全文获取类型
收费全文 | 29410篇 |
免费 | 15802篇 |
国内免费 | 189篇 |
专业分类
45401篇 |
出版年
2023年 | 41篇 |
2022年 | 168篇 |
2021年 | 530篇 |
2020年 | 2260篇 |
2019年 | 3812篇 |
2018年 | 3910篇 |
2017年 | 4163篇 |
2016年 | 4213篇 |
2015年 | 4203篇 |
2014年 | 3910篇 |
2013年 | 4354篇 |
2012年 | 2062篇 |
2011年 | 1746篇 |
2010年 | 3180篇 |
2009年 | 1960篇 |
2008年 | 866篇 |
2007年 | 447篇 |
2006年 | 409篇 |
2005年 | 438篇 |
2004年 | 427篇 |
2003年 | 392篇 |
2002年 | 381篇 |
2001年 | 372篇 |
2000年 | 269篇 |
1999年 | 207篇 |
1998年 | 60篇 |
1997年 | 40篇 |
1996年 | 35篇 |
1995年 | 31篇 |
1994年 | 50篇 |
1993年 | 25篇 |
1992年 | 65篇 |
1991年 | 64篇 |
1990年 | 36篇 |
1989年 | 36篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1978年 | 13篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 10篇 |
1974年 | 9篇 |
1969年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
941.
C. C. Taff C. R. Freeman‐Gallant P. O. Dunn L. A. Whittingham 《Journal of evolutionary biology》2013,26(7):1392-1405
In socially monogamous species, extra‐pair paternity may increase the strength of intersexual selection by allowing males with preferred phenotypes to monopolize matings. Several studies have found relationships between male signals and extra‐pair mating, but many others fail to explain variation in extra‐pair mating success. A greater appreciation for the role that ecological contingencies play in structuring behavioural processes may help to reconcile contradictory results. We studied extra‐pair mating in a spatial context in the common yellowthroat (Geothlypis trichas), a territorial wood warbler. Over the course of 6 years, we observed 158 breeding attempts by 99 males, resulting in a total of 369 nests and 520 sampled nestlings. The spatial distribution of territories varied greatly, with males having between 0 and 10 close neighbours and between three and 39 neighbouring nestlings close enough to represent extra‐pair siring opportunities. Both within‐pair and extra‐pair reproductive success increased with breeding density, but the opportunity for sexual selection and strength of selection varied with density. Total variance in reproductive success was highest at low density and was mostly explained by variation in within‐pair success. In contrast, at high density, both within‐pair and extra‐pair successes contributed substantially to variance in reproductive success. The relationships between plumage and extra‐pair mating also varied by density; plumage was under strong sexual selection via extra‐pair mating success at high density, but no selection was detected at low density. Thus, ecological factors that structure social interactions can drive patterns of sexual selection by facilitating or constraining the expression of mating preferences. 相似文献
942.
X.‐X. Li Y. Zou C.‐L. Xiao R. W. Gituru Y.‐H. Guo C.‐F. Yang 《Journal of evolutionary biology》2013,26(9):1949-1958
Self‐interference is one of the most important selective forces in shaping floral evolution. Herkogamy and dichogamy both can achieve reductions in the extent of self‐interference, but they may have different roles in minimizing self‐interference in a single species. We used four self‐incompatible Epimedium species to explore the roles of herkogamy and dichogamy in avoiding self‐interference and to test the hypothesis that herkogamy and dichogamy may be separated and become selected preferentially in the taxa. Two species (E. franchetii and E. mikinorii) expressed strong herkogamy and weak protogyny (adichogamy), whereas another two species (E. sutchuenense and E. leptorrhizum) expressed slight herkogamy and partial protandry. Field investigations indicated that there was no physical self‐interference between male function and female function regarding pollen removal and pollen deposition in all species. Self‐pollination (autonomous or facilitated) was greater in species with slight herkogamy than in those with strong herkogamy. Artificial pollination treatments revealed that self‐pollination could reduce outcrossed female fertility in all species, and we found evidence that self‐interference reduced seed set in E. sutchuenense and E. leptorrhizum in the field, but not in E. franchetii and E. mikinorii. These results indicate that well‐developed herkogamy is more effective compared with dichogamy in avoiding self‐interference in the four species. In genus Epimedium, herkogamy instead of dichogamy should be selected preferentially and evolved as an effective mechanism for avoiding self‐interference and might not need to evolve linked with dichogamy. 相似文献
943.
Kamonrat Phopin Wutigri Nimlamool Linda J. Lowe‐Krentz Elijah W. Douglass Jaclyn N. Taroni Barry S. Bean 《Molecular reproduction and development》2013,80(4):273-285
Sperm‐associated α‐L ‐fucosidases have been implicated in fertilization in many species. Previously, we documented the existence of α‐L ‐fucosidase in mouse cauda epididymal contents, and showed that sperm‐associated α‐L ‐fucosidase is cryptically stored within the acrosome and reappears within the sperm equatorial segment after the acrosome reaction. The enrichment of sperm membrane‐associated α‐L ‐fucosidase within the equatorial segment of acrosome‐reacted cells implicates its roles during fertilization. Here, we document the absence of α‐L ‐fucosidase in mouse oocytes and early embryos, and define roles of sperm associated α‐L ‐fucosidase in fertilization using specific inhibitors and competitors. Mouse sperm were pretreated with deoxyfuconojirimycin (DFJ, an inhibitor of α‐L ‐fucosidase) or with anti‐fucosidase antibody; alternatively, mouse oocytes were pretreated with purified human liver α‐L ‐fucosidase. Five‐millimolar DFJ did not inhibit sperm–zona pellucida (ZP) binding, membrane binding, or fusion and penetration, but anti‐fucosidase antibody and purified human liver α‐L ‐fucosidase significantly decreased the frequency of these events. To evaluate sperm‐associated α‐L ‐fucosidase enzyme activity in post‐fusion events, DFJ‐pretreated sperm were microinjected into oocytes, and 2‐pronuclear (2‐PN) embryos were treated with 5 mM DFJ with no significant effects, suggesting that α‐L ‐fucosidase enzyme activity does not play a role in post‐fusion events and/or early embryo development in mice. The recognition and binding of mouse sperm to the ZP and oolemma involves the glycoprotein structure of α‐L ‐fucosidase, but not its catalytic action. These observations suggest that deficits in fucosidase protein and/or the presence of anti‐fucosidase antibody may be responsible for some types of infertility. Mol. Reprod. Dev. 80: 273–285, 2013. © 2013 Wiley Periodicals, Inc. 相似文献
944.
945.
In coastal cliffs consisting of Pliocene fine‐grained clastic sediments at El Rompido, Huelva, Spain we discovered around the high water line an extensive zone of about 1.5 m in height burrows that were made by up to 4 mm long isopods Paragnathia formica (Hesse) and small 5 mm long staphylinid beetles (Bledius cf. corniger Rosenhauer). The burrowing zone was crowded with irregular burrows 2–5 mm in diameter and extended some 5 cm deep in the cliff. The base of the cliff showed a clear (bio)erosional notch due to this burrowing activity, comparable to those produced by marine boring and scraping organisms in, for example, calcareous rocks. This indicates that the activity of Paragnathia and Bledius helps in eroding the cliffs, a phenomenon not published earlier as far as we know. A comparable but less extensive burrowed zone was found farther upstream near Cartaya. Their presence around the high tide zone also might prove these burrows to be good markers of former high tide lines in older cliffs. 相似文献
946.
947.
948.
949.
950.