首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   784篇
  免费   37篇
  国内免费   15篇
  2015年   6篇
  2014年   8篇
  2013年   19篇
  2012年   16篇
  2011年   32篇
  2010年   44篇
  2009年   44篇
  2008年   27篇
  2007年   38篇
  2006年   33篇
  2005年   35篇
  2004年   28篇
  2003年   28篇
  2002年   22篇
  2001年   8篇
  1999年   6篇
  1998年   13篇
  1997年   17篇
  1996年   20篇
  1995年   13篇
  1994年   9篇
  1993年   7篇
  1992年   11篇
  1991年   11篇
  1990年   13篇
  1989年   11篇
  1988年   9篇
  1987年   17篇
  1986年   6篇
  1985年   7篇
  1979年   8篇
  1978年   7篇
  1977年   7篇
  1975年   5篇
  1974年   5篇
  1973年   10篇
  1972年   9篇
  1971年   11篇
  1959年   15篇
  1958年   31篇
  1957年   12篇
  1956年   17篇
  1955年   13篇
  1954年   9篇
  1953年   5篇
  1952年   8篇
  1951年   11篇
  1950年   14篇
  1949年   11篇
  1948年   9篇
排序方式: 共有836条查询结果,搜索用时 640 毫秒
101.
ABSTRACT Not only were used Random amplified polymorphismic DNA(RAPD) markers as input for an analysis of molecular variance (AMOVA) and cluster analysis to describe the population genetic structure, but also morphometric analysis of Formica japonica in Korea in order to investigate the relationships between local populations according to altitude. The ten populations of F. japonica in Korea did not showed any relationships according to altitude in morphological analysis, while they showed distinct relationship in RAPD analysis. It was noticed that they were clearly separated as four groups according to altitude. The first was Jirisan-Gachilbong- Gyebangsan population, the second Iksansi-Odongdo, the third Deukyusan-Hallasan, the fourth Siheungsi- Seonamsa-Jinjusi. The first group was high mountainous, the second flat and low altitude, the third high mountainous, and the fourth flat and low altitude. They occur from low altitude to comparative high altitude 1,400 m in Korea. Actually, it was found that the relationships of 10 local populations of Korean Formica japonica were not related to morphological evolution, but related to molecular evolutionary data evolved from RAPD analysis.  相似文献   
102.
103.
ABSTRACT The phytogeny of Formica ants in Korea was carried out using RAPD. Analyzing RAPD marker, the similarity degrees between species were estimated, and they were compared with the results from the measuring values of thirteen morphological characters. Morphological analysis showed as follows: all the five species were grouped with 72% or more similarity. Out-group, Camponotus was independently isolated. F. lemani and F. Candida, F. yessensis and F.japonica were grouped in a very tight cluster with 97% or more similarity, while F. sanguinea+F. yessensis and F.japonica were grouped with 88%. On the other hand, RAPD analysis showed as followes: F. sanguinea, F. yessensis were grouped at the level of 97% similarity, F. lemani and, F. Candida were also grouped with 97%. Both two groups were again grouped with 85% or more. This group and F.japonica were grouped with 72% similarity. In RAPD the lowest similarity was 72% in the five species of Formica. The positional homology of amplified fragments was identified by Southern hybridiztion of RAPD fragments. As a result, F. sanguinea and F. yessensis showed 97% degree of similarity, and also F. lemani and F. Candida showed 97%. F.japonica and above four species showed 72%. The level of similarity of F.japonica against the other four species was lower than those of four other species.  相似文献   
104.
The song of male Kentucky warblers consists of several repetitions of syllables containing 2 to 7 elements within a frequency range from 1.8 to 6.0 kHz. Each male has a single, individually distinctive song type that spans part or all of this frequency range. Song responses to playbacks of songs artificially elevated or lowered in frequency by 200 Hz show that a bird is able to alter its song in two ways. It may lower or raise the frequency range of the song or put more energy into the lower or higher frequencies within the song's elements to more closely match the energy distribution of the playback song. The syllable or element structure of the song is kept constant with only the song's energy distribution among frequencies changing. Thus, Kentucky warblers are able to alter their single song type to “match countersing” in a way similar to that of species with song repertoires who switch song types to match those of rivals. The Kentucky warbler may have a single songtype because of its ability to change its song's energy distribution. This ability may perform the same functions as song matching does in species with larger repertoires. This finding is discussed in relation to ideas on the evolution of song repertoires in intrasexual competition.  相似文献   
105.
106.
The ecological trade-off between developmental time and starvation resistance, acting in a heterogeneous environment, can promote the coexistence of competing species. Heterogeneity results from variation in the vegetation that influences both abiotic (e.g. temperature, humidity) and biotic (e.g. fruit availability during the year) aspects of the environment. In this study, we investigated whether differences between collection sites have led to local differentiation of the two life-history traits underlying the coexistence model: developmental time and starvation resistance. Drosophila were collected from four collection sites, ranging from grassland to secondary forest, along a transect of 15 km. The microclimatic and vegetation differences among these collection sites were considerable. For developmental time, different species showed similar genetic responses to the (habitat) differences between the different collection sites. The shortest developmental times were found in the secondary forest populations and the agricultural area populations, the longest in the grassland populations, and the forest edge populations were intermediate. However, there was no correlation between the habitat ranking based on disturbance and canopy cover, and the ranking of the developmental times. Furthermore, the data did not confirm the generality of the positive correlation between developmental time and starvation underlying the coexistence model.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 87 , 115–125.  相似文献   
107.
We reviewed the family Lycidae in Korea. The Korean Lycidae was recorded to fourteen species under eight genera until now. However, the taxonomy of Korean species was conducted without comparative analysis. We revised the Korean Lycidae to eleven species under seven genera of two subfamilies. Also, we provided the key to subfamilies, gernera, and species including illustrations of pronotum and aedeagus, the photos of adult habitus.  相似文献   
108.
A taxonomic review of six genera of Korean Diaperini is presented. Seven species in Korea are described here. We provide a key to the Korean species and genera, diagnoses and photos for them.  相似文献   
109.
Food webs in tropical Australian streams: shredders are not scarce   总被引:4,自引:0,他引:4  
1. Macroinvertebrates were collected in dry and wet seasons from riffles and pools in two streams in tropical north Queensland. Total biomass, abundance and species richness were higher in riffles than in pools but did not differ between streams or seasons. 2. Gut contents of all species were identified. Cluster analysis based on gut contents identified five dietary groups: I, generalist collectors; II, generalist shredders and generalist predators; III, generalist scrapers; IV, specialist shredders; and V, specialist predators. Species were allocated to functional feeding groups (FFGs) based on these dietary groups. 3. Many species were generalist in their diets, but specialist predators and shredders were particularly prominent components of the invertebrate assemblages in terms of biomass and species richness. 4. Community composition (proportions of biomass, abundance and species richness of the different FFGs) varied between habitat types, but not between streams or seasons, although differences between riffles and pools varied with season. 5. Comparison of the fauna of 20 streams showed that our study sites were similar to, or not atypical of, low‐order streams in the Queensland wet tropics.  相似文献   
110.
Very-broad-scale assessment of human impacts on river condition   总被引:2,自引:0,他引:2  
1. Management of whole rivers and river catchments requires a comprehensive set of information about river condition and use, both existing and historical, and the links between them at regional, state or national scales. This paper outlines a new approach to the assessment of river condition, using a small team was able to assess 210 000 km of rivers across more than 3 million km2 of Australia in little more than a year. 2. The approach was driven by a hierarchical model of river function, which assumed that broad‐scale catchment characteristics affect local hydrology, habitat features, water quality and, ultimately, aquatic biota. The model provided the basis for selecting important ecologically relevant features that indices should represent. For each reach of each river we derived a biological index and an environmental index based on measures quantifying catchment and hydrological condition, and habitat and water quality condition. Data came from existing state and national databases, satellite images, site measurements and process models. 3. All indices were calculated as deviation from a reference condition, were range‐standardised and were divided into equivalent bands of condition. Amalgamation of index components and of sub‐indices was determined by consideration of their ecological effects; for example, general degradation might be additive, but toxic effects of one component would override all others. 4. Several internal and external validation methods were employed, with the all‐important validation of the final assessments undertaken by comparison with a similar index based on locally measured data. 5. The environmental assessment classified 14% of reaches as largely unmodified, 67% as moderately modified and 19% as substantially modified by human impacts. The biological assessment based on site assessments and modelled data using invertebrates indicated that 70% of reaches were equivalent to reference condition and that 30% were significantly impaired. Catchment disturbance, elevated sediment and nutrient loads, and habitat degradation all contributed to these results. These impacts have all occurred during the last 200 years (post‐European settlement). 6. Partly as a result of the assessments of this study the Australian Government has begun to adopt a more environmentally sustainable approach to broad‐scale water management.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号