全文获取类型
收费全文 | 10901篇 |
免费 | 520篇 |
国内免费 | 836篇 |
出版年
2024年 | 9篇 |
2023年 | 118篇 |
2022年 | 169篇 |
2021年 | 296篇 |
2020年 | 255篇 |
2019年 | 266篇 |
2018年 | 288篇 |
2017年 | 258篇 |
2016年 | 278篇 |
2015年 | 392篇 |
2014年 | 585篇 |
2013年 | 639篇 |
2012年 | 457篇 |
2011年 | 524篇 |
2010年 | 463篇 |
2009年 | 625篇 |
2008年 | 651篇 |
2007年 | 676篇 |
2006年 | 546篇 |
2005年 | 491篇 |
2004年 | 390篇 |
2003年 | 358篇 |
2002年 | 306篇 |
2001年 | 313篇 |
2000年 | 258篇 |
1999年 | 309篇 |
1998年 | 241篇 |
1997年 | 215篇 |
1996年 | 174篇 |
1995年 | 204篇 |
1994年 | 191篇 |
1993年 | 158篇 |
1992年 | 167篇 |
1991年 | 111篇 |
1990年 | 88篇 |
1989年 | 89篇 |
1988年 | 87篇 |
1987年 | 96篇 |
1986年 | 66篇 |
1985年 | 84篇 |
1984年 | 74篇 |
1983年 | 39篇 |
1982年 | 50篇 |
1981年 | 51篇 |
1980年 | 35篇 |
1979年 | 33篇 |
1978年 | 33篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1972年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Varelius J 《Bioethics》2007,21(3):140-149
In a recent issue of this journal, David Silver and Gerald Dworkin discuss the physicians' role in execution by lethal injection. Dworkin concludes that discussion by stating that, at that point, he is unable to think of an acceptable set of moral principles to support the view that it is illegitimate for physicians to participate in execution by lethal injection that would not rule out certain other plausible moral judgements, namely that euthanasia is under certain conditions legitimate and that organ-donation surgery is sometimes permissible. This article draws attention to some problems in the views of Silver and Dworkin and suggests moral principles which support the three moral views just mentioned. 相似文献
982.
Sexual selection theory has primarily focussed on the role of mating preferences for the best individuals in the evolution
of condition-dependent ornaments, traits that signal absolute quality. Because the most suitable mate for one individual is
not always the best for others, however, we argue that non-directional mate choice can promote the evolution of alternative
morphs that are not condition-dependent in their expression (i.e. genetic polymorphism). We list the different mate-choice
rules (i.e. all individuals have the same preference; preference depends on the chooser’s morph; individuals mate preferentially
with conspecifics displaying an uncommon or the most frequent morph) and review experimental studies that investigated mate
choice in natural populations of colour-polymorphic animals. Our review emphasises that although the experimental data support
the idea that sexual selection plays an important role in the evolution of genetic colour polymorphism in many different ways,
little is known about the adaptive value of each mate-choice strategy and about their implication in the evolutionary stability
of colour polymorphism. One way of solving this problem is to determine the adaptive function of colour morphs, a worthwhile
objective, because better understanding of mate-choice rules in polymorphic species should provide important insights into
sexual-selection processes and, in turn, into the maintenance of genetic variation. 相似文献
983.
Jianping Han Wensheng Zhang Hongbing Cao Shilin Chen Yongyan Wang 《Biochemical Systematics and Ecology》2007
Gardenia jasminoides Ellis is used in traditional Chinese medicine (TCM) in China. Levels of genetic variation and patterns of population structure within and among eight wild or cultivated populations of G. jasminoides Ellis in China were investigated using amplified fragment length polymorphism (AFLP) markers. Of the 11 primers screened, four produced highly reproducible AFLP bands. Using these primers, 244 discernible DNA fragments were generated with 165 bands (67.6%), were polymorphic, indicating considerable genetic variation at the species level. In contrast, there were relatively low levels of polymorphism at the population level with the percentage of polymorphic bands (PPB) ranging from 36.89% to 59.43%. Genetic diversity within populations ranged from 0.2086 to 0.3108, averaging 0.2392 at the species level. A high level of genetic differentiation among populations was detected based on Nei's genetic diversity analysis (76.59%), Shannon's index analysis (64.8%) and AMOVA analysis (72.75%). No significant statistical differences (analysis of molecular variance [AMOVA], p = 0.0639) in AFLP variation were found between regions. However, the variance among populations and within populations differed significantly (p < 0.001). An indirect estimate of historical levels of gene flow (Nm = 1.7448) was consistent with the high mean genetic identity (mean I = 0.9263) found among populations. There is an association between geographic and genetic distances between populations. Presently gene change exists between populations. 相似文献
984.
Pedro L.R. de Moraes Claudia J. Nehme Marcelo C. Alves Maria Teresa V.C. Derbyshire Alberto J. Cavalheiro 《Biochemical Systematics and Ecology》2007
The chemical composition of leaves of 57 trees of Cryptocarya mandioccana from three populations of southeastern Brazil was investigated through HPLC, assaying six flavonoids, seven styrylpyrones, and seven unidentified compounds. From 51 of the former trees, genotypes were obtained from 40 polymorphic loci of 19 isozymes. Cluster analyses of the phytochemical and genetical variation revealed that trees exhibited four chemotypes and five clusters from isozymes, respectively. Discriminant analyses from selected variables of the isozymic and chemical data sets were performed, respectively, in relation to the four chemotypes and the five isozyme clusters. The classification of individuals presented respective error estimates of 9.16% and 13.57%, indicating that the genetic data could explain the clusters from chemotypes and vice versa at acceptable error levels. Linear regressions with Dummy variable showed significant association of locus Skdh-2 with quercetin-3-O-β-d-glucopyranoside and cryptofolione, indicating that its alleles would be responsible for the chemotype variation between individuals. 相似文献
985.
Essential oil yield and composition in seven natural populations of Lavandula latifolia from the eastern Iberian Peninsula were determined by GC/MS. Twenty-eight constituents were identified, accounting for 92.0–95.4% of the total oils. These oils were dominated by the monoterpene fraction and three of them (linalool, cineole and camphor) constituted 79.5–86.9% of the oil from flowers. Essential oil yield in leaves and flowers varied among and within populations, but hierarchic analyses of variance showed that the proportion of variation attributable to individuals was significantly higher than that attributable to population differences. Principal component and cluster analyses allowed three groups of flower essential oils to be distinguished according to their high, intermediate and low proportion of linalool. These essential oil types are respectively correlated to the Supra-, Meso- and Thermo-Mediterranean bioclimatic belts where the populations are located. A genetic analysis based on those terpenes that showed a trimodal distribution roughly corroborated the relationships between the seven populations obtained from the ordination analyses and emphasizes the distinctiveness of some of the populations. 相似文献
986.
为了获得能够携带较大外源基因的犬2型腺病毒E3区缺失性载体,以犬2型腺病毒全基因组质粒pPolyⅡ-CAV-2及E3区重组质粒pVAX-E3为基础,缺失1381bp的E3区片段(92.6%的E3区全序列),插入Linker-NF(内含NotⅠ、ClaⅠ、FseⅠ多克隆位点),获得重组载体质粒pPolyⅡ-CAV-2-ΔE3(NF)(31.9kb)。以AscⅠ和PmeⅠ双酶切,游离重组基因组,在脂质体LipofectamineTM 2000介导下,转染MDCK细胞系,获得了E3区缺失的重组病毒CAV-2-ΔE3(NF)。通过病毒的形态学观察,血凝性、生长特性、感染性实验证明,该重组病毒与母源病毒没有差异。重组病毒CAV-2-ΔE3(NF)可以作为载体表达外源基因,其外源基因插入片段不小于3.3kb。 相似文献
987.
We describe an efficient method for generating highly functional membrane proteins with variant amino acids at defined positions that couples a modified site saturation strategy with functional genetic selection. We applied this method to the production of a cysteine-less variant of the Crithidia fasciculata inosine-guanosine permease CfNT2 to facilitate biochemical studies using thiol-specific modifying reagents. Of 10 endogenous cysteine residues in CfNT2, two cannot be replaced with serine or alanine without loss of function. High-quality single- and double-mutant libraries were produced by combining a previously reported site saturation mutagenesis scheme based on the Stratagene Quikchange method with a novel gel purification step that effectively eliminated template DNA from the products. Following selection for functional complementation in Saccharomyces cerevisiae cells auxotrophic for purines, several highly functional noncysteine substitutions were efficiently identified at each desired position, allowing the construction of cysteine-less variants of CfNT2 that retained wild-type affinity for inosine. This combination of an improved site saturation mutagenesis technique and positive genetic selection provides a simple and efficient means to identify functional and perhaps unexpected amino acid variants at a desired position. 相似文献
988.
The intentional introduction of red king crab, Paralithodes camtschatica (Tilesius, 1815) in the Barents Sea represent one of a few successful cases and one that now supports a commercial fishery.
Introductions of alien species into new environments are often associated with genetic bottlenecks, which cause a reduction
in the genetic variation, and this could be important for the spreading potential of the species in the Atlantic Ocean. Red
king crab samples collected in the Varangerfjord located on the Barents Sea (northern Norway) were compared with reference
crab samples collected from the Bering Sea and Kamchatka regions in the Pacific Ocean. All samples were screened for eleven
microsatellite loci, based on the development of species-specific primers. The observed number of alleles per locus was similar,
and no reduction in genetic variation, including gene diversity and allelic richness, was detected between the Varangerfjord
sample and the reference sample from Okhotsk Sea near Kamchatka, indicating no genetic bottlenecking at least for the microsatellite
loci investigated. The same results were found in comparison with the sample from Bering Sea. The level of genetic differentiation
among the samples, measured as overall F
ST
across all loci, was relatively low (0.0238) with a range of 0.0035–0.1000 for the various loci investigated. The largest
pairwise F
ST
values were found between the Bering Sea and Varangerfjord/Barents Sea samples, with a value of 0.0194 across all loci tested.
The lowest value (0.0101) was found between the Varangerfjord and Kamchatka samples. Genetic differentiation based on exact
tests on allele frequencies revealed highly significant differences between all pairwise comparisons. The high level of genetic
variation found in the Varangerfjord/Barents Sea sample could be of significance with respect to further spreading of the
species to other regions in the North Atlantic Ocean. 相似文献
989.
Knut E. Jørstad P. A. Prodohl A.-L. Agnalt M. Hughes E. Farestveit A. F. Ferguson 《Hydrobiologia》2007,590(1):103-114
American lobsters (Homarus americanus H. Milne Edwards, 1837) are imported live to Europe and should according regulations be kept in land-based tanks until sold. In spite of the strict regulations aimed specifically at preventing the introduction of this species into the NE Atlantic, several specimens of H. americanus have been captured in the wild, especially in Oslofjord, Norway since 1999. One of the great concerns is interbreeding between the introduced American species and the local European lobster, H. gammarus (Linnaeus, 1758). For this reason an awareness campaign was launched in 2000 focusing on morphologically “unusual” lobsters caught in local waters. Morphological characters have been based on colour and sub-ventral spines on the rostrum. Two samples of H. americanus were used for comparisons, as well as samples of European lobster from Oslofjord collected in 1992. Previous genetic analyses (allozymes, mtDNA and microsatellite DNA) have demonstrated that the American lobster is distinct from its European counterpart, with several additional alleles at many loci in addition to different allelic frequency distribution of alleles of “shared” alleles. During the present study, thirteen microsatellite loci were tested in the initial screening, and the three most discriminating loci (Hgam98, Hgam197b and Hgam47b) were used in a detailed comparison between the two species. A total of 45 unusual lobsters were reported captured from Ålesund (west) to Oslofjord (southeast) from 2001 to 2005 and these were analysed for the three microsatellite loci. Nine specimens were identified as American lobsters. Comparisons between morphological and genetic characteristics revealed that morphological differences are not reliable in discrimination the two species, or to identify hybrids. Further, some loci display almost no overlapping in allele frequency distribution for the reference samples analysed, thus providing a reliable tool to identify hybrids. 相似文献
990.
Enrique Arbeláez-Cortes María Fernanda Castillo-Cárdenas Nelson Toro-Perea Heiber Cárdenas-Henao 《Hydrobiologia》2007,583(1):321-330
Rhizophora mangle, one of the five species of the genus Rhizophora, is found widely distributed along the American and West African coasts. This species is one of the principal constituents
of the mangrove ecosystem in Colombia and is also found within the most important economic activities for the communities
that inhabit the littoral. In order to assess the degree of genetic diversity of R. mangle in five populations of the Colombian Pacific, nuclear microsatellite molecular markers were used. In 92 individuals sampled,
it was found that 100% of the loci were polymorphic
, and no private alleles were detected. The population structure of R. mangle in the Colombian Pacific, was highly significant (P < 0.001); however, the greatest differentiation was detected at the within-population level (94.62%). For the populations
of La Plata, Virudó and Charambirá, the tendency toward panmixia could be the cause of the low differentiation among these
three locations. Within populations, the genetic diversity revealed a deviation from Hardy–Weinberg equilibrium with high
significance in Virudó and Tumaco, where it appears the intense anthropogenic activity has exercised strong pressure on the
red mangrove, resulting in the possible fragmentation of the local landscape and therefore an increase in the rate of endogamy
within these populations. Despite this situation, our study―one of the first developed in genetics of the red mangrove in
Colombia―did not show evidence of recent bottleneck effects or deterioration in its genetic composition, which could be exploited
to propose management and restoration programs for the zones where the forests of this species are degraded.
Handling editor: K. Martens 相似文献