1,720,959 research outputs found
What is hidden behind the concept of ecosystem efficiency in energy transformation?
No full textThe number of energy transformation levels in trophic webs is usually below five, but can be extended up to ten when parasites and hyper-parasites are included. Research on the structure and function of food webs is relevant to the complexity–stability–productivity debate. The aim of this theoretical analysis is to link energetic and connectional aspects of ecosystems with information theory. Updating an energetic model reported by Ricklefs [Ecologia, Zanichelli Editore S.p.A., Bologna, Italy, 1993, p. 896], our approach is integrated with a static analysis of food webs. The length of food webs is theoretically associated with the average ecological efficiency which can be empirically correlated with the effective connectance between species. Furthermore, the advantage of greater complexity when applied to a signalling network is qualitatively addressed.
The overall efficiency of energy transformation into biomass throughout a trophic web, in an ecosystem with a given number of species, is the resultant of the various ecological efficiencies, η, at the transitions between the trophic levels. However, we propose that an increment in effective connectance and interspecies connectivity based on a superimposed signalling web may increase the η values, despite the fact that signalling per se has an energetic cost. According to this hypothesis, ecosystem stability would not be necessarily reduced by increasing the number of trophic levels, N, whenever stability in terms of persistence is improved by a cost-efficient regulatory network
Quasi specie virale: modello matematico e considerazioni biologiche
No full textIl modello della “quasi-specie virale” (Eigen, Naturwissenschaften Vol.58: 465, 1971) è stato molto spesso
citato, ma la condizione Q·S31, dove Q è la fedeltà replicativa ed S è il coefficiente di selezione su cui è basato
non è sufficientemente chiara. Tentativi di trovare una corrispondenza tra modelli dinamici ed un sistema
bidimensionale di Ising sono già stati riportati (Leuthäusser, J.Chem.Phys., 84(3):1884, 1986; Galzigna et al.,
Biologica 1, 1988; Eigen, J.Phys.Chem., 92:6881, 1988), ma la condizione Q·S31 sembra abbisognare di alcune
considerazioni. Questa relazione iperbolica semplice ricorre in molti fenomeni che hanno una soglia. Un
modello di percolazione, applicato in campi diversi tra cui epidemiologia, studio dei polimeri, conduzione,
propagazione di un incendio, galassie a spirale, può essere utilizzato in questo contesto.
Alcuni commenti sono fatti sulla condizione n·p31, dove nel modello di percolazione applicato al caso di
conduzione di carica in un solido, n è il numero dei contatti e p è la concentrazione degli atomi di materiale
conduttore. Si può dimostrare che per 0<n×p<1, la conduzione per quanto improbabile può avvenire, anche nel
caso limite di n×p = e, con e numero positivo minimizzabile ad arbitrio nel caso semplice di un albero di Bethe
infinitamente esteso; inoltre è interessante notare che la conduzione è permessa, ma non sempre garantita per
n×p>1.
Le considerazioni di interesse per la Biologia sono molteplici. Anche se in natura si trovano virus con
molte caratteristiche diverse, un modello minimo della quasi-specie ci aiuta a descrivere le lunghezze dei relativi
genomi e a fare predizioni sulla loro origine ed evoluzione:
N = Lmax < ln S/(1-q), ove N è il numero di basi nucleotidiche in un filamento.
I filamenti di un antico “mondo ad RNA” dovevano essere molto corti, dato che, plausibilmente, i
meccanismi di replicazione non erano particolarmente efficienti. La teoria degli ipercicli risolve il problema
della catastrofe da errore e del deficit informazionale, anche se non senza rischi. Un virus è un parassita delle
cellule costituito di un filamento di acido nucleico, che è in grado di replicarsi molto rapidamente e cambiare
aspetto, prima che il sistema immunitario possa riconoscerlo ed impedirne l’invasione. Un processo di mutazione
casuale origina uno spettro di mutanti, cioè virus con alcune lettere copiate erroneamente. La quasispecie è
assimilabile ad una sequenza di 4 lettere che non è esattamente costante e che può essere rappresentata come una
nuvola in uno spazio multidimensionale. La condizione di selezione xm+xj = cost. impone che un possibile
aumento del numero dei mutanti (j) corrisponda ad una diminuzione nel numero delle sequenze master (m)
(Maynard Smith & Szathmáry, Freeman, Oxford, 1995). Un tasso di mutazione o frequenza di errore prossima a
questo valore critico o soglia, fa si che il sistema risponda più prontamente ai cambiamenti ambientali.
Riteniamo che il modello di quasispecie sia utile alla descrizione di certi aspetti del processo evolutivo; tuttavia
talvolta sorgono alcuni sani dubbi su una eventuale strategia di terapia antivirale basata sull’aumento della
frequenza di mutazione fino alla dispersione della quasispecie che questa teoria suggerisce
Isomorphism of quasispecies and percolation models
No full textPopulation dynamics of tRNA-like macromolecules and viruses have been interpreted by
Eigen (1971, Naturwissenschaften 58, 465}526) on the basis of the &&quasispecies'' model. The
present paper contains a qualitative analysis of the similarities between Eigen's quasispecies
model and percolation models. In fact, di!erent phenomena characterized by an analogous
inner structure can conceivably be described by quite similar mathematical formalisms. The
occurrence of a threshold in speci"c processes predicted by the models is considered "rst.
Secondly, Ising's model of ferromagnetism is taken into account in the last section. An
interpretation of the above-mentioned biological theory in terms of percolation, implying
a zeroth-order approximation to the real situation, might be a point of departure to a deeper
insight obtainable with more re"ned approaches. A better comprehension of biological
phenomena might in any case arise from a percolative approach, even if the description of the
systems is simpli"ed. An overview of some quasispecies results and some plausible applications
are presented
Relevance of lunar periodicity in human spontaneous abortions
No full textThe effect of the moon on human reproduction has been scarcely investigated and
with controversial results. The present analysis describes a significant effect
of extreme perigeal lunar positions on the number of hospitalized spontaneous
abortions (n = 1,329) recorded at two university clinics and a public hospital at
Padova, Italy, during the years 2000-2003. Spectral analysis evidenced a 205-day
period which appears to be correlated with the 206-day periodicity in extreme
lunar distances. Circa-septan and circa-annual periodicities were also observed.
Peak significances were determined by a Monte Carlo approach (circa-septan and
205-day periodicities: p < 0.001; circa-annual periodicity: p < 0.05). Our study
indicates that the occurrence of human abortions displays suggestive
periodicities that may be of relevance for gynecological and obstetrical
practice
Potenza cinematica dell'arto superiore: modello a minimo jerk
No full textLa potenza cinematica dell'arto superiore viene descritta utilizzando il modello di minimo jerk per l'individuazione del dominio dei movimenti permessi e del dominio dei movimenti ottimali
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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