全文获取类型
收费全文 | 1749篇 |
免费 | 53篇 |
国内免费 | 5篇 |
出版年
2013年 | 29篇 |
2012年 | 33篇 |
2011年 | 40篇 |
2010年 | 56篇 |
2009年 | 95篇 |
2008年 | 41篇 |
2007年 | 75篇 |
2006年 | 45篇 |
2005年 | 50篇 |
2004年 | 26篇 |
2003年 | 18篇 |
1999年 | 11篇 |
1998年 | 27篇 |
1997年 | 46篇 |
1996年 | 40篇 |
1995年 | 35篇 |
1994年 | 27篇 |
1993年 | 26篇 |
1992年 | 34篇 |
1991年 | 19篇 |
1990年 | 27篇 |
1989年 | 24篇 |
1988年 | 22篇 |
1987年 | 15篇 |
1985年 | 13篇 |
1984年 | 15篇 |
1983年 | 17篇 |
1982年 | 14篇 |
1980年 | 14篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1976年 | 17篇 |
1975年 | 25篇 |
1974年 | 13篇 |
1973年 | 19篇 |
1972年 | 16篇 |
1971年 | 14篇 |
1959年 | 35篇 |
1958年 | 68篇 |
1957年 | 81篇 |
1956年 | 69篇 |
1955年 | 55篇 |
1954年 | 64篇 |
1953年 | 62篇 |
1952年 | 60篇 |
1951年 | 43篇 |
1950年 | 18篇 |
1949年 | 33篇 |
1948年 | 19篇 |
排序方式: 共有1807条查询结果,搜索用时 393 毫秒
191.
Williams BR VAN Heerwaarden B Dowling DK Sgrò CM 《Journal of evolutionary biology》2012,25(7):1415-1426
Exposure to extreme temperatures is increasingly likely to impose strong selection on many organisms in their natural environments. The ability of organisms to adapt to such selective pressures will be determined by patterns of genetic variation and covariation. Despite increasing interest in thermal adaptation, few studies have examined the extent to which the genetic covariance between traits might constrain thermal responses. Furthermore, it remains unknown whether sex‐specific genetic architectures will constrain responses to climatic selection. We used a paternal half‐sibling breeding design to examine whether sex‐specific genetic architectures and genetic covariances between traits might constrain evolutionary responses to warming climates in a population of Drosophila melanogaster. Our results suggest that the sexes share a common genetic underpinning for heat tolerance as indicated by a strong positive inter‐sexual genetic correlation. Further, we found no evidence in either of the sexes that genetic trade‐offs between heat tolerance and fitness will constrain responses to thermal selection. Our results suggest that neither trade‐offs, nor sex‐specific genetics, will significantly constrain an evolutionary response to climatic warming, at least in this population of D. melanogaster. 相似文献
192.
S Folloni DM Kagkli B Rajcevic NC Guimarães B VAN Droogenbroeck FH Valicente G VAN den Eede M VAN den Bulcke 《Molecular ecology resources》2012,12(5):810-821
The cultivation of genetically modified (GM) crops has raised numerous concerns in the European Union and other parts of the world about their environmental and economic impact. Especially outcrossing of genetically modified organisms (GMO) was from the beginning a critical issue as airborne pollen has been considered an important way of GMO dispersal. Here, we investigate the use of airborne pollen sampling combined with microscopic analysis and molecular PCR analysis as an approach to monitor GM maize cultivations in a specific area. Field trial experiments in the European Union and South America demonstrated the applicability of the approach under different climate conditions, in rural and semi-urban environment, even at very low levels of airborne pollen. The study documents in detail the sampling of GM pollen, sample DNA extraction and real-time PCR analysis. Our results suggest that this 'GM pollen monitoring by bioaerosol sampling and PCR screening' approach might represent an useful aid in the surveillance of GM-free areas, centres of origin and natural reserves. 相似文献
193.
ABBEY K. THOMPSON MICHAEL D. SAMUEL TIMOTHY R. VAN DEELEN 《The Journal of wildlife management》2008,72(2):416-421
ABSTRACT We conducted experimental feeding using 3 feeding methods (pile, spread, trough) and 2 quantities (rationed, ad libitum) of shelled corn to compare deer activity and behavior with control sites and evaluate potential direct and indirect transmission of infectious disease in white-tailed deer (Odocoileus virginianus) in central Wisconsin, USA. Deer use was higher at 2 of the feeding sites than at natural feeding areas (P ≤ 0.02). Deer spent a higher proportion of time (P < 0.01) feeding at pile (49%) and spread (61%) treatments than at natural feeding areas (36%). We found higher deer use for rationed than ad libitum feeding quantities and feeding intensity was greatest at rationed piles and lowest at ad libitum spreads. We also observed closer pairwise distances (≤0.3 m) among deer when corn was provided in a trough relative to spread (P=0.03). Supplemental feeding poses risks for both direct and indirect disease transmission due to higher deer concentration and more intensive use relative to control areas. Concentrated feeding and contact among deer at feeding sites can also increase risk for disease transmission. Our results indicated that restrictions on feeding quantity would not mitigate the potential for disease transmission. None of the feeding strategies we evaluated substantially reduced the potential risk for disease transmission and banning supplemental feeding to reduce transmission is warranted. 相似文献
194.
GABRIELA GARCÍA-MARMOLEJO GRISELDA ESCALONA-SEGURA HANS VAN DER WAL 《The Journal of wildlife management》2008,72(5):1194-1202
Abstract: Since 1997 the Mexican government has promoted Management Units for Conservation and Sustainable Use of Wildlife (UMAs) on private and community lands as an economically attractive mean for the conservation of biodiversity. To date, compliance of UMAs with stated sustainability goals has not been evaluated. Thus, we designed multicriteria evaluation framework based on the stated objectives of the national UMA program and applied it to a sample of 6 UMAs operating in peasant communities near or in the Calakmul Biosphere Reserve in Campeche, Mexico. Evaluation criteria covered thematic areas of environment, economy, social development, and laws and rules. We formulated 15 criteria with 29 indicators, and 133 verifiers. Data for the verifiers were based on direct observation, interviews with key actors in the peasant communities, participation in regional meetings, scrutiny of government reports, and databases. We calculated sustainability indexes by weighting, summing, and standardizing verifiers to percentage scales, and aggregating to the successive higher hierarchical levels of the evaluation framework. We found an average overall sustainability index of 45.7% for the UMAs evaluated. Scores were highest for the environment thematic area (55.2%), followed by economy (43.4%), social development (42.3%), and laws and rules (41.7%). We observed particularly low indicators for management strategies, habitat monitoring, environmental education, and knowledge of wildlife laws. We conclude that the contribution of UMAs to the conservation and sustainable use of biodiversity could be improved by a number of actions. These include developing wildlife management educational programs with a strong environmental legislation component, the correct enforcement of wildlife laws, and creating country-wise links among academic and governmental institutions promoting the work with regional and national wildlife experts. 相似文献
195.
NANCY D. MONCRIEF RONALD A. VAN DEN BUSSCHE RAYMOND D. DUESER DUSTIN LOFTIS NOELLE E. COCKETT MELANIE CULVER 《The Journal of wildlife management》2008,72(1):320-323
Abstract The Delmarva fox squirrel (Sciurus niger cinereus) has been listed as endangered by the United States Department of Interior since 1967. A high-priority task for the recovery of this taxon is to determine its current geographic distribution. Toward this end, we have identified a microsatellite locus that unambiguously differentiates Delmarva fox squirrels from eastern gray squirrels (S. carolinensis), which frequently co-occur with Delmarva fox squirrels. Analysis of this marker in noninvasively collected hair samples will allow unequivocal identification of localities occupied by Delmarva fox squirrels with a minimum investment of funds, time, and effort because handling individuals will be unnecessary. This protocol will expedite site review in connection with the Endangered Species Act consultation process. 相似文献
196.
JONATHAN BRECKO KATLEEN HUYGHE BIEKE VANHOOYDONCK ANTHONY HERREL IRENA GRBAC RAOUL VAN DAMME 《Biological journal of the Linnean Society. Linnean Society of London》2008,94(2):251-264
Within populations, individual animals may vary considerably in morphology and ecology. The degree to which variation in morphology is related to ecological variation within a population remains largely unexplored. We investigated whether variation in body size and shape among sexes and age classes of the lizard Podarcis melisellensis translates in differential whole-animal performance (sprint speed, bite force), escape and prey attack behaviour in the field, microhabitat use and diet. Male and female adult lizards differed significantly in body size and head and limb proportions. These morphological differences were reflected in differences in bite strength, but not in sprint speed. Accordingly, field measurements of escape behaviour and prey attack speed did not differ between the sexes, but males ate larger, harder and faster prey than females. In addition to differences in body size, juveniles diverged from adults in relative limb and head dimensions. These shape differences may explain the relatively high sprint and bite capacities of juvenile lizards. Ontogenetic variation in morphology and performance is strongly reflected in the behaviour and ecology in the field, with juveniles differing from adults in aspects of their microhabitat use, escape behaviour and diet. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 251–264. 相似文献
197.
Parasite-mediated sexual selection and species divergence in Lake Victoria cichlid fish 总被引:1,自引:0,他引:1
MARTINE E. MAAN ANNE M. C. VAN ROOIJEN JACQUES J. M. VAN ALPHEN OLE SEEHAUSEN 《Biological journal of the Linnean Society. Linnean Society of London》2008,94(1):53-60
We investigate the role of parasite-mediated sexual selection in the divergence of two species of Lake Victoria cichlids. Pundamilia pundamilia and Pundamilia nyererei represent a common pattern of male nuptial colour divergence between haplochromine sister species: metallic grey–blue in P. pundamilia and bright yellow and red in P. nyererei . Female mating preferences for different male colours maintain the genetic and phenotypic differentiation of the two species in clear water. Previous work indicated that the red coloration of P. nyererei males, which is subject to directional sexual selection, may be a carotenoid-dependent signal of parasite infestation rate. In the present study, we find a parallel result for P. pundamilia : bright blue males are infected with fewer species of parasites. We also find that parasite infestation rates differ quantitatively between the two species in a way that is consistent with species differences in diet and microhabitat. We conclude that parasite-mediated sexual selection may have contributed to the divergence of female mating preferences between P. pundamilia and P. nyererei , and may currently strengthen reproductive isolation between these species. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94 , 53–60. 相似文献
198.
DE Gelas K Janko K Volckaert FA DE Charleroy D VAN Houdt JK 《Molecular ecology resources》2008,8(5):1001-1003
Nine polymorphic microsatellite loci were developed for the spined loach, Cobitis taenia (Teleostei: Cobitidae). The loci were validated using 50 individuals from a population in Belgium. Moderate to high levels of polymorphism were detected (two to 11 alleles). In addition, most markers amplified successfully in three closely related taxa that are known to hybridize with C. taenia: C. elongatoides, C. taurica and C. tanaitica. Some of the loci are most likely diagnostic among species. These markers will be valuable for the study of the historical and contemporary interactions within C. taenia and the Cobitis species complex. 相似文献
199.
Mészáros LA Frauenfelder N VAN DER Velde M Komdeur J Szabad J 《Molecular ecology resources》2008,8(3):692-694
To describe the exceptional mating system of the penduline tit, Remiz pendulinus, we aim to combine field observation records with DNA analysis based on polymorphic microsatellite DNA markers. Here we describe features of nine loci and their corresponding polymerase chain reaction primers. The observed number of alleles varied from four to seven and the observed heterozygosity ranged from 0.419 to 0.802. Neither of the loci is sex-linked and as linkage disequilibrium analysis showed they assort independently. Seven of the nine loci were polymorphic in the Cape penduline tit, Anthoscopus minutus. 相似文献
200.
Leaf digestibility and litter decomposability are related in a wide range of subarctic plant species and types 总被引:7,自引:2,他引:5