全文获取类型
收费全文 | 26668篇 |
免费 | 2197篇 |
国内免费 | 20篇 |
专业分类
28885篇 |
出版年
2023年 | 86篇 |
2022年 | 189篇 |
2021年 | 440篇 |
2020年 | 262篇 |
2019年 | 328篇 |
2018年 | 411篇 |
2017年 | 342篇 |
2016年 | 589篇 |
2015年 | 1023篇 |
2014年 | 1202篇 |
2013年 | 1435篇 |
2012年 | 1915篇 |
2011年 | 1940篇 |
2010年 | 1254篇 |
2009年 | 1082篇 |
2008年 | 1691篇 |
2007年 | 1746篇 |
2006年 | 1599篇 |
2005年 | 1558篇 |
2004年 | 1579篇 |
2003年 | 1509篇 |
2002年 | 1395篇 |
2001年 | 272篇 |
2000年 | 174篇 |
1999年 | 300篇 |
1998年 | 357篇 |
1997年 | 264篇 |
1996年 | 219篇 |
1995年 | 221篇 |
1994年 | 234篇 |
1993年 | 227篇 |
1992年 | 181篇 |
1991年 | 175篇 |
1990年 | 149篇 |
1989年 | 131篇 |
1988年 | 153篇 |
1987年 | 121篇 |
1986年 | 127篇 |
1985年 | 163篇 |
1984年 | 165篇 |
1983年 | 171篇 |
1982年 | 192篇 |
1981年 | 183篇 |
1980年 | 158篇 |
1979年 | 105篇 |
1978年 | 113篇 |
1977年 | 90篇 |
1976年 | 87篇 |
1975年 | 71篇 |
1974年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Peter R. Johnston Stephen J. Walker Jari A.K. Hyttinen David Kilpatrick 《Mathematical biosciences》1994,120(2)
The inverse problem of electrocardiography, the computation of epicardial potentials from body surface potentials, is influenced by the desired resolution on the epicardium, the number of recording points on the body surface, and the method of limiting the inversion process. To examine the role of these variables in the computation of the inverse transform, Tikhonov's zero-order regularization and singular value decomposition (SVD) have been used to invert the forward transfer matrix. The inverses have been compared in a data-independent manner using the resolution and the noise amplification as endpoints. Sets of 32, 50, 192, and 384 leads were chosen as sets of body surface data, and 26, 50, 74, and 98 regions were chosen to represent the epicardium.The resolution and noise were both improved by using a greater number of electrodes on the body surface. When 60% of the singular values are retained, the results show a trade-off between noise and resolution, with typical maximal epicardial noise levels of less than 0.5% of maximum epicardial potentials for 26 epicardial regions, 2.5% for 50 epicardial regions, 7.5% for 74 epicardial regions, and 50% for 98 epicardial regions. As the number of epicardial regions is increased, the regularization technique effectively fixes the noise amplification but markedly decreases the resolution, whereas SVD results in an increase in noise and a moderate decrease in resolution. Overall the regularization technique performs slightly better than SVD in the noise-resolution relationship.There is a region at the posterior of the heart that was poorly resolved regardless of the number of regions chosen. The variance of the resolution was such as to suggest the use of variable-size epicardial regions based on the resolution. 相似文献
942.
Aerial Dispersal and Epiphytic Survival of Pseudomonas syringae during a Pretest for the Release of Genetically Engineered Strains into the Environment 下载免费PDF全文
Steven E. Lindow Guy R. Knudsen Ramon J. Seidler Michael V. Walter Victor W. Lambou Penny S. Amy David Schmedding Valerie Prince Stephen Hern 《Applied microbiology》1988,54(6):1557-1563
Prospective experimental field evaluation of genetically engineered microorganisms, such as microbial pest control agents, raises issues of how to properly ascertain their fate and survival in the environment. Field trials with recombinant organisms must reflect requirements for sampling and monitoring. Field trials were conducted at Tulelake, Calif., to monitor the numbers of viable cells of a nonrecombinant strain of Pseudomonas syringae that entered the atmosphere and landed on plants and soil during and after an aerosol spray application. An exponential decrease in numbers of viable cells deposited at increasing distances from three sprayed plots was observed. The relative rate of survival of cells sprayed directly on plants was more than 10 times higher than that of cells dispersed through the air to similar adjacent plants. Results are being used to gain experience with the characteristics of a release site that influence containment or dispersal and to develop appropriate sampling methodologies for evaluating survival and dispersal characteristics of genetically engineered bacteria released into the environment. The ability to make predictions about microbial dispersal and survival will reduce the uncertainties associated with environmental releases of recombinant organisms. 相似文献
943.
Nicholas M. A. Smith Claire Wade Michael H. Allsopp Brock A. Harpur Amro Zayed Stephen A. Rose Jan Engelstdter Nadine C. Chapman Boris Yagound Benjamin P. Oldroyd 《Journal of evolutionary biology》2019,32(2):144-152
Inbreeding (the mating between closely related individuals) often has detrimental effects that are associated with loss of heterozygosity at overdominant loci, and the expression of deleterious recessive alleles. However, determining which loci are detrimental when homozygous, and the extent of their phenotypic effects, remains poorly understood. Here, we utilize a unique inbred population of clonal (thelytokous) honey bees, Apis mellifera capensis, to determine which loci reduce individual fitness when homozygous. This asexual population arose from a single worker ancestor approximately 20 years ago and has persisted for at least 100 generations. Thelytokous parthenogenesis results in a 1/3 of loss of heterozygosity with each generation. Yet, this population retains heterozygosity throughout its genome due to selection against homozygotes. Deep sequencing of one bee from each of the three known sub‐lineages of the population revealed that 3,766 of 10,884 genes (34%) have retained heterozygosity across all sub‐lineages, suggesting that these genes have heterozygote advantage. The maintenance of heterozygosity in the same genes and genomic regions in all three sub‐lineages suggests that nearly every chromosome carries genes that show sufficient heterozygote advantage to be selectively detrimental when homozygous. 相似文献
944.
SSU‐rRNA Gene Sequencing Survey of Benthic Microbial Eukaryotes from Guaymas Basin Hydrothermal Vent
Alexis Pasulka Sarah K. Hu Peter D. Countway Kathryn J. Coyne Stephen C. Cary Karla B. Heidelberg David A. Caron 《The Journal of eukaryotic microbiology》2019,66(4):637-653
Microbial eukaryotes have important roles in marine food webs, but their diversity and activities in hydrothermal vent ecosystems are poorly characterized. In this study, we analyzed microbial eukaryotic communities associated with bacterial (Beggiatoa) mats in the 2,000 m deep‐sea Guaymas Basin hydrothermal vent system using 18S rRNA gene high‐throughput sequencing of the V4 region. We detected 6,954 distinct Operational Taxonomic Units (OTUs) across various mat systems. Of the sequences that aligned with known protistan phylotypes, most were affiliated with alveolates (especially dinoflagellates and ciliates) and cercozoans. OTU richness and community structure differed among sediment habitats (e.g. different mat types and cold sediments away from mats). Additionally, full‐length 18S rRNA genes amplified and cloned from single cells revealed the identities of some of the most commonly encountered, active ciliates in this hydrothermal vent ecosystem. Observations and experiments were also conducted to demonstrate that ciliates were trophically active and ingesting fluorescent bacteria or Beggiatoa trichomes. Our work suggests that the active and diverse protistan community at the Guaymas Basin hydrothermal vent ecosystem likely consumes substantial amounts of bacterial biomass, and that the different habitats, often defined by distances of just a few 10s of cm, select for particular assemblages and levels of diversity. 相似文献
945.
946.
947.
948.
Paul A. De Luca Stephen Buchmann Candace Galen Andrew C. Mason Mario Vallejo‐Marín 《Ecology and evolution》2019,9(8):4875-4887
Body size is an important trait linking pollinators and plants. Morphological matching between pollinators and plants is thought to reinforce pollinator fidelity, as the correct fit ensures that both parties benefit from the interaction. We investigated the influence of body size in a specialized pollination system (buzz‐pollination) where bees vibrate flowers to release pollen concealed within poricidal stamens. Specifically, we explored how body size influences the frequency of buzz‐pollination vibrations. Body size is expected to affect frequency as a result of the physical constraints it places on the indirect flight muscles that control the production of floral vibrations. Larger insects beat their wings less rapidly than smaller‐bodied insects when flying, but whether similar scaling relationships exist with floral vibrations has not been widely explored. This is important because the amount of pollen ejected is determined by the frequency of the vibration and the displacement of a bee's thorax. We conducted a field study in three ecogeographic regions (alpine, desert, grassland) and recorded flight and floral vibrations from freely foraging bees from 27 species across four families. We found that floral vibration frequencies were significantly higher than flight frequencies, but never exceeded 400 Hz. Also, only flight frequencies were negatively correlated with body size. As a bee's size increased, its buzz ratio (floral frequency/flight frequency) increased such that only the largest bees were capable of generating floral vibration frequencies that exceeded double that of their flight vibrations. These results indicate size affects the capacity of bees to raise floral vibration frequencies substantially above flight frequencies. This may put smaller bees at a competitive disadvantage because even at the maximum floral vibration frequency of 400 Hz, their inability to achieve comparable thoracic displacements as larger bees would result in generating vibrations with lower amplitudes, and thus less total pollen ejected for the same foraging effort. 相似文献
949.
Ben Swallow Stephen T. Buckland Ruth King Mike P. Toms 《Ecology and evolution》2019,9(21):12182-12192
The factors governing the recent declines observed in many songbirds have received much research interest, in particular whether increases of avian predators have had a negative effect on any of their prey species. In addition, further discussion has centered on whether or not the choice of model formulation has an effect on model inference. The study goal was to evaluate changes in the number of 10 songbird species in relation to a suite of environmental covariates, testing for any evidence in support of a predator effect using multiple model formulations to check for consistency in the results. We compare two different approaches to the analysis of long‐term garden bird monitoring data. The first approach models change in the prey species between 1970 and 2005 as a function of environmental covariates, including the abundance of an avian predator, while the second uses a change–change approach. Significant negative relationships were found between Eurasian Sparrowhawk Accipiter nisus and three of the 10 species analyzed, namely house Sparrow Passer domesticus, starling Sturnus vulgaris, and blue tit Cyanistes caeruleus. The results were consistent under both modeling approaches. It is not clear if this is a direct negative impact on the overall populations of these species or a behavioral response of the prey species to avoid feeding stations frequented by Sparrowhawks (which may in turn have population consequences, by reducing available resources). The species showing evidence of negative effects of Sparrowhawks were three of the four species most at risk to Sparrowhawk predation according to their prevalence in the predator's diet. The associations could be causal in nature, although in practical terms the reduction in the rate of change in numbers visiting gardens accredited to Sparrowhawks is relatively small, and so unlikely to be the main driver of observed population declines. 相似文献
950.
Sexual selection on spontaneous mutations strengthens the between‐sex genetic correlation for fitness 下载免费PDF全文
Scott L. Allen Katrina McGuigan Tim Connallon Mark W. Blows Stephen F. Chenoweth 《Evolution; international journal of organic evolution》2017,71(10):2398-2409
A proposed benefit to sexual selection is that it promotes purging of deleterious mutations from populations. For this benefit to be realized, sexual selection, which is usually stronger on males, must purge mutations deleterious to both sexes. Here, we experimentally test the hypothesis that sexual selection on males purges deleterious mutations that affect both male and female fitness. We measured male and female fitness in two panels of spontaneous mutation‐accumulation lines of the fly, Drosophila serrata, each established from a common ancestor. One panel of mutation accumulation lines limited both natural and sexual selection (LS lines), whereas the other panel limited natural selection, but allowed sexual selection to operate (SS lines). Although mutation accumulation caused a significant reduction in male and female fitness in both the LS and SS lines, sexual selection had no detectable effect on the extent of the fitness reduction. Similarly, despite evidence of mutational variance for fitness in males and females of both treatments, sexual selection had no significant impact on the amount of mutational genetic variance for fitness. However, sexual selection did reshape the between‐sex correlation for fitness: significantly strengthening it in the SS lines. After 25 generations, the between‐sex correlation for fitness was positive but considerably less than one in the LS lines, suggesting that, although most mutations had sexually concordant fitness effects, sex‐limited, and/or sex‐biased mutations contributed substantially to the mutational variance. In the SS lines this correlation was strong and could not be distinguished from unity. Individual‐based simulations that mimick the experimental setup reveal two conditions that may drive our results: (1) a modest‐to‐large fraction of mutations have sex‐limited (or highly sex‐biased) fitness effects, and (2) the average fitness effect of sex‐limited mutations is larger than the average fitness effect of mutations that affect both sexes similarly. 相似文献