Sexual selection and the maintenance of sexual reproduction

The maintenance of sexual reproduction is a problem in evolutionary theory because, all else being equal, asexual populations have a twofold fitness advantage over their sexual counterparts and should rapidly outnumber a sexual population because every individual has the potential to reproduce. The twofold cost of sex exists because of anisogamy or gamete dimorphism-egg-producing females make a larger contribution to the zygote compared with the small contribution made by the sperm of males, but both males and females contribute 50% of the genes. Anisogamy also generates the conditions for sexual selection, a powerful evolutionary force that does not exist in asexual populations. The continued prevalence of sexual reproduction indicates that the 'all else being equal' assumption is incorrect. Here I show that sexual selection can mitigate or even eliminate the cost of sex. If sexual selection causes deleterious mutations to be more deleterious in males than females, then deleterious mutations are maintained at lower equilibrium frequency in sexual populations relative to asexual populations. The fitness of sexual females is higher than asexuals because there is no difference in the fecundity of sexual females and asexuals of the same genotype, but the equilibrium frequency of deleterious mutations is lower in sexual populations. The results are not altered by synergistic epistasis in males.     

通信电源系统的选择与维护

首先介绍了通信电源的基本概念 ,然后分别介绍了作者对通信电源系统选择标准的见解和维护     

利用维护代价约束的物化视图选择方法

为了在一定维护代价约束条件下,使查询过程中花费的总查询成本最优化,提出了最小/最大候选集变换算法.该算法构造最大候选视图集和最小候选视图集,基于最小有效的极大基数配比技术,并通过单位维护代价内的查询收益而设计的代价计算模型来获得最佳物化视图集.理论分析和实验结果表明该算法是高效、动态、近似最优的.与以往算法相比,在数据维度大、维层次复杂的情况下,该算法有着更优的执行效率.     

Sexual selection for sensory exploitation in the frog Physalaemus pustulosus

The sensory bases of species and population mate preferences are well known; in frogs properties of the female auditory system influence such preferences. By contrast, there is little understanding of how sensory characteristics could result in sexual selection within a population. One possible mechanism is that females are more sensitive to male courtship signals that deviate from the population mean. We document this mechanism in the frog Physalaemus pustulosus. Female basilar papilla tuning is biased toward lower-than-average frequencies in the 'chuck' portion of the male's call, explaining female preference for the lower-frequency chucks produced by larger males. The tuning does not differ between P. pustulosus and its close relative P. coloradorum, a species in which males never evolved the ability to produce chucks; thus the female tuning evolved before the chuck and therefore the chuck played no role in the evolution of the preference. This allows us to reject two popular hypotheses for the evolution of this female preference (runaway sexual selection and natural selection) in favour of a third: sexual selection for sensory exploitation.     

Sexual selection enables long-term coexistence despite ecological equivalence

Empirical data indicate that sexual preferences are critical for maintaining species boundaries, yet theoretical work has suggested that, on their own, they can have only a minimal role in maintaining biodiversity. This is because long-term coexistence within overlapping ranges is thought to be unlikely in the absence of ecological differentiation. Here we challenge this widely held view by generalizing a standard model of sexual selection to include two ubiquitous features of populations with sexual selection: spatial variation in local carrying capacity, and mate-search costs in females. We show that, when these two features are combined, sexual preferences can single-handedly maintain coexistence, even when spatial variation in local carrying capacity is so slight that it might go unnoticed empirically. This theoretical study demonstrates that sexual selection alone can promote the long-term coexistence of ecologically equivalent species with overlapping ranges, and it thus provides a novel explanation for the maintenance of species diversity.     

环境协调材料的内涵与研究方法

探讨了材料科学的发展与生态环境的关系．分析了环境协调材料的内涵，提出了它的定义．系统地论述了环境协调材料的研究内容和研究方法．     

A gradient of sex linkage in the pseudoautosomal region of the human sex chromosomes

Three independent pseudoautosomal loci are linked to sex determination at frequencies which define a gradient of linkage. The segregation patterns of these loci indicate that X/Y recombination results from a single obligatory meiotic crossing-over in the pseudoautosomal region. Recombination in male germ cells in the terminal regions of the short arms of the X and Y chromosomes in 10-fold greater than between the same regions of the X chromosomes in female germ cells.     

电梯电气控制系统故障分析与检修

电梯在使用过程中经常频繁出现电气控制系统故障,为保证电梯的可靠运行,迅速地排除电气故障,延长电气线路和电气设备的使用寿命,文章对电梯电气控制系统常见故障原因进行了分析,给出了简捷,快速的检修方法。对具有偶然性、突发性的故障通过事例进行了分析处理。并采用电梯远程监测系统对系统进行监测,实现电梯运行状态的在线监测,保证电梯安全可靠运行。     

高校计算机实验室的管理与维护

高等学校的计算机实验室是进行科学研究和实验教学的重要场所,有计算机数量多、使用率高、软件系统复杂等特点。安全和舒适的实验环境,软硬件的及时维护,是建设计算机实验室的基础和保证,科学地管理高校计算机房,可以延长设备的使用寿命和提高设备的利用率。     

Metabolic trade-offs and the maintenance of the fittest and the flattest

How is diversity maintained? Environmental heterogeneity is considered to be important, yet diversity in seemingly homogeneous environments is nonetheless observed. This, it is assumed, must either be owing to weak selection, mutational input or a fitness advantage to genotypes when rare. Here we demonstrate the possibility of a new general mechanism of stable diversity maintenance, one that stems from metabolic and physiological trade-offs. The model requires that such trade-offs translate into a fitness landscape in which the most fit has unfit near-mutational neighbours, and a lower fitness peak also exists that is more mutationally robust. The 'survival of the fittest' applies at low mutation rates, giving way to 'survival of the flattest' at high mutation rates. However, as a consequence of quasispecies-level negative frequency-dependent selection and differences in mutational robustness we observe a transition zone in which both fittest and flattest coexist. Although diversity maintenance is possible for simple organisms in simple environments, the more trade-offs there are, the wider the maintenance zone becomes. The principle may be applied to lineages within a species or species within a community, potentially explaining why competitive exclusion need not be observed in homogeneous environments. This principle predicts the enigmatic richness of metabolic strategies in clonal bacteria and questions the safety of lethal mutagenesis as an antimicrobial treatment.     

酸度计的使用与维护

酸度计是化学实验室和工农业生产的常用仪器,其型号和性能颇多,工作原理不尽相同。因此能否正确的使用和维护,直接影响着测试结果的准确性。     

性教育的必要性和意义

简介了国内外性教育概况,从性是人类的一种自然生理现象;婚前性行为、未婚怀孕增多要求开展性教育;性知识缺乏导致心理疾病和家庭悲剧;性教育有助预防艾滋病及其它性病的传播;性教育可以增进家庭的和谐和社会稳定等方面来说明性教育的必要性和意义．