全文获取类型
收费全文 | 1485篇 |
免费 | 140篇 |
出版年
2022年 | 17篇 |
2021年 | 33篇 |
2020年 | 17篇 |
2019年 | 22篇 |
2018年 | 32篇 |
2017年 | 18篇 |
2016年 | 34篇 |
2015年 | 53篇 |
2014年 | 67篇 |
2013年 | 79篇 |
2012年 | 74篇 |
2011年 | 80篇 |
2010年 | 43篇 |
2009年 | 45篇 |
2008年 | 63篇 |
2007年 | 61篇 |
2006年 | 45篇 |
2005年 | 59篇 |
2004年 | 34篇 |
2003年 | 45篇 |
2002年 | 44篇 |
2001年 | 23篇 |
2000年 | 41篇 |
1999年 | 28篇 |
1998年 | 17篇 |
1997年 | 14篇 |
1996年 | 12篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 9篇 |
1992年 | 15篇 |
1991年 | 31篇 |
1990年 | 24篇 |
1989年 | 20篇 |
1988年 | 17篇 |
1987年 | 12篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 29篇 |
1983年 | 25篇 |
1982年 | 14篇 |
1981年 | 20篇 |
1979年 | 20篇 |
1978年 | 20篇 |
1977年 | 22篇 |
1976年 | 24篇 |
1975年 | 13篇 |
1974年 | 11篇 |
1973年 | 19篇 |
1972年 | 15篇 |
排序方式: 共有1625条查询结果,搜索用时 31 毫秒
61.
62.
63.
64.
The Red-necked Falcon in southern Zambia is a year-long resident in its breeding territory. The study was conducted on a floodplain and the adjacent acacia savannah in an undisturbed natural environment. The falcon principally nests in natural depressions on frond bases on the leeward side of Borassus Palms but also utilizes old crow and raptor nests. Incubation averaged 33 days and was undertaken by the female. The young fledged after a 36-day nestling period and remained under parental care for up to three weeks after that. Nesting success averaged 1.3 young per nest or 44% of the eggs laid. The post-juvenal moult commenced when the young were five to six months old and continued for six months. Birds comprised 98% of the diet of the falcons. The adults selected larger prey as the nestlings increased in size. The 69-day incubation and nesting period is up to 14 days longer than in similar-sized falcons. 相似文献
65.
66.
67.
Matheus E. Bianconi Luke T. Dunning Emma V. Curran Oriane Hidalgo Robyn F. Powell Sahr Mian Ilia J. Leitch Marjorie R. Lundgren Sophie Manzi Maria S. Vorontsova Guillaume Besnard Colin P. Osborne Jill K. Olofsson Pascal-Antoine Christin 《Proceedings. Biological sciences / The Royal Society》2020,287(1938)
C4 photosynthesis evolved multiple times independently in angiosperms, but most origins are relatively old so that the early events linked to photosynthetic diversification are blurred. The grass Alloteropsis semialata is an exception, as this species encompasses C4 and non-C4 populations. Using phylogenomics and population genomics, we infer the history of dispersal and secondary gene flow before, during and after photosynthetic divergence in A. semialata. We further analyse the genome composition of individuals with varied ploidy levels to establish the origins of polyploids in this species. Detailed organelle phylogenies indicate limited seed dispersal within the mountainous region of origin and the emergence of a C4 lineage after dispersal to warmer areas of lower elevation. Nuclear genome analyses highlight repeated secondary gene flow. In particular, the nuclear genome associated with the C4 phenotype was swept into a distantly related maternal lineage probably via unidirectional pollen flow. Multiple intraspecific allopolyploidy events mediated additional secondary genetic exchanges between photosynthetic types. Overall, our results show that limited dispersal and isolation allowed lineage divergence, with photosynthetic innovation happening after migration to new environments, and pollen-mediated gene flow led to the rapid spread of the derived C4 physiology away from its region of origin. 相似文献
68.
Edgar L. Y. Wong Bruno Nevado Owen G. Osborne Alexander S. T. Papadopulos Jon R. Bridle Simon J. Hiscock Dmitry A. Filatov 《Molecular ecology》2020,29(2):394-412
Recently diverged species present particularly informative systems for studying speciation and maintenance of genetic divergence in the face of gene flow. We investigated speciation in two closely related Senecio species, S. aethnensis and S. chrysanthemifolius, which grow at high and low elevations, respectively, on Mount Etna, Sicily and form a hybrid zone at intermediate elevations. We used a newly generated genome‐wide single nucleotide polymorphism (SNP) dataset from 192 individuals collected over 18 localities along an elevational gradient to reconstruct the likely history of speciation, identify highly differentiated SNPs, and estimate the strength of divergent selection. We found that speciation in this system involved heterogeneous and bidirectional gene flow along the genome, and species experienced marked population size changes in the past. Furthermore, we identified highly‐differentiated SNPs between the species, some of which are located in genes potentially involved in ecological differences between species (such as photosynthesis and UV response). We analysed the shape of these SNPs’ allele frequency clines along the elevational gradient. These clines show significantly variable coincidence and concordance, indicative of the presence of multifarious selective forces. Selection against hybrids is estimated to be very strong (0.16–0.78) and one of the highest reported in literature. The combination of strong cumulative selection across the genome and previously identified intrinsic incompatibilities probably work together to maintain the genetic and phenotypic differentiation between these species – pointing to the importance of considering both intrinsic and extrinsic factors when studying divergence and speciation. 相似文献
69.
Troy J. Gibson Charles W. Mason Jade Y. Spence Heather Barker Neville G. Gregory 《Journal of applied animal welfare science : JAAWS》2013,16(3):222-238
Captive bolt stunning is used for rendering livestock insensible at slaughter. The mechanical factors relating to performance of 6 penetrating captive bolt gun (CBG) models were examined. The Matador Super Sécurit 3000 and the .25 Cash Euro Stunner had the highest kinetic energy values (443 J and 412 J, respectively) of the CBGs tested. Ninety percent (27/30) of CBGs held at a government gun repository (United Kingdom) were found to have performed at a normal standard for the model, while 53% (10/19) of commercial contractor CBGs tested were found to underperform for the gun model. When the .22 Cash Special was fired 500 times at 4 shots per min, the gun reached a peak temperature of 88.8°C after 2.05 hr. Repeat firing during extended periods significantly reduced the performance of the CBG. When deciding on the appropriate CBG/cartridge combination, the kinetic energy delivered to the head of the nonhuman animal, bolt penetration depth, and species/animal type must be considered. It is recommended that CBGs are routinely checked for wear to the bolt and barrel if they are repeatedly fired in a session. 相似文献
70.
Humaira Rasheed Amanda Phipps-Green Ruth Topless Jade E Hollis-Moffatt Jennie Harré Hindmarsh Christopher Franklin Nicola Dalbeth Peter B Jones Douglas HN White Lisa K Stamp Tony R Merriman 《Arthritis research & therapy》2013,15(6):R177