全文获取类型
收费全文 | 4993篇 |
免费 | 563篇 |
专业分类
5556篇 |
出版年
2022年 | 41篇 |
2021年 | 73篇 |
2020年 | 46篇 |
2019年 | 48篇 |
2018年 | 61篇 |
2017年 | 66篇 |
2016年 | 112篇 |
2015年 | 171篇 |
2014年 | 176篇 |
2013年 | 267篇 |
2012年 | 277篇 |
2011年 | 318篇 |
2010年 | 200篇 |
2009年 | 169篇 |
2008年 | 239篇 |
2007年 | 229篇 |
2006年 | 209篇 |
2005年 | 195篇 |
2004年 | 195篇 |
2003年 | 183篇 |
2002年 | 199篇 |
2001年 | 115篇 |
2000年 | 103篇 |
1999年 | 114篇 |
1998年 | 59篇 |
1997年 | 49篇 |
1996年 | 67篇 |
1995年 | 54篇 |
1994年 | 57篇 |
1993年 | 61篇 |
1992年 | 65篇 |
1991年 | 53篇 |
1990年 | 70篇 |
1989年 | 87篇 |
1988年 | 56篇 |
1987年 | 41篇 |
1986年 | 46篇 |
1985年 | 49篇 |
1984年 | 50篇 |
1983年 | 40篇 |
1982年 | 50篇 |
1981年 | 40篇 |
1980年 | 37篇 |
1979年 | 49篇 |
1978年 | 33篇 |
1977年 | 35篇 |
1976年 | 49篇 |
1975年 | 33篇 |
1974年 | 52篇 |
1973年 | 42篇 |
排序方式: 共有5556条查询结果,搜索用时 0 毫秒
61.
Sarah R. Grant Sabine Hardenack Stefan Trentmann Heinz Saedler 《Molecular genetics and genomics : MGG》1993,241(1-2):153-160
TNPA, one of the two transposition proteins encoded by the En/Spm transposable elements of Zea mays, suppresses the expression of genes that contain an appropriate cis element. Suppression can be monitored in tobacco protoplasts in a transient expression assay as follows. The plant promoter-driven expression of the Escherichia coli-glucuronidase (GUS)-encoding gene, uidA, is repressed in the presence of TNPA if the GUS gene contains a functional cis element in the untranslated RNA leader sequence. Earlier, we found that the minimal cis element is composed of two 12 by sequences in a tail-to-tail inverted orientation. Each 12 by sequence is sufficient to bind TNPA in vitro and can be thought of as a half-site in the cis element. Here, we investigated the sequence requirements of the minimal cis element. Our observations support our expectations that a functional cis element must provide a template to which two TNPA molecules can bind in the correct orientation. Sequences within the half-sites can be altered as long as the eight bases that make up the consensus binding sites are not changed. However, we found the following unexpected sequence specificities. Firstly, some changes to the consensus binding sequence can be tolerated in one half-site, as long as the other site matches the consensus. Secondly, although the region between the half-sites can vary in sequence and in length between two and four bases, a thymidine residue is not tolerated directly 5′ preceding the second half-site. Since many variants of the cis element sequence remain functional, the suppressor response element provides a flexible tool for artificially manipulating the expression of genes. 相似文献
62.
63.
H. Lisle Gibbs Peter R. Grant 《Evolution; international journal of organic evolution》1989,43(6):1273-1284
We studied the frequency and causes of inbreeding and its effect on reproductive success in a population of Darwin's Medium Ground Finches (Geospiza fortis) on Isla Daphne Major, Galápagos, during four breeding seasons (1981, 1983, 1984, and 1987). Pedigree analysis showed that levels of inbreeding were low but comparable with those observed in other passerine birds. For pairs with at least half of their grandparents known, approximately 20% of all pairings were between detectably related birds. The frequency of pairings between closely related birds (coefficient of kinship [φ] ≥ 0.250) among all pairs was 0.6%. We detected no effect of inbreeding on reproductive success, although sample sizes were small. The observed reproductive output of related pairs was not significantly different from the output of unrelated pairs, and there was no correlation between a pair's kinship coefficient and an estimate of the potential magnitude of inbreeding depression. Comparisons with a study of Great Tits (Parus major) by van Noordwijk and Scharloo (1981) suggest that, even if present, the fitness costs of inbreeding in this population of G. fortis would be low. Observed levels of inbreeding in each breeding episode were accurately predicted by simulations of random mating in which relatedness had no influence on pairing between individuals. This result suggests that levels of inbreeding in this population are determined more by demographic factors than by behavioral avoidance of mating with kin. 相似文献
64.
Little is known about the patterns and dynamics of exotic species invasions at landscape to regional spatial scales. We quantified the presence (identity, abundance, and richness) and characteristics of native and exotic species in estuarine strandline plant communities at 24 sites in Narragansett Bay, Rhode Island, USA. Our results do not support several fundamental predictions of invasion biology. Established exotics (79 of 147 recorded plant species) were nearly indistinguishable from the native plant species (i.e. in terms of growth form, taxonomic grouping, and patterns of spatial distribution and abundance) and essentially represent a random sub-set of the current regional species pool. The cover and richness of exotic species varied substantially among quadrats and sites but were not strongly related to any site-level physical characteristics thought to affect invasibility (i.e. the physical disturbance regime, legal status, neighboring habitat type, and substrate characteristics). Native and exotic cover or richness were not negatively related within most sites. Across sites, native and exotic richness were positively correlated and exotic cover was unrelated to native richness. The colonization and spread of exotics does not appear to have been substantially reduced at sites with high native diversity. Furthermore, despite the fact that the Rhode Island strandline system is one of the most highly-invaded natural plant communities described to date, exotic species, both individually and as a group, currently appear to pose little threat to native plant diversity. Our findings are concordant with most recent, large-scale investigations that do not support the theoretical foundation of invasion biology and generally contradict small-scale experimental work. 相似文献
65.
66.
67.
68.
Peter R. Grant B. Rosemary Grant 《Evolution; international journal of organic evolution》1995,49(2):229-240
We report the natural colonization of the small Galápagos island Daphne Major by the large ground finch (Geospiza magnirostris). Immigrants of this species were present in every year of a 22-yr study, 1973–1994. Typically they arrived after a breeding season and left at the beginning of the next one. Geospiza magnirostris bred on the island for the first time in the exceptionally wet El Niño year of 1982–1983, and bred in all subsequent years except drought years. In agreement with theoretical expectations the frequency of inbreeding was unusually high. Pronounced fluctuating asymmetry in tarsus length, together with slightly reduced breeding success of inbreeding pairs, suggests a low level of inbreeding depression. Despite this, the population increased from 5 breeding individuals in 1983 to 20 breeding individuals in 1992, and probably more than twice that number in 1993, largely through recruitment of locally born birds. 相似文献
69.
70.