全文获取类型
收费全文 | 6951篇 |
免费 | 796篇 |
国内免费 | 195篇 |
出版年
2024年 | 12篇 |
2023年 | 170篇 |
2022年 | 123篇 |
2021年 | 203篇 |
2020年 | 281篇 |
2019年 | 331篇 |
2018年 | 297篇 |
2017年 | 318篇 |
2016年 | 304篇 |
2015年 | 292篇 |
2014年 | 365篇 |
2013年 | 488篇 |
2012年 | 281篇 |
2011年 | 329篇 |
2010年 | 265篇 |
2009年 | 347篇 |
2008年 | 383篇 |
2007年 | 396篇 |
2006年 | 284篇 |
2005年 | 267篇 |
2004年 | 246篇 |
2003年 | 194篇 |
2002年 | 213篇 |
2001年 | 178篇 |
2000年 | 147篇 |
1999年 | 137篇 |
1998年 | 108篇 |
1997年 | 78篇 |
1996年 | 85篇 |
1995年 | 70篇 |
1994年 | 61篇 |
1993年 | 60篇 |
1992年 | 56篇 |
1991年 | 79篇 |
1990年 | 49篇 |
1989年 | 42篇 |
1988年 | 42篇 |
1987年 | 48篇 |
1986年 | 39篇 |
1985年 | 38篇 |
1984年 | 38篇 |
1983年 | 15篇 |
1982年 | 54篇 |
1981年 | 24篇 |
1980年 | 21篇 |
1979年 | 16篇 |
1978年 | 12篇 |
1975年 | 9篇 |
1973年 | 12篇 |
1972年 | 15篇 |
排序方式: 共有7942条查询结果,搜索用时 15 毫秒
101.
L. P. Ruse 《Hydrobiologia》1995,315(2):135-142
Species abundances of Chironomidae (Insecta: Diptera) have often been excluded from studies of benthic river communities because of difficulties associated with sampling and identifying larvae. Chironomid pupal exuviae are easier to collect and identify and could be used to determine community structure if shown to be representative of local larval assemblages. Larvae were sampled along a 20 m chain secured over mid-channel gravels, upstream of two collection points for pupal exuviae. Proportional taxa abundances of pupal exuviae and larvae sampled from 130 m of stream were directly compared by a 2 test of independence and also separately fitted to four models of species abundance distribution. Observed proportions of taxa were not independent of the life stage sampled. The greatest discrepancies occurred with species of pupal exuviae that were absent as larvae from the gravel. The log series model provided the best fit with both pupal and larval data. Collections of pupal exuviae had greater species richness and evenness than samples of larvae. This was considered to be a consequence of sampling larvae from the gravel habitat alone. 相似文献
102.
Frances K. Skinner Nancy Kopell Brian Mulloney 《Journal of computational neuroscience》1997,4(2):151-160
Rhythmic movements of crayfish swimmerets are coordinated by a neural circuit that links their four abdominal ganglia. Each swimmeret is driven by its own small local circuit, or pattern-generating module. We modeled this networkas a chain of four oscillators, bidirectionally coupled to their nearest neighbors, and tested the models ability to reproduce experimentally observed changes in intersegmental phases and in period caused by differential excitation of selected abdominal ganglia. The choices needed to match the experimental data lead to the followingpredictions: coupling between ganglia is asymmetric; the ascending and descending coupling have approximately equal strengths; intersegmental coupling does not significantly affect the frequency of the system; and excitation affects the intrinsic frequencies of the oscillators and might also change properties ofintersegmental coupling. 相似文献
103.
104.
Molecular evolution of a portion of the mitochondrial 16S ribosomal gene region in scleractinian corals 总被引:1,自引:0,他引:1
Relationships among families and suborders of scleractinian corals are poorly understood because of difficulties 1) in making
inferences about the evolution of the morphological characters used in coral taxonomy and 2) in interpreting their 240-million-year
fossil record. Here we describe patterns of molecular evolution in a segment of the mitochondrial (mt) 16S ribosomal gene from taxa of 14 families of corals and the use of this gene segment in a phylogenetic analysis of relationships
within the order. We show that sequences obtained from scleractinians are homologous to other metazoan 16S ribosomal sequences
and fall into two distinct clades defined by size of the amplified gene product. Comparisons of sequences from the two clades
demonstrate that both sets of sequences are evolving under similar evolutionary constraints: they do not differ in nucleotide
composition, numbers of transition and transversion substitutions, spatial patterns of substitutions, or in rates of divergence.
The characteristics and patterns observed in these sequences as well as the secondary structures, are similar to those observed
in mt 16S ribosomal DNA sequences from other taxa. Phylogenetic analysis of these sequences shows that they are useful for evaluating
relationships within the order. The hypothesis generated from this analysis differs from traditional hypotheses for evolutionary
relationships among the Scleractinia and suggests that a reevaluation of evolutionary affinities in the order is needed.
Received: 4 September 1996 / Accepted: 7 April 1997 相似文献
105.
Francesco Frati Chris Simon Jack Sullivan David L. Swofford 《Journal of molecular evolution》1997,44(2):145-158
The sequence of the mitochondrial COII gene has been widely used to estimate phylogenetic relationships at different taxomonic
levels across insects. We investigated the molecular evolution of the COII gene and its usefulness for reconstructing phylogenetic
relationships within and among four collembolan families. The collembolan COII gene showed the lowest A + T content of all
insects so far examined, confirming that the well-known A + T bias in insect mitochondrial genes tends to increase from the
basal to apical orders. Fifty-seven percent of all nucleotide positions were variable and most of the third codon positions
appeared free to vary. Values of genetic distance between congeneric species and between families were remarkably high; in
some cases the latter were higher than divergence values between other orders of insects. The remarkably high divergence levels
observed here provide evidence that collembolan taxa are quite old; divergence levels among collembolan families equaled or
exceeded divergences among pterygote insect orders. Once the saturated third-codon positions (which violated stationarity
of base frequencies) were removed, the COII sequences contained phylogenetic information, but the extent of that information
was overestimated by parsimony methods relative to likelihood methods. In the phylogenetic analysis, consistent statistical
support was obtained for the monophyly of all four genera examined, but relationships among genera/families were not well
supported. Within the genus Orchesella, relationships were well resolved and agreed with allozyme data. Within the genus Isotomurus, although three pairs of populations were consistently identified, these appeared to have arisen in a burst of evolution from
an earlier ancestor. Isotomurus italicus always appeared as basal and I. palustris appeared to harbor a cryptic species, corroborating allozyme data.
Received: 12 January 1996 / Accepted: 10 August 1996 相似文献
106.
Louis Prusse Yvon C. Chagnon France T. Dionne Claude Bouchard 《Obesity (Silver Spring, Md.)》1997,5(1):49-61
An update of the human obesity gene map up to October 1996 is presented. Evidence from Mendelian disorders exhibiting obesity as a clinical feature, single-gene mutation rodent models, quantitative trait loci uncovered in crossbreeding experiments with mouse, rat, and pig models, association and case-control studies with candidate genes, and linkage studies with genes and other markers is reviewed. All chromosomal locations of the animal loci are converted into human genome locations based on syntenic relationships between the genomes. A complete listing of all these loci reveals that only 4 of the 24 human chromosomes are not yet represented, i.e., 9, 18, 21, and Y. Several chromosome arms are characterized by the presence of several putative loci. The following arms include at least three such loci: 1p, 1q, 3p, 4q, 6p, 7q, 8p, 8q, 11p, 11q, 15q, 20q, and Xq. Studies with negative association and linkage results are also reviewed. 相似文献
107.
Global stability of an SIR epidemic model with time delays 总被引:11,自引:0,他引:11
An SIR disease transmission model is formulated under the assumption that the force of infection at the present time depends on the number of infectives at the past. It is shown that a disease free equilibrium point is globally stable if no endemic equilibrium point exists. Further the endemic point (if it exists) is globally stable with respect to the whole state space except the neighborhood of the disease free state.Research partly supported by the Ministry of Education, Science and Culture, Japan, Grant 05640256 相似文献
108.
Lattice models of proteins were used to examine the role of local propensities in stabilizing the native state of a protein, using techniques drawn from spin-glass theory to characterize the free-energy landscapes. In the strong evolutionary limit, optimal conditions for folding are achieved when the contributions from local interactions to the stability of the native state is small. Further increasing the local interactions rapidly decreases the foldability. © 1995 Wiley-Liss, Inc. 相似文献
109.
Brief overview of control of genetic expression by antisense oligonucleotides and in vivo applications 总被引:7,自引:0,他引:7
Gerald Zon 《Molecular neurobiology》1995,10(2-3):219-229
Over the past several years, the use of synthetic oligonucleotides and functional analogs thereof as a possibly general means
of controlling genetic expression has received widespread attention. Following a brief overview of some of the basic principles
and strategies for this approach, attention is focused here on summarizing some recent reports of in vitro and, in particular,
in vivo investigations in various animal models using phosphorothioate analogs of 2′-deoxyoligo-nucleotides. In view of these
findings, which include studies related to neurobiology, this field should find significant utility in applications of the
antisense method for controlling genetic expression. 相似文献
110.
R. S. DeWitte S. W. Michnick E. I. Shakhnovich 《Protein science : a publication of the Protein Society》1995,4(9):1780-1791
We present an efficient new algorithm that enumerates all possible conformations of a protein that satisfy a given set of distance restraints. Rapid growth of all possible self-avoiding conformations on the diamond lattice provides construction of alpha-carbon representations of a protein fold. We investigated the dependence of the number of conformations on pairwise distance restraints for the proteins crambin, pancreatic trypsin inhibitor, and ubiquitin. Knowledge of between one and two contacts per monomer is shown to be sufficient to restrict the number of candidate structures to approximately 1,000 conformations. Pairwise RMS deviations of atomic position comparisons between pairs of these 1,000 structures revealed that these conformations can be grouped into about 25 families of structures. These results suggest a new approach to assessing alternative protein folds given a very limited number of distance restraints. Such restraints are available from several experimental techniques such as NMR, NOESY, energy transfer fluorescence spectroscopy, and crosslinking experiments. This work focuses on exhaustive enumeration of protein structures with emphasis on the possible use of NOESY-determined distance restraints. 相似文献