全文获取类型
收费全文 | 3899篇 |
免费 | 359篇 |
国内免费 | 142篇 |
出版年
2024年 | 8篇 |
2023年 | 68篇 |
2022年 | 34篇 |
2021年 | 97篇 |
2020年 | 166篇 |
2019年 | 155篇 |
2018年 | 125篇 |
2017年 | 151篇 |
2016年 | 157篇 |
2015年 | 152篇 |
2014年 | 180篇 |
2013年 | 279篇 |
2012年 | 139篇 |
2011年 | 170篇 |
2010年 | 161篇 |
2009年 | 246篇 |
2008年 | 237篇 |
2007年 | 215篇 |
2006年 | 195篇 |
2005年 | 150篇 |
2004年 | 159篇 |
2003年 | 107篇 |
2002年 | 124篇 |
2001年 | 108篇 |
2000年 | 86篇 |
1999年 | 77篇 |
1998年 | 68篇 |
1997年 | 59篇 |
1996年 | 56篇 |
1995年 | 37篇 |
1994年 | 37篇 |
1993年 | 42篇 |
1992年 | 37篇 |
1991年 | 27篇 |
1990年 | 27篇 |
1989年 | 31篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 24篇 |
1985年 | 25篇 |
1984年 | 27篇 |
1983年 | 14篇 |
1982年 | 24篇 |
1981年 | 15篇 |
1980年 | 13篇 |
1979年 | 14篇 |
1978年 | 14篇 |
1977年 | 9篇 |
1975年 | 7篇 |
1973年 | 6篇 |
排序方式: 共有4400条查询结果,搜索用时 15 毫秒
81.
82.
83.
通过对20世纪80年代以来具有代表性的华北夏谷区审(鉴)定的20个主栽谷子品种进行SSR标记多态性分析,研究了华北地区育成谷子品种的遗传多样性。49对引物在20个谷子品种中扩增出具有多态性的条带,共检测出142个等位变异,平均每个位点检测出的等位变异数为2.96个。标记位点多态性信息含量(PIC)变幅为0.0904~0.6896,平均为0.4168。20份材料间的遗传距离变幅为0.0173~0.9000,聚类分析将其分成3个类群,其中谷丰1号自成一类,表明谷丰1号的遗传距离较其他品种大。不同年代谷子品种间遗传距离分析结果显示,各年代品种之间的平均遗传距离由大到小依次是1980s>1990s>2000s>2010s,表明随着年代的递进育成品种的遗传差异减小,亲缘关系增近。 相似文献
84.
Traditional phylogenies based on analysis of multiple genes have failed to obtain a well‐resolved evolutionary history for the backbone of the Asian Palmate group of Araliaceae, the largest clade of the family. In this study, we applied the genome skimming approach of next‐generation sequencing to address whether the lack of resolution at the base of the Asian Palmate tree is due to molecular sampling error or the footprint of ancient radiation. Twenty‐nine complete plastid genomes of Araliaceae (17 newly sequenced) were analyzed (RAxML, Beast, Lagrange, and BioGeoBears) to provide the first phylogenomic reconstruction of the group (95% of genera included). As a result, the early divergences of the Asian Palmate group have been clarified but the backbone of its core is not totally resolved, with short internal branches pointing to an ancient radiation scenario. East Asia is inferred as the most likely ancestral area for the Asian Palmate group (from late Paleocene to Eocene) from which early colonization of the Neotropics is inferred during the Eocene. The radiation of the core Palmate group took place during the late Eocene, most likely in the context of the Boreotropical hypothesis. Recurrent episodes of southward migration (to the tropics) coupled with northern latitude local extinctions (promoting geographic isolation of lineages) followed by northward expansion (promoting contact of lineages that erased the trace of preceding geographic isolation) are hypothesized to have linked to the alternation of the cold and warm periods of the Eocene. 相似文献
85.
86.
Long-term, high-resolution proxy records containing temperature signals are relatively scarce over Central China, limiting our understanding of regional climate and the potential driving forces. In this study, a robust tree-ring width chronology is developed based on Pinus massoniana woods from two sites on the northeast Wuling Mountains, spanning from 1815 to 2014. The presented chronology is used to reconstruct regional long-term winter-spring (January-May) temperature changes over the past two centuries in Central China. The reconstruction reveals that cold epochs occurred in 1830 s–1840 s, 1850 s–1860 s, 1900 s–1910 s and 1960 s–1970 s. The correlation patterns between the reconstruction and global sea surface temperature (SST) are suggestive of the influence of SST of the western North Pacific Ocean (especially the Japan Current region) and the North Atlantic Ocean on regional temperature. In addition, persistent close matches are observed between the chronology and the Atlantic Multidecadal Oscillation (AMO) reconstructions over the past two centuries, suggesting the AMO may have been a key modulator of long-term temperature variations over Central China. 相似文献
87.
Chengfeng Le Shuyu Wu Chuanmin Hu Marcus W. Beck Xuchao Yang 《Global Change Biology》2019,25(10):3485-3493
Global climate change can significantly influence oceanic phytoplankton dynamics, and thus biogeochemical cycles and marine food webs. However, associative explanations based on the correlation between chlorophyll‐a concentration (Chl‐a) and climatic indices is inadequate to describe the mechanism of the connection between climate change, large‐scale atmospheric dynamics, and phytoplankton variability. Here, by analyzing multiple satellite observations of Chl‐a and atmospheric conditions from National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis datasets, we show that high‐latitude atmospheric blocking events over Alaska are the primary drivers of the recent decline of Chl‐a in the eastern North Pacific transition zone. These blocking events were associated with the persistence of large‐scale atmosphere pressure fields that decreased westerly winds and southward Ekman transport over the subarctic ocean gyre. Reduced southward Ekman transport leads to reductions in nutrient availability to phytoplankton in the transition zone. The findings describe a previously unidentified climatic factor that contributed to the recent decline of phytoplankton in this region and propose a mechanism of the top‐down teleconnection between the high‐latitude atmospheric circulation anomalies and the subtropical oceanic primary productivity. The results also highlight the importance of understanding teleconnection among atmosphere–ocean interactions as a means to anticipate future climate change impacts on oceanic primary production. 相似文献
88.
Ronan James O'Sullivan Tutku Aykanat Susan E. Johnston Adam Kane Russell Poole Ger Rogan Paulo A. Prodhl Craig R. Primmer Philip McGinnity Thomas Eric Reed 《Ecology and evolution》2019,9(12):7096-7111
Comparing observed versus theoretically expected evolutionary responses is important for our understanding of the evolutionary process, and for assessing how species may cope with anthropogenic change. Here, we document directional selection for larger female size in Atlantic salmon, using pedigree‐derived estimates of lifetime reproductive success as a fitness measure. We show the trait is heritable and, thus, capable of responding to selection. The Breeder's Equation, which predicts microevolution as the product of phenotypic selection and heritability, predicted evolution of larger size. This was at odds, however, with the observed lack of either phenotypic or genetic temporal trends in body size, a so‐called “paradox of stasis.” To investigate this paradox, we estimated the additive genetic covariance between trait and fitness, which provides a prediction of evolutionary change according to Robertson's secondary theorem of selection (STS) that is unbiased by missing variables. The STS prediction was consistent with the observed stasis. Decomposition of phenotypic selection gradients into genetic and environmental components revealed a potential upward bias, implying unmeasured factors that covary with trait and fitness. These results showcase the power of pedigreed, wild population studies—which have largely been limited to birds and mammals—to study evolutionary processes on contemporary timescales. 相似文献
89.
Marlene Jahnke Martin Gullstrm Josefine Larsson Maria E. Asplund Said Mgeleka Mathew Ogalo Silas Arielle Hoamby Jamal Mahafina Lina Mtwana Nordlund 《Ecology and evolution》2019,9(16):8953-8964
This study is the first large‐scale genetic population study of a widespread climax species of seagrass, Thalassia hemprichii, in the Western Indian Ocean (WIO). The aim was to understand genetic population structure and connectivity of T. hemprichii in relation to hydrodynamic features. We genotyped 205 individual seagrass shoots from 11 sites across the WIO, spanning over a distance of ~2,700 km, with twelve microsatellite markers. Seagrass shoots were sampled in Kenya, Tanzania (mainland and Zanzibar), Mozambique, and Madagascar: 4–26°S and 33–48°E. We assessed clonality and visualized genetic diversity and genetic population differentiation. We used Bayesian clustering approaches (TESS) to trace spatial ancestry of populations and used directional migration rates (DivMigrate) to identify sources of gene flow. We identified four genetically differentiated groups: (a) samples from the Zanzibar channel; (b) Mozambique; (c) Madagascar; and (d) the east coast of Zanzibar and Kenya. Significant pairwise population genetic differentiation was found among many sites. Isolation by distance was detected for the estimated magnitude of divergence (DEST), but the three predominant ocean current systems (i.e., East African Coastal Current, North East Madagascar Current, and the South Equatorial Current) also determine genetic connectivity and genetic structure. Directional migration rates indicate that Madagascar acts as an important source population. Overall, clonality was moderate to high with large differences among sampling sites, indicating relatively low, but spatially variable sexual reproduction rates. The strongest genetic break was identified for three sites in the Zanzibar channel. Although isolation by distance is present, this study suggests that the three regionally predominant ocean current systems (i.e., East African Coastal Current, North East Madagascar Current, and the South Equatorial Current) rather than distance determine genetic connectivity and structure of T. hemprichii in the WIO. If the goal is to maintain genetic connectivity of T. hemprichii within the WIO, conservation planning and implementation of marine protection should be considered at the regional scale—across national borders. 相似文献
90.
Yuqiu Wei Guicheng Zhang Ju Chen Jing Wang Changling Ding Xiaodong Zhang Jun Sun 《Ecology and evolution》2019,9(8):5003-5017
Picophytoplankton were investigated during spring 2015 and 2016 extending from near‐shore coastal waters to oligotrophic open waters in the eastern Indian Ocean (EIO). They were typically composed of Prochlorococcus (Pro), Synechococcus (Syn), and picoeukaryotes (PEuks). Pro dominated most regions of the entire EIO and were approximately 1–2 orders of magnitude more abundant than Syn and PEuks. Under the influence of physicochemical conditions induced by annual variations of circulations and water masses, no coherent abundance and horizontal distributions of picophytoplankton were observed between spring 2015 and 2016. Although previous studies reported the limited effects of nutrients and heavy metals around coastal waters or upwelling zones could constrain Pro growth, Pro abundance showed strong positive correlation with nutrients, indicating the increase in nutrient availability particularly in the oligotrophic EIO could appreciably elevate their abundance. The exceptional appearance of picophytoplankton with high abundance along the equator appeared to be associated with the advection processes supported by the Wyrtki jets. For vertical patterns of picophytoplankton, a simple conceptual model was built based upon physicochemical parameters. However, Pro and PEuks simultaneously formed a subsurface maximum, while Syn generally restricted to the upper waters, significantly correlating with the combined effects of temperature, light, and nutrient availability. The average chlorophyll a concentrations (Chl a) of picophytoplankton accounted for above 49.6% and 44.9% of the total Chl a during both years, respectively, suggesting that picophytoplankton contributed a significant proportion of the phytoplankton community in the whole EIO. 相似文献