An Analytically Tractable Model for Competitive Speciation

Several recent models have shown that frequency-dependent disruptive selection created by intraspecific competition can lead to the evolution of assortative mating 4 and, thus, to competitive sympatric speciation. However, since most results from these 5 models rely on limited numerical analyses, their generality has been subject to considerable debate. Here, we consider one of the standard models, the so-called Roughgarden model, with a simplified genetics where the selected trait is determined by a single diallelic locus. This model is sufficiently complex to maintain key properties of the general multilocus case, but still simple enough to allow for a comprehensive analytical treatment. By means of invasion fitness analysis, we describe the impact of all model parameters on the evolution of assortative mating. Depending on (1) the strength and (2) shape of stabilizing selection, (3) the strength and (4) shape of pairwise competition, (5) the shape of the mating function, and (6) the type of assortative mating, which may or may not lead to sexual selection, we find five different evolutionary regimes. In one of these regimes, the evolution of complete reproductive isolation is possible through arbitrarily small steps in the strength of assortative mating. Our approach provides a mechanistic understanding of several phenomena that have been found in previous models. The results demonstrate how, even in a simple model of competitive speciation, results depend in a complex way on ecological and genetic parameters

Ultracompact monolithic integration of balanced, polarization diversity photodetectors for coherent lightwave receivers

The authors have monolithically integrated an optical front-end on InP for balanced, polarization-diversity coherent lightwave reception which is only 1.3-mm long. Low on-chip insertion loss (<4.5 dB) and balanced photoresponse (1.05:1 or better) are achieved at 1.5-μm wavelength using straightforward, regrowth-free fabrication. Low-capacitance photodetectors (≤0.15 pF) are employed for high bandwidth operation

Biologische reiniging van grond vervuild met gasolie en ruwe olie volgens de landfarming - methode

Biologische reiniging van grond volgens de landfarming-methode is een relatief eenvoudige en goedkope reinigingsmethode voor grond verontreinigd met afbreekbare koolwaterstoffen. De vervuilde grond wordt uitgespreid op een speciaal ingericht terrein voorzien van een bodemafdichting en een drainagesysteem, waar vervolgens de microbiologische afbraak van de verontreinigingen wordt gestimuleerd door grondbewerking en bemesting. De toepasbaarheid is onderzocht op proefvelden van de VAM te Wijste

Direct growth of graphene on GaN via plasma-enhanced chemical vapor deposition under N₂ atmosphere

One of the bottlenecks in the implementation of graphene as a transparent electrode in modern opto-electronic devices is the need for complicated and damaging transfer processes of high-quality graphene sheets onto the desired target substrates. Here, we study the direct, plasma-enhanced chemical vapor deposition (PECVD) growth of graphene on GaN-based light-emitting diodes (LEDs). By replacing the commonly used hydrogen (H2) process gas with nitrogen (N2), we were able to suppress GaN surface decomposition while simultaneously enabling graphene deposition at lt;800 °C in a single-step growth process. Optimizing the methane (CH4) flow and varying the growth time between 0.5 h and 8 h, the electro-optical properties of the graphene layers could be tuned to sheet resistances as low as ∼1 kΩ/D with a maximum transparency loss of ∼12. The resulting high-quality graphene electrodes show an enhanced current spreading effect and an increase of the emission area by a factor of ∼8 in operating LEDs. © 2020 The Author(s)

Ultracompact, low-loss directional couplers on InP based on self-imaging by multimode interference

We report extremely compact (494-µm-long 3 dB splitters, including input/output bends), polarization-insensitive, zero-gap directional couplers on InP with a highly multimode interference region that are based on the self-imaging effect. We measured cross-state extinctions better than 28 dB and on-chip insertion losses of 0.5 dB/coupler plus 1 dB/cm guide propagation loss at 1523 nm wavelength

De Bevordering van Zelfregulatie Tijdens Afstandsonderwijs en Contactonderwijs: ervaringen tijdens de Covid-19 pandemie

Zelfregulatie is essentieel voor leren, zeker tijdens afstandsonderwijs. Een onderwijsvorm waarmee leerlingen en leerkrachten te maken kregen tijdens de Covid-19 pandemie. Leerkrachten zijn belangrijke initiators voor de ontwikkeling van zelfregulatie. Met het huidige onderzoek hebben we in kaart gebracht welke praktische en theoretische kennis omtrent zelfregulatie leerkrachten hebben en in welke mate zij de ontwikkeling van zelfregulatie ondersteunen tijdens contactonderwijs en afstandsonderwijs. Daartoe vulden 122 primair onderwijs leerkrachten een online vragenlijst in. De vragenlijst bestond uit de Nederlandse versie van de Self-Regulated Learning Inventory for Teachers aangevuld met open vragen ter verdieping. Uit de analyses bleek dat leerkrachten enige theoretische kennis van zelfregulatie hebben, maar dat deze niet volledig is. Daarnaast bleken de meeste leerkrachten zowel directe als indirecte interventies voor het bevorderen van zelfregulatie te kennen. Tijdens afstandsonderwijs werd significant minder aandacht besteed aan vaardigheden gerelateerd aan de voorbereidende en de evaluatiefase dan tijdens contactonderwijs, maar juist meer aan vaardigheden van belang voor de uitvoerende fase. De antwoorden op de open vragen gaven meer inzicht in de herkomst van deze verschillen. De resultaten geven inzicht in de verschillen in ondersteuning van zelfregulatie tijdens afstandsonderwijs en contactonderwijs en kunnen aanleiding geven tot het ontwikkelen van professionele ontwikkelingstrajecten voor leerkrachten

Polymer reptation and nucleosome repositioning

We consider how beads can diffuse along a chain that wraps them, without becoming displaced from the chain; our proposed mechanism is analogous to the reptation of "stored length" in more familiar situations of polymer dynamics. The problem arises in the case of globular aggregates of proteins (histones) that are wound by DNA in the chromosomes of plants and animals; these beads (nucleosomes) are multiply wrapped and yet are able to reposition themselves over long distances, while remaining bound by the DNA chain.Comment: 9 pages, including 2 figures, to be published in Phys. Rev. Let

Effects of mangrove cover on coastal erosion during a hurricane in Texas, USA

We tested the hypothesis that mangroves provide better coastal protection than salt marsh vegetation using 10 1,008-m2 plots in which we manipulated mangrove cover from 0 to 100%. Hurricane Harvey passed over the plots in 2017. Data from erosion stakes indicated up to 26 cm of vertical and 970 cm of horizontal erosion over 70 months in the plot with 0% mangrove cover, but relatively little erosion in other plots. The hurricane did not increase erosion, and erosion decreased after the hurricane passed. Data from drone images indicated 196 m2 of erosion in the 0% mangrove plot, relatively little erosion in other plots, and little ongoing erosion after the hurricane. Transects through the plots indicated that the levee (near the front of the plot) and the bank (the front edge of the plot) retreated up to 9 m as a continuous function of decreasing mangrove cover. Soil strength was greater in areas vegetated with mangroves than in areas vegetated by marsh plants, or nonvegetated areas, and increased as a function of plot-level mangrove cover. Mangroves prevented erosion better than marsh plants did, but this service was nonlinear, with low mangrove cover providing most of the benefits

The Influence of a Fibrin-Coating Inside a Biodegradable Poly(DL-Lactide-ε-Caprolactone) Nerve Guide on Peripheral Nerve Regeneration

The aim of this study was to evaluate the effect of a fibrin-coating on the inner surface of a biodegradable poly(DL-lactide-E-caprolactone) nerve guide on the speed and quality of the nerve regeneration. The nerve regeneration and orientation of the nerve fibers, as well as the fibrous tissue formation were evaluated. On the short term, nerve regeneration was slightly faster in the non-coated nerve guide. After longer implantation periods (≥ 4 weeks), nerve regeneration in the fibrin-coated nerve guides was characterized by a severe inflammatory response with large numbers of macrophages and polymorphonuclear cells (PMN\u27s). This study clearly demonstrates that nerve regeneration in a fibrin-coated nerve guide is not faster when compared with a non-coated nerve guide, and that nerve regeneration in the fibrin-coated nerve guide is even worse after longer implantation periods