX, 47 figures (available on request
Multifractal Behaviour of n-Simplex Lattice
We study the asymptotic behaviour of resistance scaling and fluctuation of
resistance that give rise to flicker noise in an {\em n}-simplex lattice. We
propose a simple method to calculate the resistance scaling and give a
closed-form formula to calculate the exponent, , associated with
resistance scaling, for any n. Using current cumulant method we calculate the
exact noise exponent for n-simplex lattices.Comment: Latex, 9 pages including one figur
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