全文获取类型
收费全文 | 182篇 |
免费 | 11篇 |
国内免费 | 3篇 |
出版年
2022年 | 2篇 |
2021年 | 4篇 |
2020年 | 4篇 |
2019年 | 2篇 |
2018年 | 1篇 |
2017年 | 4篇 |
2016年 | 3篇 |
2015年 | 9篇 |
2014年 | 6篇 |
2013年 | 10篇 |
2012年 | 10篇 |
2011年 | 8篇 |
2010年 | 21篇 |
2009年 | 11篇 |
2008年 | 12篇 |
2007年 | 15篇 |
2006年 | 6篇 |
2005年 | 6篇 |
2004年 | 5篇 |
2003年 | 7篇 |
2002年 | 5篇 |
2001年 | 7篇 |
2000年 | 1篇 |
1999年 | 3篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1996年 | 3篇 |
1995年 | 3篇 |
1994年 | 2篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1988年 | 1篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1977年 | 2篇 |
1968年 | 1篇 |
排序方式: 共有196条查询结果,搜索用时 15 毫秒
31.
America G. Valiente Francis Juanes Pablo Nuñez Eva Garcia-Vazquez 《Biological invasions》2010,12(3):451-462
Brown trout of German origin were introduced into Patagonian National Parks in 1905, where they acclimatized and underwent
population expansion endangering populations of native species like Galaxiidae. Spawning adults of two populations were sampled
in 2004. Their age, length-at-age and migratory behaviour were assessed from scale samples, as well as their variation at
the coding LDH-C1* and eight non coding microsatellite loci. Between-population differentiation for life history (spawning time, migratory
behaviour, length and weight at age) and reduced genetic variation were revealed. Based on genetic variation, effective population
size smaller than 50 individuals has been estimated for the founder stock, and its German origin has been genetically traced.
Flexibility in migratory behaviour and spawning time were identified as key factors conferring competitive advantage on those
brown trout populations. 相似文献
32.
Deforestation usually results in significant losses of soil organic carbon (SOC). The rate and factors determining the recovery of this C pool with afforestation are still poorly understood. This paper provides a review of the influence of afforestation on SOC stocks based on a meta-analysis of 33 recent publications (totaling 120 sites and 189 observations), with the aim of determining the factors responsible for the restoration of SOC following afforestation. Based on a mixed linear model, the meta-analysis indicates that the main factors that contribute to restoring SOC stocks after afforestation are: previous land use, tree species planted, soil clay content, preplanting disturbance and, to a lesser extent, climatic zone. Specifically, this meta-analysis (1) indicates that the positive impact of afforestation on SOC stocks is more pronounced in cropland soils than in pastures or natural grasslands; (2) suggests that broadleaf tree species have a greater capacity to accumulate SOC than coniferous species; (3) underscores that afforestation using pine species does not result in a net loss of the whole soil-profile carbon stocks compared with initial values (agricultural soil) when the surface organic layer is included in the accounting; (4) demonstrates that clay-rich soils (> 33%) have a greater capacity to accumulate SOC than soils with a lower clay content (< 33%); (5) indicates that minimizing preplanting disturbances may increase the rate at which SOC stocks are replenished; and (6) suggests that afforestation carried out in the boreal climate zone results in small SOC losses compared with other climate zones, probably because trees grow more slowly under these conditions, although this does not rule out gains over time after the conversion. This study also highlights the importance of the methodological approach used when developing the sampling design, especially the inclusion of the organic layer in the accounting. 相似文献
33.
Spatial and temporal dynamics of macroinvertebrate communities have usually been linked to several environmental and anthropic
factors. The aim of this study is to elucidate how important are these factors in structuring macroinvertebrate communities
from temperate regions. Regarding the macroinvertebrate number of taxa, the Habitat Template Model, the Dynamic Equilibrium
Hypothesis and the Intermediate Disturbance Hypothesis will be tested in order to know how important the diversity of instream
elements and the hydrological disturbance frequency are in defining the macroinvertebrate taxonomic richness. Thus, the structure
and composition of macroinvertebrate communities were analysed in nine sites of the Pas River basin, a temperate Atlantic
basin in northern Spain, during winter, spring, summer and autumn 2005, together with water physicochemical and environmental
characteristics. Macroinvertebrate abundance increased downstream and during summer, probably favoured by lower hydraulic
stress and water organic enrichment. As predicts the Habitat Template Model, the macroinvertebrate number of taxa was related
to habitat heterogeneity. However, no clear relationship amongst macroinvertebrate richness and water quality was found. The
macroinvertebrate taxonomic richness did not correspond exactly with the Dynamic Equilibrium Hypothesis and the Intermediate
Disturbance Hypothesis because it was relatively high in the absence of hydrological disturbances (summer). Thus, disturbance
events may play a secondary role in determining the seasonal dynamic of the number of taxa. However, hydrological disturbances
can be considered the most important factors explaining the seasonal pattern of macroinvertebrate abundance. On the other
hand, spatial patterns of macroinvertebrate community structure and composition were mainly determined by resource availability,
hydraulic conditions, habitat heterogeneity and human alterations, whilst hydrological predictability and resource availability
might play a major role in determining seasonal dynamics. 相似文献
34.
ABSTRACT: Large-scale sequencing of genomes has enabled the inference of phylogenies based on the evolution of genomic architecture, under such events as rearrangements, duplications, and losses. Many evolutionary models and associated algorithms have been designed over the last few years and have found use in comparative genomics and phylogenetic inference. However, the assessment of phylogenies built from such data has not been properly addressed to date. The standard method used in sequence-based phylogenetic inference is the bootstrap, but it relies on a large number of homologous characters that can be resampled; yet in the case of rearrangements, the entire genome is a single character. Alternatives such as the jackknife suffer from the same problem, while likelihood tests cannot be applied in the absence of well established probabilistic models. We present a new approach to the assessment of distance-based phylogenetic inference from whole-genome data; our approach combines features of the jackknife and the bootstrap and remains nonparametric. For each feature of our method, we give an equivalent feature in the sequence-based framework; we also present the results of extensive experimental testing, in both sequence-based and genome-based frameworks. Through the feature-by-feature comparison and the experimental results, we show that our bootstrapping approach is on par with the classic phylogenetic bootstrap used in sequence-based reconstruction, and we establish the clear superiority of the classic bootstrap for sequence data and of our corresponding new approach for rearrangement data over proposed variants. Finally, we test our approach on a small dataset of mammalian genomes, verifying that the support values match current thinking about the respective branches. Our method is the first to provide a standard of assessment to match that of the classic phylogenetic bootstrap for aligned sequences. Its support values follow a similar scale and its receiver-operating characteristics are nearly identical, indicating that it provides similar levels of sensitivity and specificity. Thus our assessment method makes it possible to conduct phylogenetic analyses on whole genomes with the same degree of confidence as for analyses on aligned sequences. Extensions to search-based inference methods such as maximum parsimony and maximum likelihood are possible, but remain to be thoroughly tested. 相似文献
35.
36.
Contribution of sucrose synthase, ADP-glucose pyrophosphorylase and starch synthase to starch synthesis in developing pea seeds 总被引:11,自引:0,他引:11
Using genetic variability existing amongst nine pea genotypes (Pisum sativum L.), the biochemical basis of sink strength in developing pea seeds was investigated. Sink strength was considered to be reflected by the rate of starch synthesis (RSS) in the embryo, and sink activity in the seed was reflected by the relative rate of starch synthesis (RRSS). These rates were compared to the activities of three enzymes of the starch biosynthetic pathway [sucrose synthase (Sus), ADP-glucose pyrophosphorylase and starch synthase] at three developmental stages during seed filling (25, 50 and 75% of the dry seed weight). Complete sets of data collected during seed filling for the nine genotypes showed that, for all enzyme activities (expressed on a protein basis), only Sus in the embryo and seed coat was linearly and significantly correlated to RRSS. The contribution of the three enzyme activities to the variability in RSS and RRSS was evaluated by multiple regression analysis for the first two developmental stages. Only Sus activity in the embryo could explain, at least in part, the significant variability observed for both the RSS and the RRSS at each developmental stage. We conclude that Sus activity is a reliable marker of sink activity in developing pea seeds. 相似文献
37.
Sea urchin Hox genes: insights into the ancestral Hox cluster 总被引:3,自引:0,他引:3
We describe the Hox cluster in the radially symmetric sea urchin and
compare our findings to what is known from clusters in bilaterally
symmetric animals. Several Hox genes from the direct-developing sea urchin
Heliocidaris erythrogramma are described. CHEF gel analysis shows that the
Hox genes are clustered on a < or = 300 kilobase (kb) fragment of DNA,
and only a single cluster is present, as in lower chordates and other
nonvertebrate metazoans. Phylogenetic analyses of sea urchin, amphioxus,
Drosophila, and selected vertebrate Hox genes confirm that the H.
erythrogramma genes, and others previously cloned from other sea urchins,
belong to anterior, central, and posterior groups. Despite their radial
body plan and lack of cephalization, echinoderms retain at least one of the
anterior group Hox genes, an orthologue of Hox3. The structure of the
echinoderm Hox cluster suggests that the ancestral deuterostome had a Hox
cluster more similar to the current chordate cluster than was expected Sea
urchins have at least three Abd-B type genes, suggesting that Abd-B
expansion began before the radiation of deuterostomes.
相似文献
38.
39.
Exploitation of the commercially-important species of Gelidium in Spain (G. sesquipedale and G. latifolium) and development of the industry occurred after World War II, as a consequence of their use as resources for the extraction of agar. This resulted in the implementation of several harvesting methods, the most important of which is the gathering of cast seaweeds, both from the shore and the sea. From the very beginning, direct exploitation of these species (i.e. plucking) was controversial because of possible adverse ecological effects. Consequently, several biological and ecological studies of both species of Gelidium were begun. This included such aspects as growth, biomass production, productivity, reproduction, regeneration capacity and agar yield. Recently, the growing interest in using and conserving this resource has led to increased knowledge of the biology and ecology of these species. In this paper, we provide an overview of the research carried out by different Spanish groups and suggest guidelines for rational management of these resources. 相似文献
40.
Thérèse S Lapperre Jacob K Sont Annemarie van Schadewijk Margot ME Gosman Dirkje S Postma Ingeborg M Bajema Wim Timens Thais Mauad Pieter S Hiemstra 《Respiratory research》2007,8(1):85-9