全文获取类型
收费全文 | 3619篇 |
免费 | 467篇 |
国内免费 | 673篇 |
出版年
2024年 | 18篇 |
2023年 | 92篇 |
2022年 | 92篇 |
2021年 | 160篇 |
2020年 | 180篇 |
2019年 | 197篇 |
2018年 | 173篇 |
2017年 | 179篇 |
2016年 | 199篇 |
2015年 | 168篇 |
2014年 | 210篇 |
2013年 | 285篇 |
2012年 | 190篇 |
2011年 | 182篇 |
2010年 | 165篇 |
2009年 | 185篇 |
2008年 | 183篇 |
2007年 | 207篇 |
2006年 | 187篇 |
2005年 | 174篇 |
2004年 | 118篇 |
2003年 | 124篇 |
2002年 | 109篇 |
2001年 | 86篇 |
2000年 | 84篇 |
1999年 | 98篇 |
1998年 | 77篇 |
1997年 | 65篇 |
1996年 | 56篇 |
1995年 | 53篇 |
1994年 | 56篇 |
1993年 | 56篇 |
1992年 | 50篇 |
1991年 | 53篇 |
1990年 | 31篇 |
1989年 | 35篇 |
1988年 | 23篇 |
1987年 | 18篇 |
1986年 | 22篇 |
1985年 | 21篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 20篇 |
1981年 | 15篇 |
1980年 | 12篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 3篇 |
1976年 | 3篇 |
1958年 | 1篇 |
排序方式: 共有4759条查询结果,搜索用时 15 毫秒
161.
Early Late Miocene coral assemblages from five outcrops of Gavdos Island, Southern Greece, are investigated with respect to their palaeoecological implications. Small patch reefs with Porites assemblage are a common feature of the low-diversity coral occurrences. The determined hermatypic colonies indicate a nearshore palaeoecosystem prevailing in a tropical to subtropical coastal sea at depths ranging from 5 to 50 m with an average temperature of 22–26 °C. Microfacially, the studied Scleractinian patch reefs are represented by Coral Framestones-Floatstones. The reefal facies has been affected by syndepositional processes (boring activities-micritization), as well as by post-depositional diagenesis in the meteoric realm (dissolution, cementation and intense pedogenesis). The palaeoecological and sedimentological analysis indicates a restricted to open-marine inner platform setting of moderate to high energy, possibly of ramp-type (inner-mid ramp). Coral reef growth took place mainly during stages of accommodation (i.e., transgressive episode, cycle 3.1 of Vail curve) and of relatively low siliciclastic input. 相似文献
162.
Thomas Mitchell‐Olds 《Molecular ecology》2013,22(13):3427-3429
Understanding genetic variation for complex traits in heterogeneous environments is a fundamental problem in biology. In this issue of Molecular Ecology, Fournier‐Level et al. ( 2013 ) analyse quantitative trait loci (QTL) influencing ecologically important phenotypes in mapping populations of Arabidopsis thaliana grown in four habitats across its native European range. They used causal modelling to quantify the selective consequences of life history and morphological traits and QTL on components of fitness. They found phenology QTL colocalizing with known flowering time genes as well as novel loci. Most QTL influenced fitness via life history and size traits, rather than QTL having direct effects on fitness. Comparison of phenotypes among environments found no evidence for genetic trade‐offs for phenology or growth traits, but genetic trade‐offs for fitness resulted because flowering time had opposite fitness effects in different environments. These changes in QTL effects and selective consequences may maintain genetic variation among populations. 相似文献
163.
164.
An ongoing new synthesis in evolutionary theory is expanding our view of the sources of heritable variation beyond point mutations of fixed phenotypic effects to include environmentally sensitive changes in gene regulation. This expansion of the paradigm is necessary given ample evidence for a heritable ability to alter gene expression in response to environmental cues. In consequence, single genotypes are often capable of adaptively expressing different phenotypes in different environments, i.e. are adaptively plastic. We present an individual-based heuristic model to compare the adaptive dynamics of populations composed of plastic or non-plastic genotypes under a wide range of scenarios where we modify environmental variation, mutation rate and costs of plasticity. The model shows that adaptive plasticity contributes to the maintenance of genetic variation within populations, reduces bottlenecks when facing rapid environmental changes and confers an overall faster rate of adaptation. In fluctuating environments, plasticity is favoured by selection and maintained in the population. However, if the environment stabilizes and costs of plasticity are high, plasticity is reduced by selection, leading to genetic assimilation, which could result in species diversification. More broadly, our model shows that adaptive plasticity is a common consequence of selection under environmental heterogeneity, and hence a potentially common phenomenon in nature. Thus, taking adaptive plasticity into account substantially extends our view of adaptive evolution. 相似文献
165.
Zoé Dumas Adin Ross-Gillespie Rolf Kümmerli 《Proceedings. Biological sciences / The Royal Society》2013,280(1764)
Bacteria often possess multiple siderophore-based iron uptake systems for scavenging this vital resource from their environment. However, some siderophores seem redundant, because they have limited iron-binding efficiency and are seldom expressed under iron limitation. Here, we investigate the conundrum of why selection does not eliminate this apparent redundancy. We focus on Pseudomonas aeruginosa, a bacterium that can produce two siderophores—the highly efficient but metabolically expensive pyoverdine, and the inefficient but metabolically cheap pyochelin. We found that the bacteria possess molecular mechanisms to phenotypically switch from mainly producing pyoverdine under severe iron limitation to mainly producing pyochelin when iron is only moderately limited. We further show that strains exclusively producing pyochelin grew significantly better than strains exclusively producing pyoverdine under moderate iron limitation, whereas the inverse was seen under severe iron limitation. This suggests that pyochelin is not redundant, but that switching between siderophore strategies might be beneficial to trade off efficiencies versus costs of siderophores. Indeed, simulations parameterized from our data confirmed that strains retaining the capacity to switch between siderophores significantly outcompeted strains defective for one or the other siderophore under fluctuating iron availabilities. Finally, we discuss how siderophore switching can be viewed as a form of collective decision-making, whereby a coordinated shift in behaviour at the group level emerges as a result of positive and negative feedback loops operating among individuals at the local scale. 相似文献
166.
167.
168.
Many studies have examined the effects of spatial heterogeneity in light supply on clonal plants in terrestrial environments, but few have examined those in aquatic conditions. In a greenhouse experiment, we grew the rhizomatous submerged macrophyte Vallisneria spiralis L. in containers in three homogeneous light treatments (100%, 65%, and 30% of ambient light coded as high, medium, and low light, respectively) and two heterogeneous ones differing in patch scale (small and large patch). The growth of V. spiralis decreased significantly with decreasing light availability. In the low light conditions, V. spiralis allocated greater biomass to shoots and developed elongated leaves. In the patchy treatments, ramets distributed in the light‐rich patches had significant costs in the large patch treatment, but not in the small patch treatment, while both small and large patch treatments had no significant benefits in the light‐poor patches. We conclude that V. spiralis could escape from adverse habitats and occupy the favorable habitats. Providing the same amount of light, responses of V. spiralis to different patch scales were different at the patch level, but not at the whole clone level. Together, growth of V. spiralis could not benefit from different patch scales in heterogeneous environments at the patch as well as the whole clone level. 相似文献
169.
《Journal of Plant Interactions》2013,8(1):262-269
The effect of fan-forced wind on the severity and growth of Fusarium oxysporum f. sp. radicis-lycopersici (FORL) was examined in this study. The discoloration severity of the total root system was significantly reduced in plants treated with air blasting for 30?min at a wind speed of 4?m/s compared with the control. In addition, the number of colony-forming units of FORL per gram of fresh root weight was significantly reduced (p?≤?0.05) in plants treated with air blasting at a wind speed of 4?m/s for 30?min, and the root extracts of these plants had a significantly lower production of FORL budding cells. Booster wind treatments significantly reduced the severity and growth of FORL compared with single and control treatments. Furthermore, RT-PCR analysis indicated that the expression of defense-related genes was induced in the leaves of seedlings treated with air blasting at a wind speed of 4?m/s. 相似文献
170.
《Animal : an international journal of animal bioscience》2013,7(4):655-664
Data from 113 Dutch organic farms were analysed to determine the effect of cross-breeding on production and functional traits. In total, data on 33 788 lactations between January 2003 and February 2009 from 15 015 cows were available. Holstein–Friesian pure-bred cows produced most kg of milk in 305 days, but with the lowest percentages of fat and protein of all pure-bred cows in the data set. Cross-breeding Holstein dairy cows with other breeds (Brown Swiss, Dutch Friesian, Groningen White Headed, Jersey, Meuse Rhine Yssel, Montbéliarde or Fleckvieh) decreased milk production, but improved fertility and udder health in most cross-bred animals. In most breeds, heterosis had a significant effect (P < 0.05) on milk (kg in 305 days), fat and protein-corrected milk production (kg in 305 days) and calving interval (CI) in the favourable direction (i.e. more milk, shorter CI), but unfavourably for somatic cell count (higher cell count). Recombination was unfavourable for the milk production traits, but favourable for the functional traits (fertility and udder health). Farm characteristics, like soil type or housing system, affected the regression coefficients on breed components significantly. The effect of the Holstein breed on milk yield was twice as large in cubicle housing as in other housing systems. Jerseys had a negative effect on fertility only on farms on sandy soils. Hence, breed effects differ across farming systems in the organic farming and farmers can use such information to dovetail their farming system with the type of cow they use. 相似文献