1,720,979 research outputs found
Exploring models in population biology through the simulation of species invasions, natural selection and market-mediated gene flow
No full textIn this thesis, I apply simulation techniques to investigate three questions in population biology, which focus on movement and natural selection. The first model assesses the theoretical implications of long-range dispersal in species invasions, identifying an important interaction between the representation of a finite population and the rate of population spread. The second investigates the genetic impact of movement distortions among domestic animals due to human economic activity, suggesting that the marketing of animals could fundamentally impact their genetic variation and distribution. My third model considers the problem of detecting evidence of positive natural selection in the genome, refining and testing statistics designed to identify which genes have offered a reproductive advantage in the past using population genetic data. These three simulation studies use very different approaches, and, separately, identify the critical and practical importance of assumptions frequently encountered in population models. Such assumptions - infinite population size, unbiased migration, and constant recombination rate - each lead to interesting properties of model behaviour, and may be relevant to interpretation and prediction in real world problems
Long-range dispersal, stochasticity and the broken accelerating wave of advance
Full text linkRare long distance dispersal events are thought to have a disproportionate impact on the spread of invasive species. Modelling using integrodifference equations suggests that, when long distance contacts are represented by a fat-tailed dispersal kernel, an accelerating wave of advance can ensue. Invasions spreading in this manner could have particularly dramatic effects. Recently, various authors have suggested that demographic stochasticity disrupts wave acceleration. Integrodifference models have been widely used in movement ecology, and as such a clearer understanding of stochastic effects is needed. Here, we present a stochastic non-linear one-dimensional lattice model in which demographic stochasticity and the dispersal regime can be systematically varied. Extensive simulations show that stochasticity has a profound effect on model behaviour, and usually breaks acceleration for fat-tailed kernels. Exceptions are seen for some power law kernels, with $\bet
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Effect of multidisciplinary cardiac rehabilitation on the response to cardiac resynchronization therapy
No full textBackground Both cardiac resynchronization therapy (CRT) and Multidisciplinary Cardiac Rehabilitation (CR) beneficially influence symptomatic status, exercise capacity, quality of life, and heart failure readmission rates. However, the interaction between both therapies remain incompletely addressed. Methods Consecutive CRT patients implanted in a single tertiary care center were retrospectively analyzed. Patients were dived according to the participation in a structured CR-program following CRT-implant. The effect on functional status (New York Heart Association; NYHA-class), reverse remodeling (change in left ventricular ejection fraction; LVEF), and the combined endpoint of heart failure readmission and all-cause mortality was assessed after multivariate correction. Results A total of 655 patients were analyzed of whom 223(34%) did and 432(66%) did not participate in a structured multidisciplinary CR-program following implant. No adverse events relating to exercise training occurred during the CR-program. Patients who participated in the CR-program had a more pronounced improvement in NYHA-class at 6-months (P = 0.006), even after multivariate correction (beta = -0.144; 95% CI = [-0.270; -0.018]; P = 0.025). Maximal workload and VO2max on CPET at 6 months improved significantly even after adjustment (P < 0.001, respectively P = 0.017). At 6-months, CR associated with more improvement in LVEF (+11.9 +/- 13 vs +14.5 +/- 11; P = 0.008), however, this relationship was lost after multivariate correction (P = 0.136). During 36 +/- 22 months follow-up, patients in the CR group had a higher event-free survival for the combined endpoint (P = 0.001), even after multivariate correction (adjusted HR = 0.547; CI = 0.366-0.818; P = 0.003). Conclusions Following CRT-implant, the participation in a structured CR-program is safe and beneficially influences symptomatic response and clinical outcome. The beneficial effects of exercise training are potentially independent and additive to the beneficial reverse remodeling effect induced by CRT itself.Pieter Martens is supported by a doctoral fellowship by the Research Foundation-Flanders (FWO, grant-number: 1127917N). Pieter Martens and Wilfried Mullens are researchers for the Limburg Clinical Research Program (LCRP) UHasselt-ZOL-Jessa, supported by the foundation Limburg Sterk Merk (LSM), Hasselt University, Ziekenhuis Oost-Limburg, and Jessa Hospital
Refining the use of linkage disequilibrium as a robust signature of selective sweeps
No full textDuring a selective sweep, characteristic patterns of linkage disequilibrium can arise in the genomic region surrounding a selected locus. These have been used to infer past selective sweeps. However, the recombination rate is known to vary substantially along the genome for many species. We here investigate the effectiveness of current (Kelly's ZnS and ωmax) and novel statistics at inferring hard selective sweeps based on linkage disequilibrium distortions under different conditions, including a human-realistic demographic model and recombination rate variation. When the recombination rate is constant, Kelly's ZnS offers high power, but is outperformed by a novel statistic that we test, which we call Zα. We also find this statistic to be effective at detecting sweeps from standing variation. When recombination rate fluctuations are included, there is a considerable reduction in power for all linkage disequilibrium-based statistics. However, this can largely be reversed by appropriately controlling for expected linkage disequilibrium using a genetic map. To further test these different methods, we perform selection scans on well-characterized HapMap data, finding that all three statistics—ωmax; Kelly’s ZnS; and Zα—are able to replicate signals at regions previously identified as selection candidates based on population differentiation or the site frequency spectrum. While ωmax replicates most candidates when recombination map data are not available, the ZnS and Zα statistics are more successful when recombination rate variation is controlled for. Given both this and their higher power in simulations of selective sweeps, these statistics are preferred when information on local recombination rate variation is available.</p
Variations on the Author
Full text link“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
Full text linkWe 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
Full text linkWe 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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