20 research outputs found
Strategies for the evolution of sex
We find that the hypothesis made by Jan, Stauffer and Moseley [Theory in
Biosc., 119, 166 (2000)] for the evolution of sex, namely a strategy devised to
escape extinction due to too many deleterious mutations, is sufficient but not
necessary for the successful evolution of a steady state population of sexual
individuals within a finite population. Simply allowing for a finite
probability for conversion to sex in each generation also gives rise to a
stable sexual population, in the presence of an upper limit on the number of
deleterious mutations per individual. For large values of this probability, we
find a phase transition to an intermittent, multi-stable regime. On the other
hand, in the limit of extremely slow drive, another transition takes place to a
different steady state distribution, with fewer deleterious mutations within
the asexual population.Comment: RevTeX, 11 pages, multicolumn, including 12 figure
Punctuated equilibria and 1/f noise in a biological coevolution model with individual-based dynamics
We present a study by linear stability analysis and large-scale Monte Carlo
simulations of a simple model of biological coevolution. Selection is provided
through a reproduction probability that contains quenched, random interspecies
interactions, while genetic variation is provided through a low mutation rate.
Both selection and mutation act on individual organisms. Consistent with some
current theories of macroevolutionary dynamics, the model displays
intermittent, statistically self-similar behavior with punctuated equilibria.
The probability density for the lifetimes of ecological communities is well
approximated by a power law with exponent near -2, and the corresponding power
spectral densities show 1/f noise (flicker noise) over several decades. The
long-lived communities (quasi-steady states) consist of a relatively small
number of mutualistically interacting species, and they are surrounded by a
``protection zone'' of closely related genotypes that have a very low
probability of invading the resident community. The extent of the protection
zone affects the stability of the community in a way analogous to the height of
the free-energy barrier surrounding a metastable state in a physical system.
Measures of biological diversity are on average stationary with no discernible
trends, even over our very long simulation runs of approximately 3.4x10^7
generations.Comment: 20 pages RevTex. Minor revisions consistent with published versio
Current Status of Simulations
As the title suggests, the purpose of this chapter is to review the current
status of numerical simulations of black hole accretion disks. This chapter
focuses exclusively on global simulations of the accretion process within a few
tens of gravitational radii of the black hole. Most of the simulations
discussed are performed using general relativistic magnetohydrodynamic (MHD)
schemes, although some mention is made of Newtonian radiation MHD simulations
and smoothed particle hydrodynamics. The goal is to convey some of the exciting
work that has been going on in the past few years and provide some speculation
on future directions.Comment: 15 pages, 14 figures, to appear in the proceedings of the ISSI-Bern
workshop on "The Physics of Accretion onto Black Holes" (8-12 October 2012
Confirmação de infecção por Brucella abortus em um rebanho bovino certificado livre em Minas Gerais: relato de caso
Relata-se a ocorrência de um surto de brucelose em um rebanho de aproximadamente 1000 animais, livre da doença há 18 anos, certificado pelo Ministério da Agricultura, Pecuária e Abastecimento desde 2006. Dois animais reagiram aos testes sorológicos de diagnóstico por ocasião dos procedimentos de recertificação em 2008. Após o sacrifício deles, Brucella abortus, biovariedade 1, amostra não vacinal, foi isolada e identificada por meio de provas bioquímicas e de biologia molecular (PCR AMOS). A origem do agente no rebanho é de difícil determinação. No entanto, a adoção de procedimentos preconizados pelo Programa Nacional de Controle e Erradicação da Brucelose permitiu evitar a disseminação da enfermidade. Ocorrências como essas, em que rebanhos livres foram infectados após anos sem a ocorrência de brucelose, nunca haviam sido relatadas no Brasil
Physiochemical parameters optimization for enhanced nisin production by Lactococcus lactis (MTCC 440)
The influence of various physiochemical parameters on the growth of Lactococcus lactis sub sp. lactis MTCC 440 was studied at shake flask level for 20 h. Media optimization (MRS broth) was studied to achieve enhanced growth of the organism and also nisin production. Bioassay of nisin was done with agar diffusion method using Streptococcus agalactae NCIM 2401 as indicator strain. MRS broth (6%, w/v) with 0.15μg/ml of nisin supplemented with 0.5% (v/v) skimmed milk was found to be the best for nisin production as well as for growth of L lactis. The production of nisin was strongly influenced by the presence of skimmed milk and nisin in MRS broth. The production of nisin was affected by the physical parameters and maximum nisin production was at 30(0)C while the optimal temperature for biomass production was 37(0)C