Sex releases the speed limit on evolution

Explaining the evolutionary maintenance of sex remains a key problem in evolutionary biology. One potential benefit of sex is that it may allow a more rapid adaptive response when environmental conditions change, by increasing the efficiency with which selection can fix beneficial mutations. Here I show that sex can increase the rate of adaptation in the facultatively sexual single-celled chlorophyte Chlamydomonas reinhardtii, but that the benefits of sex depend crucially on the size of the population that is adapting: sex has a marked effect in large populations but little effect in small populations. Several mechanisms have been proposed to explain the benefits of sex in a novel environment, including stochastic effects in small populations, clonal interference and epistasis between beneficial alleles. These results indicate that clonal interference is important in this system.     

高速公路车速限制实施方法研究

通过对分车型与分车道车速限制的优缺点及适用性进行综合分析,应用关于最高车速限制基准值、车道数及车道位置修正系数及最低车速限制方法的研究成果,给出了不同车道高速公路的分车型与分车道车速限制方法与建议值,提出了车速限制区间和车速限制连续性指数的概念,以车速限制连续性指数小于0.85作为车速限制区间的划分依据,给出了高速公路车速限制区间划分结果。     

基于车速平稳过渡的高速公路出口限速方案

研究能够满足出口车速平稳过渡的逐级限速方案,对提高出口区域车辆平稳运行及安全提升有重要意义。利用无人机采集高速公路出口车辆运行参数,分析出口车辆运行特征,考虑驾驶员对相邻限速标志的认知反应,构建了连续限速标志设置间距计算模型,建立了高速公路出口不同级别的限速方案,利用驾驶模拟试验对设计的不同限速方案进行分析及评价。模型的计算结果显示,二级限速方案中,限速值为60-40km/h的限速标志间距为160m;三级限速方案中,限速值为80-60-40km/h的限速标志间距分别为：300,80m,限速值为90-60-40km/h的限速标志间距分别为300,85m;四级限速方案中,限速值为100-80-60-40km/h的限速标志间距分别为295,160,80m。利用驾驶模拟器对模型结果进行验证,试验结果表明：逐级限速方案的平均减速度指标小于1.3 m/s2,均在驾驶舒适性阈值内;随着限速级数增加,车辆离散幅度显著降低,表明逐级限速方案对出口车辆运行速度有明显管控效果;四级限速方案可使在分流鼻端车速标准差、平均减速度、V85分别控制在5.23km/h、0.52m/s2、54km/h左右,极大了满足车速平稳过渡要求。可见,借助模型定量优化的逐级限速方案可以显著提高出口车速过渡的平稳性和安全性。     

普通公路路侧限速标志设置位置的确定方法

路侧限速标志的设置位置是影响其有效性的一个重要因素。基于驾驶人的视觉反应、认知特性,分析了驾驶人视认交通标志过程中主要参数的计算方法,根据驾驶人视认限速标志和完成减速操作要求的最小时间,建立了最小前置距离计算模型;并根据短时记忆特性,以驾驶人对限速标志信息短时记忆即将消失时发现限速路段起点处设置的限速标志为原则,建立了最大前置距离计算模型,进而确定了路侧限速标志的最佳设置位置,并通过算例对模型进行了验证。结果表明,所提出的模型可以帮助确定路侧限速标志的最佳设置位置,避免以往依靠经验设置的随意性,提高了限速标志的有效性。     

公路限速标志设置策略

公路交通标志是公路使用的说明书,是公路的一种无声语言,公路限速标志是交通标志的一种。文章针对安徽省现有公路限速标志设置现状和存在的问题,系统总结了公路限速标志的设计方法和策略,对新时期下的公路运营管理具有重要意义。     

Asymptotic prey profitability drives star-nosed moles to the foraging speed limit

Foraging theory provides models for predicting predator diet choices assuming natural selection has favoured predators that maximize their rate of energy intake during foraging. Prey profitability (energy gained divided by prey handling time) is an essential variable for estimating the optimal diet. Time constraints of capturing and consuming prey generally result in handling times ranging from minutes to seconds, yet profitability increases dramatically as handling time approaches zero, providing the potential for strong directional selection for increasing predator speed at high encounter rates (tiny increments in speed increase profitability markedly, allowing expanded diets of smaller prey). We provide evidence that the unusual anatomical and behavioural specializations characterizing star-nosed moles resulted from progressively stronger selection for speed, allowing the progressive addition of small prey to their diet. Here we report handling times as short as 120 ms (mean 227 ms) for moles identifying and eating prey. 'Double takes' during prey identification suggest that star-nosed moles have reached the speed limit for processing tactile information. The exceptional speed of star-nosed moles, coupled with unusual specializations for finding and eating tiny prey, provide new support for optimal foraging theory.     

考虑风作用的公路限速值

为研究风对交通安全的影响,选择小客车和集装箱车为典型车型,采用风洞试验方法,进行了均匀流的风场模拟,得到了小客车和集装箱车受到的风作用力.以汽车动力学原理为基础,分析了汽车在弯道路段的行驶状态,建立了考虑风作用的汽车安全行驶的车速计算模型,得到了不同风速和设计速度条件下典型车型的限速值.研究结果表明:汽车受到的风作用力随风等级增加而增加;相同等级的风作用下,集装箱车受到的风作用力远大于小客车受到的力;公路设计速度越高,风力对限速值影响越大.研究成果量化了风对行驶车辆的影响,可为大风频发区的公路安全运营提供理论基础和指导.     

Biomechanics: no force limit on greyhound sprint speed

Maximum running speed is constrained by the speed at which the limbs can be swung forwards and backwards, and by the force they can withstand while in contact with the ground. Humans sprinting around banked bends change the duration of foot contact to spread the time over which the load is applied, thereby keeping the force on their legs constant. We show here that, on entering a tight bend, greyhounds do not change their foot-contact timings, and so have to withstand a 65% increase in limb forces. This supports the idea that greyhounds power locomotion by torque about the hips, so--just as in cycling humans--the muscles that provide the power are mechanically divorced from the structures that support weight.     

基于演化硬件的道路限速标志识别方法

针对当前模式识别技术存在的学习和识别时间长、学习结果可读性差等缺点,提出了一种新型识别方法.首先对4类常见的道路限速标志图像进行定位与特征提取,经预处理的特征向量作为系统训练集和测试集数据;然后在Xilinx Virtex xcv2000E FPGA硬件平台上采用VHDL设计演化硬件识别系统,完成对特征向量数据的学习与...     

扩张映射的带有收敛速度的高维中心极限定理

摘要：设M是紧致连通的光滑的黎曼流形,X∪→U∪→M,T：X→X上的扩张映射,g是X上的Holder连续函数,m是g的平衡态,假设f：X→R^d,其每个分量fi是Holder连续函数,且∫Xfidm=0。如果f是每个分量fi是上同调不相关的,那么存在一个正定对称矩阵σ^2,使得f^n/√n=f＋f。T＋…＋f。T^n-1/√n关于m依分布收敛于期望向量为0、协方差矩阵为σ^2的n维Gauss随机变量,进一步,存在一个实数A〉0使得,对任意整数n≥1,有不等式 П（m＊（f^n/√n）,N（0,σ^2）≤A/√n,其中,m＊（f^n/√n）表示f^n/√n关于m的分布,П（＊,＊）是Prokhorov度量。     

山区高速公路车辆速度特性与区间限速方法

为了合理确定事故多发路段的区间测速值,以G5515张南高速事故多发路段(K556+200 m～K578+800 m)为例,提出了高速公路区间限速值的设置方法和工作流程.以该路段为对象,利用移动式测速系统和ETC龙门架分别获取了该路段小型车和大型车的瞬时速度和行程速度数据,明确了高速公路典型车型的运行特性和速度分布特征,...     

直接转矩控制系统低速下转速特性的研究

针对直接转矩控制技术控制下的异步电动机在低速时存在较大转速脉动,提出了一种新的控制方案.该方案是基于直接转矩控制的原理,在异步电动机低速时,保证输出负载不变,减小定子电压矢量幅值,提高非零电压矢量的作用效果,可以降低转矩低频脉动,提高直接转矩控制系统低速下转速的控制特性.最后给出了控制机理及有效改善转速脉动的仿真图形.     

Testing the speed of 'spooky action at a distance'

Correlations are generally described by one of two mechanisms: either a first event influences a second one by sending information encoded in bosons or other physical carriers, or the correlated events have some common causes in their shared history. Quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This reveals itself in correlations that violate Bell inequalities (implying that they cannot be described by common causes) between space-like separated events (implying that they cannot be described by classical communication). Many Bell tests have been performed, and loopholes related to locality and detection have been closed in several independent experiments. It is still possible that a first event could influence a second, but the speed of this hypothetical influence (Einstein's 'spooky action at a distance') would need to be defined in some universal privileged reference frame and be greater than the speed of light. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test over more than 24 hours between two villages separated by 18 km and approximately east-west oriented, with the source located precisely in the middle. We continuously observed two-photon interferences well above the Bell inequality threshold. Taking advantage of the Earth's rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of the influence. For example, if such a privileged reference frame exists and is such that the Earth's speed in this frame is less than 10(-3) times that of the speed of light, then the speed of the influence would have to exceed that of light by at least four orders of magnitude.     

无速度传感器异步电动机极低转速下的矢量控制

在较低转速下,对异步电机转子磁通位置的观测比较困难,因此在定子边注入与电机控制量无关的高频信号,是提取转子磁通位置信息的有效方法之一。为此,该文分析了异步电机高频脉振信号注入下的电机模型,得到了异步电机同步参考坐标系下的阻抗不对称的特点,提出了磁通跟踪的方法,通过检测高频脉振信号注入时的响应电流,来观测转子的磁通位置,并以此为基础实现了无速度传感器异步电机极低转速下的矢量控制。实验结果表明,该方法在异步电机极低转速(包括零速)下能准确地观测出转子磁通位置,对电机的参数和负载变化具有鲁棒性,并且能够在零速时输出额定转矩。     

轮胎高速直线稳定性研究

针对不同型号的轮胎进行力学特性试验,建立轮胎模型和车辆动力学模型。实验结果显示,所建模型是正确的,可以进行车辆操纵稳定性仿真实验。通过中心区和回正性仿真试验,研究不同轮胎对车辆直线性的影响。在中立感实验中,轮胎的侧偏刚度越大,侧向加速度、横摆角速度和侧倾角越小。在回正实验中,轮胎的侧偏刚度越大,方向盘转角、横摆角速度和侧倾角的残余越小。仿真结果显示不同侧偏特性的轮胎表现出不同的直线稳定性。     

Minimum speed limit for ocean ridge magmatism from 210Pb-226Ra-230Th disequilibria

Although 70 per cent of global crustal magmatism occurs at mid-ocean ridges-where the heat budget controls crustal structure, hydrothermal activity and a vibrant biosphere-the tempo of magmatic inputs in these regions remains poorly understood. Such timescales can be assessed, however, with natural radioactive-decay-chain nuclides, because chemical disruption to secular equilibrium systems initiates parent-daughter disequilibria, which re-equilibrate by the shorter half-life in a pair. Here we use 210Pb-226Ra-230Th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the Earth's mantle can be transported, accumulated and erupted in a few decades. This implies that magmatic conditions can fluctuate rapidly at ridge volcanoes. 210Pb deficits of up to 15 per cent relative to 226Ra occur in normal mid-ocean ridge basalts, with the largest deficits in the most magnesium-rich lavas. The 22-year half-life of 210Pb requires very recent fractionation of these two uranium-series nuclides. Relationships between 210Pb-deficits, (226Ra/230Th) activity ratios and compatible trace-element ratios preclude crustal-magma differentiation or daughter-isotope degassing as the main causes for the signal. A mantle-melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly. The likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied.