全文获取类型
收费全文 | 1932篇 |
免费 | 9篇 |
国内免费 | 18篇 |
专业分类
系统科学 | 33篇 |
丛书文集 | 2篇 |
教育与普及 | 11篇 |
理论与方法论 | 49篇 |
现状及发展 | 229篇 |
研究方法 | 310篇 |
综合类 | 1216篇 |
自然研究 | 109篇 |
出版年
2021年 | 6篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 15篇 |
2017年 | 20篇 |
2016年 | 21篇 |
2015年 | 15篇 |
2014年 | 19篇 |
2013年 | 27篇 |
2012年 | 173篇 |
2011年 | 296篇 |
2010年 | 72篇 |
2009年 | 14篇 |
2008年 | 136篇 |
2007年 | 201篇 |
2006年 | 163篇 |
2005年 | 177篇 |
2004年 | 159篇 |
2003年 | 150篇 |
2002年 | 164篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 11篇 |
1998年 | 2篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1982年 | 5篇 |
1980年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1971年 | 2篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1968年 | 4篇 |
1967年 | 2篇 |
1966年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有1959条查询结果,搜索用时 390 毫秒
171.
172.
Bird AW Yu DY Pray-Grant MG Qiu Q Harmon KE Megee PC Grant PA Smith MM Christman MF 《Nature》2002,419(6905):411-415
173.
174.
175.
Hon WC Wilson MI Harlos K Claridge TD Schofield CJ Pugh CW Maxwell PH Ratcliffe PJ Stuart DI Jones EY 《Nature》2002,417(6892):975-978
176.
Widespread use of antimalarial agents can profoundly influence the evolution of the human malaria parasite Plasmodium falciparum. Recent selective sweeps for drug-resistant genotypes may have restricted the genetic diversity of this parasite, resembling effects attributed in current debates to a historic population bottleneck. Chloroquine-resistant (CQR) parasites were initially reported about 45 years ago from two foci in southeast Asia and South America, but the number of CQR founder mutations and the impact of chlorquine on parasite genomes worldwide have been difficult to evaluate. Using 342 highly polymorphic microsatellite markers from a genetic map, here we show that the level of genetic diversity varies substantially among different regions of the parasite genome, revealing extensive linkage disequilibrium surrounding the key CQR gene pfcrt and at least four CQR founder events. This disequilibrium and its decay rate in the pfcrt-flanking region are consistent with strong directional selective sweeps occurring over only approximately 20-80 sexual generations, especially a single resistant pfcrt haplotype spreading to very high frequencies throughout most of Asia and Africa. The presence of linkage disequilibrium provides a basis for mapping genes under drug selection in P. falciparum. 相似文献
177.
178.
Stöck M Lamatsch DK Steinlein C Epplen JT Grosse WR Hock R Klapperstück T Lampert KP Scheer U Schmid M Schartl M 《Nature genetics》2002,30(3):325-328
Green toads are common in the Palaearctic region, where they have differentiated into several taxa. The toads exist with variable amounts of ploidy, similar to other anuran species or reptiles. In vertebrate biology, the very rare occurrence of triploidy is coupled with infertility or unisexuality, or requires the coexistence of individuals of different ploidy in a reproductive community. The reproduction of naturally occurring triploids has been reported to occur only through parthenogenesis, gynogenesis or hybridogenesis. The bisexual reproduction of pure triploids has been considered to be impossible because of the problem of equally distributing three chromosome sets in meiosis. Here we report geographically isolated populations of green toads (Bufo viridis complex) that are all-triploid and reproduce bisexually. 相似文献
179.
Decadal temperature fluctuations in the Pacific Ocean have a significant effect on marine ecosystems and the climate of North America. The physical mechanisms responsible for these fluctuations are poorly understood. Some theories ascribe a central role to the wind-driven meridional overturning circulation between the tropical and subtropical oceans. Here we show, from observations over the past 50 years, that this overturning circulation has been slowing down since the 1970s, causing a decrease in upwelling of about 25% in an equatorial strip between 9 degrees N and 9 degrees S. This reduction in equatorial upwelling of relatively cool water, from 47 x 10(6) to 35 x 10(6) m3 s(-1), is associated with a rise in equatorial sea surface temperatures of about 0.8 degrees C. Another effect of the slowing circulation is a reduction in the outgassing of CO2 from the equatorial Pacific Ocean-at present the largest oceanic source of carbon dioxide to the atmosphere. 相似文献
180.