全文获取类型
收费全文 | 3693篇 |
免费 | 278篇 |
国内免费 | 5篇 |
专业分类
3976篇 |
出版年
2023年 | 9篇 |
2022年 | 18篇 |
2021年 | 53篇 |
2020年 | 30篇 |
2019年 | 44篇 |
2018年 | 51篇 |
2017年 | 57篇 |
2016年 | 94篇 |
2015年 | 144篇 |
2014年 | 152篇 |
2013年 | 225篇 |
2012年 | 277篇 |
2011年 | 212篇 |
2010年 | 180篇 |
2009年 | 129篇 |
2008年 | 230篇 |
2007年 | 235篇 |
2006年 | 182篇 |
2005年 | 201篇 |
2004年 | 196篇 |
2003年 | 191篇 |
2002年 | 177篇 |
2001年 | 52篇 |
2000年 | 45篇 |
1999年 | 42篇 |
1998年 | 50篇 |
1997年 | 37篇 |
1996年 | 36篇 |
1995年 | 42篇 |
1994年 | 45篇 |
1993年 | 39篇 |
1992年 | 33篇 |
1991年 | 31篇 |
1990年 | 24篇 |
1989年 | 23篇 |
1988年 | 25篇 |
1987年 | 34篇 |
1986年 | 20篇 |
1985年 | 33篇 |
1984年 | 25篇 |
1983年 | 22篇 |
1982年 | 26篇 |
1981年 | 26篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 12篇 |
1977年 | 11篇 |
1975年 | 17篇 |
1974年 | 12篇 |
1973年 | 8篇 |
排序方式: 共有3976条查询结果,搜索用时 0 毫秒
91.
Interaction between parental care and sibling competition: parents enhance offspring growth and exacerbate sibling competition 总被引:2,自引:0,他引:2
Smiseth PT Lennox L Moore AJ 《Evolution; international journal of organic evolution》2007,61(10):2331-2339
Species with elaborate parental care often also show intense sibling competition over resources provided by parents, suggesting joint evolution of these two traits. Despite this, the evolution of elaborate parental care and the evolution of intense sibling competition are often studied separately. Here, we examine the interaction between parental food provisioning and sibling competition for resources through the joint manipulation of the presence or absence of parents and brood size in a species with facultative parental care: the burying beetle Nicrophorus vespilloides. The effect of the interaction between the presence or absence of parents and brood size was strong; brood size had a strong effect on growth when parents provided care, but no effect when parents were absent. As in previous studies, offspring grew faster when parents were present than when parents were absent, and offspring grew faster in smaller broods than in larger broods. Our behavioral observations showed that brood size had a negative effect on both the amount of time parents spent providing resources to individual offspring and the offspring's effectiveness of begging, confirming that the level of sibling competition increased with brood size. Furthermore, offspring in larger broods shifted more from begging toward self-feeding as they grew older compared to offspring in small broods. Our study provides novel insights into the joint evolution of parental care and sibling competition, and the evolution of offspring begging signals. We discuss the implications of our results in light of recent theoretical work on the evolution of parental care, sibling competition, and offspring begging signals. 相似文献
92.
Studies using HeLa cells expressing 5-HT7 receptors showed that detergent resistant membranous lipid rafts purified by sucrose gradient centrifugation contained 5-HT7 receptors and caveolin-1. Caveolin-1 siRNA-mediated knockdown or serotonin (5-HT) treatment caused significant reductions of maximum [3H]5-HT binding to 5-HT7 receptors and total immunoreactivity of membranous 5-HT7 receptors in intact cells. Co-treatment with 5-HT, caveolin-1 siRNA and methyl-beta-cyclodextrin had no additive effects on reducing maximum binding of [3H]5-HT to 5-HT7 receptors. 5-HT-mediated reduction of [3H]5-HT binding to 5-HT7 receptors was counteracted by genistein, but not by sucrose. Caveolin-1, specifically localized in cholesterol-enriched lipid rafts, appears to regulate constitutive and agonist-stimulated cell surface levels of 5-HT7 receptors via a clathrin-independent mechanism. 相似文献
93.
Determination of drug permeability and prediction of drug absorption in Caco-2 monolayers 总被引:2,自引:0,他引:2
Permeability coefficients across monolayers of the human colon carcinoma cell line Caco-2, cultured on permeable supports, are commonly used to predict the absorption of orally administered drugs and other xenobiotics. This protocol describes our method for the cultivation, characterization and determination of permeability coefficients of xenobiotics (which are, typically, drug-like compounds) in the Caco-2 model. A few modifications that have been introduced over the years are incorporated in the protocol. The method can be used to trace the permeability of a test compound in two directions, from the apical to the basolateral side or vice versa, and both passive and active transport processes can be studied. The permeability assay can be completed within one working day, provided that the Caco-2 monolayers have been cultured and differentiated on the permeable supports 3 weeks in advance. 相似文献
94.
Liedvogel M Maeda K Henbest K Schleicher E Simon T Timmel CR Hore PJ Mouritsen H 《PloS one》2007,2(10):e1106
Cryptochromes (Cry) have been suggested to form the basis of light-dependent magnetic compass orientation in birds. However, to function as magnetic compass sensors, the cryptochromes of migratory birds must possess a number of key biophysical characteristics. Most importantly, absorption of blue light must produce radical pairs with lifetimes longer than about a microsecond. Cryptochrome 1a (gwCry1a) and the photolyase-homology-region of Cry1 (gwCry1-PHR) from the migratory garden warbler were recombinantly expressed and purified from a baculovirus/Sf9 cell expression system. Transient absorption measurements show that these flavoproteins are indeed excited by light in the blue spectral range leading to the formation of radicals with millisecond lifetimes. These biophysical characteristics suggest that gwCry1a is ideally suited as a primary light-mediated, radical-pair-based magnetic compass receptor. 相似文献
95.
96.
Barbora Trubenov Martin S. Krejca Per Kristian Lehre Timo Ktzing 《Evolution; international journal of organic evolution》2019,73(7):1356-1374
The environment changes constantly at various time scales and, in order to survive, species need to keep adapting. Whether these species succeed in avoiding extinction is a major evolutionary question. Using a multilocus evolutionary model of a mutation‐limited population adapting under strong selection, we investigate the effects of the frequency of environmental fluctuations on adaptation. Our results rely on an “adaptive‐walk” approximation and use mathematical methods from evolutionary computation theory to investigate the interplay between fluctuation frequency, the similarity of environments, and the number of loci contributing to adaptation. First, we assume a linear additive fitness function, but later generalize our results to include several types of epistasis. We show that frequent environmental changes prevent populations from reaching a fitness peak, but they may also prevent the large fitness loss that occurs after a single environmental change. Thus, the population can survive, although not thrive, in a wide range of conditions. Furthermore, we show that in a frequently changing environment, the similarity of threats that a population faces affects the level of adaptation that it is able to achieve. We check and supplement our analytical results with simulations. 相似文献
97.
98.
Martin Kundrát John Nudds Benjamin P. Kear Junchang Lü Per Ahlberg 《Historical Biology》2019,31(1):3-63
From an initial isolated position as the oldest evolutionary prototype of a bird, Archaeopteryx has, as a result of recent fossil discoveries, become embedded in a rich phylogenetic context of both more and less crownward stem-group birds. This has prompted debate over whether Archaeopteryx is simply a convergently bird-like non-avialan theropod. Here we show, using the first synchrotron microtomographic examination of the genus, that the eighth or Daiting specimen of Archaeopteryx possesses a character suite that robustly constrains it as a basal avialan (primitive bird). The specimen, which comes from the Mörnsheim Formation and is thus younger than the other specimens from the underlying Solnhofen Formation, is distinctive enough to merit designation as a new species, Archaeopteryx albersdoerferi sp. nov., but is recovered in close phylogenetic proximity to Archaeopteryx lithographica. Skeletal innovations of the Daiting specimen, such as fusion and pneumatization of the cranial bones, well vascularized pectoral girdle and wing elements, and a reinforced configuration of carpals and metacarpals, suggest that it may have had more characters seen in flying birds than the older Archaeopteryx lithographica. These innovations appear to be convergent on those of more crownward avialans, suggesting that Bavarian archaeopterygids independently acquired increasingly bird-like traits over time. Such mosaic evolution and iterative exploration of adaptive space may be typical for major functional transitions like the origin of flight. 相似文献
99.
A high capacity for long‐distance dispersal is a key to success for species confronted with environmental heterogeneity, habitat modification, fragmentation and loss. However, dispersal capacity is difficult to quantify and therefore poorly known in most taxa. Here, we report on a test for an association of variation in dispersal capacity with variable colouration of noctuid moths. First, using data from 12 experienced lepidopterologists, we showed that despite variation among experts in average assessments, different species are consistently classified as having non‐variable, variable or highly variable colour patterns when assessed by different experts. We then compared the incidence of non‐resident species with high inter‐individual variation in colour patterns recorded on the isolated island Utklippan (n = 47), with that in a species pool of potential long‐distance dispersers from the nearest mainland (n = 295). Species with high inter‐individual colour pattern variation were over‐represented on the island compared with species having non‐variable colouration. This finding constitutes rare evidence from the wild of long‐distance dispersal, measured on a spatial scale relevant for moths when tracking habitats in fragmented and changing landscapes or when keeping pace with environmental challenges associated with climate change. Finally, we showed that Swedish noctuid moths classified as agricultural pests (n = 28) had more variable colour patterns compared with non‐pests (n = 368). The majority of agricultural pests were also recorded on the isolated island, an outcome that is indicative of pest species having high dispersal capacity. Data on colour pattern variation may thus offer a simple and cost‐effective proxy to estimate dispersal capacity and can also help identify potential pest species. Our findings are potentially useful when modelling and predicting population and range dynamics of species in spatiotemporally heterogeneous environments, with direct implications for conservation biology and pest management. 相似文献
100.
Per Heydorn Olle Anderbrant Lars J. Jonsson Glenn P. Svensson 《Journal of Applied Entomology》2019,143(10):1193-1195
The pest Plodia interpunctella (Hübner) is reared in many research laboratories. In a culture established in 1996, attraction of males to the female‐produced sex pheromone in flight tunnel assays gradually decreased after ≈15 years of rearing. A new culture was established to enable comparison with the old culture regarding traits associated with mate finding. Female calling activity, pheromone titre and male antennal response to pheromone components did not differ between cultures. In contrast, very few males from the old culture reached the pheromone source in flight tunnel assays compared with 61%–81% of males from the other culture. Our results highlight the importance of maintaining viable insect cultures for research purposes and suggest frequent evaluation of traits involved in chemical communication in such cultures to ensure reliable results in experiments. 相似文献