全文获取类型
收费全文 | 5004篇 |
免费 | 362篇 |
国内免费 | 366篇 |
出版年
2023年 | 56篇 |
2022年 | 78篇 |
2021年 | 112篇 |
2020年 | 104篇 |
2019年 | 138篇 |
2018年 | 133篇 |
2017年 | 167篇 |
2016年 | 156篇 |
2015年 | 157篇 |
2014年 | 232篇 |
2013年 | 215篇 |
2012年 | 130篇 |
2011年 | 217篇 |
2010年 | 176篇 |
2009年 | 287篇 |
2008年 | 354篇 |
2007年 | 316篇 |
2006年 | 281篇 |
2005年 | 238篇 |
2004年 | 193篇 |
2003年 | 153篇 |
2002年 | 111篇 |
2001年 | 121篇 |
2000年 | 103篇 |
1999年 | 121篇 |
1998年 | 99篇 |
1997年 | 94篇 |
1996年 | 96篇 |
1995年 | 90篇 |
1994年 | 79篇 |
1993年 | 82篇 |
1992年 | 79篇 |
1991年 | 62篇 |
1990年 | 60篇 |
1989年 | 62篇 |
1988年 | 65篇 |
1987年 | 76篇 |
1986年 | 46篇 |
1985年 | 48篇 |
1984年 | 31篇 |
1983年 | 15篇 |
1982年 | 31篇 |
1981年 | 31篇 |
1980年 | 30篇 |
1979年 | 26篇 |
1978年 | 18篇 |
1977年 | 19篇 |
1976年 | 22篇 |
1975年 | 14篇 |
1973年 | 14篇 |
排序方式: 共有5732条查询结果,搜索用时 609 毫秒
1.
The brown algal genus, Padina, has a worldwide distribution in tropical and subtropical climate zones. Padina individuals are common and sometimes dominant in both the intertidal and shallow subtidal regions associated with coral reefs. We investigated the population structure and recruitment of two populations of Padina boryana at an exposed shore in Sirinart National Park (SNP) and a sheltered shore of Tang Khen Bay (TKB), Phuket Province, Thailand. From September 2005 to August 2006, the number of released spores and the height and radius of fronds were measured monthly. New recruits were recorded and monitored on manipulated permanent plots 0.25 m2. The experiment was carried out monthly over a year. We found Padina recruits 1 month after the plots were cleared at both sites. There was a significant difference in percentage cover by new individuals between the two locations (P < 0.05) and also at the shore levels within the sites. At SNP, the highest recruitment was found in the uppermost zone, while at TKB high recruitment occurred at all shore levels except at 80-100 m from the shore. The factors influencing recruitment of P. boryana include wave motion, competition with other earlier successional species and sediment. The high availability of reproductive spores throughout the year, the successful recruitment and the Dictyerpa stage promote the successful establishment of Padina populations. 相似文献
2.
3.
Mercedes González-Wangüemert Francisca Giménez-Casalduero Ángel Pérez-Ruzafa 《Biochemical Systematics and Ecology》2006
Genetic variation at 10 enzyme loci was analysed in Elysia timida sacoglossan mollusc samples, originating from both coastal lagoon and marine sites. The observed heterozygosity ranged from 0.390 (Los Urrutias) to 0.277 (Tabarca). Marine and coastal lagoon populations were characterised by exclusive alleles. 相似文献
4.
Eleutheronema tetradactylum is an economically important fish species in China water. To investigate the genetic diversity and describe population structure of it, an 1151 base pair (bp) fragment of the mitochondrial DNA Cytb sequence was analyzed in 120 individuals from four populations in the East China Sea and the South China Sea. A total of 16 haplotypes were defined by 24 variable nucleotide sites. High level of haplotype diversity and low nucleotide diversity were observed in all populations. The results of AMOVA detected that 89.44% of the genetic variation occurred within populations. Significant genetic differentiations were detected among populations (0.05097, P < 0.05), but no large-scale regional differences were detected. Analysis of neutral evolution and mismatch distribution suggested no recent population expansion happened. The present results provided new information for genetic assessment, fishery management and conservation of this species. 相似文献
5.
Aim To describe a protocol for incorporating a temporal dimension into historical biogeographical analysis, while maintaining the essential independence of all datasets, involving the generation of general area cladograms. Location Global. Methods General area cladograms (GACs) are a reconstruction of the evolutionary history of a set of areas and unrelated clades within those areas. Nodes on a GAC correspond to speciation events in a group of taxa; general nodes are those at which multiple unrelated clades speciate. We undertake temporal calibration of GACs using molecular clock estimates of splitting events between extant taxa as well as first appearance data from the fossil record. We present two examples based on re‐analysis of previously published data: first, a temporally calibrated GAC generated from secondary Brooks parsimony analysis (BPA) of six extant bird clades from the south‐west of North America using molecular clock estimates of divergence times; and second, an analysis of African Neogene mammals based on a phylogenetic analysis for comparing trees (PACT) analysis. Results A hypothetical example demonstrates how temporal calibration reveals potentially critical information about the timing of both unique and general events, while also illustrating instances of incongruence between dates generated from molecular clock estimates and fossils. For the African Neogene mammal dataset, our analysis reveals that most mammal clades underwent geodispersal associated with the Neogene climatic optimum (c. 16 Ma) and vicariant speciation in central Africa correlated with increased aridity and cooler temperatures around 2.5 Ma. Main conclusions Temporally calibrated GACs are valuable tools for assessing whether coordinated patterns of speciation are associated with large‐scale climatic or tectonic phenomena. 相似文献
6.
For the first time the expression of C3 and CAM in the leaves of different age of Marrubium frivaldszkyanum Boiss, is reported. With increasing leaf age a typical C3 photosynthesis pattern and high transpiration rate were found. In older leaves a shift to CAM occurred and the 24-h transpiration water loss decreased. A correlation was established between leaf area and accumulation of malate. Water loss at early stages of leaf expansion may be connected with the shift to CAM and the water economy of the whole plant. 相似文献
7.
8.
R. C. Griffiths S. W. McKechnie J. A. McKenzie 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1982,62(1):89-96
Summary Mother-offspring data for alcohol dehydrogenase genotypes of a vineyard cellar population of D. melanogaster are best explained by a model that allows 21% of females in the population to mate twice with an 83% level of sperm displacement. A population model with multiple mating and sperm displacement is examined theoretically. A formula for the effective population size is derived under this model. Multiple mating increases the effective population size relative to single mating. 相似文献
9.
Syunro Utida 《Population Ecology》1952,1(1):166-172
Contributions from the Entomological Laboratory, Kyoto University, No. 110. 相似文献
10.
Human migration is nonrandom. In small scale societies of the past, and in the modern world, people tend to move to wealthier, safer, and more just societies from poorer, more violent, less just societies. If immigrants are assimilated, such nonrandom migration can increase the occurrence of culturally transmitted beliefs, values, and institutions that cause societies to be attractive to immigrants. Here we describe and analyze a simple model of this process. This model suggests that long run outcomes depend on the relative strength of migration and local adaptation. When local adaption is strong enough to preserve cultural variation among groups, cultural variants that make societies attractive always predominate, but never drive alternative variants to extinction. When migration predominates, outcomes depend both on the relative attractiveness of alternative variants and on the initial sizes of societies that provide and receive immigrants. 相似文献