全文获取类型
收费全文 | 96篇 |
免费 | 5篇 |
出版年
2020年 | 1篇 |
2019年 | 2篇 |
2017年 | 1篇 |
2016年 | 4篇 |
2015年 | 5篇 |
2014年 | 5篇 |
2013年 | 10篇 |
2012年 | 3篇 |
2011年 | 8篇 |
2010年 | 4篇 |
2009年 | 3篇 |
2008年 | 4篇 |
2007年 | 6篇 |
2006年 | 4篇 |
2005年 | 4篇 |
2004年 | 2篇 |
2003年 | 1篇 |
2001年 | 1篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1995年 | 1篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 2篇 |
1983年 | 2篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 3篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1958年 | 1篇 |
1944年 | 1篇 |
1931年 | 1篇 |
1927年 | 1篇 |
1916年 | 1篇 |
1911年 | 2篇 |
1909年 | 1篇 |
1908年 | 1篇 |
排序方式: 共有101条查询结果,搜索用时 46 毫秒
61.
Sabine Milde Georg Hemmrich Friederike Anton-Erxleben Konstantin Khalturin Jörg Wittlieb Thomas CG Bosch 《Genome biology》2009,10(1):R8-16
Background
Despite decades of research, the molecular mechanisms responsible for the evolution of morphological diversity remain poorly understood. While current models assume that species-specific morphologies are governed by differential use of conserved genetic regulatory circuits, it is debated whether non-conserved taxonomically restricted genes are also involved in making taxonomically relevant structures. The genomic resources available in Hydra, a member of the early branching animal phylum Cnidaria, provide a unique opportunity to study the molecular evolution of morphological novelties such as the nematocyte, a cell type characteristic of, and unique to, Cnidaria. 相似文献62.
Beneficial exotic trees and shrubs have been widely spread throughout semiarid and arid regions of the world. These trees and shrubs can however cause severe negative impacts. Mesquite (Prosopis species), native to the New World, is one example which continues to be promoted despite causing serious impacts both in its native and introduced ranges. We describe the population structure of the largest population of fire-tolerant hybrid mesquite (P. velutina × P. glandulosa var. glandulosa × P. pallida) in Australia, which was intentionally established in the 1930s. We compare it with invasive populations within its native range, and consider the implications for managing exotic mesquite invasions. We found relatively high juvenile densities at all levels of canopy cover (<30% to 90–100%), and low mortality rates for both juveniles and adults (<2%/y), which suggests that populations are still in an early phase of invasion. Exotic populations differed from native range populations in being more dense (average 4,859 adults/ha), having a sizable sapling (seedling and juvenile) bank that can remain quiescent under canopy cover (average 10,914 seedlings and juveniles/ha), failing to act as nurse plants for native shrubs (<8 native shrubs/ha), and almost totally excluding the herbaceous (grass) layer (average 0.3% cover). Our results suggest that ecosystem impacts in the introduced range are likely to be even worse, and management even more difficult, than has already been reported for invasions within its native range. The lack of feasible means for managing highly invasive, broad-scale mesquite populations need to be addressed, and needs to be considered explicitly when promoting mesquite as a beneficial plant. 相似文献
63.
Rolf Weinzierl Gil Bohrer Bart Kranstauber Wolfgang Fiedler Martin Wikelski Andrea Flack 《Ecology and evolution》2016,6(24):8706-8718
The flight performance of birds is strongly affected by the dynamic state of the atmosphere at the birds' locations. Studies of flight and its impact on the movement ecology of birds must consider the wind to help us understand aerodynamics and bird flight strategies. Here, we introduce a systematic approach to evaluate wind speed and direction from the high‐frequency GPS recordings from bird‐borne tags during thermalling flight. Our method assumes that a fixed horizontal mean wind speed during a short (18 seconds, 19 GPS fixes) flight segment with a constant turn angle along a closed loop, characteristic of thermalling flight, will generate a fixed drift for each consequent location. We use a maximum‐likelihood approach to estimate that drift and to determine the wind and airspeeds at the birds' flight locations. We also provide error estimates for these GPS‐derived wind speed estimates. We validate our approach by comparing its wind estimates with the mid‐resolution weather reanalysis data from ECMWF, and by examining independent wind estimates from pairs of birds in a large dataset of GPS‐tagged migrating storks that were flying in close proximity. Our approach provides accurate and unbiased observations of wind speed and additional detailed information on vertical winds and uplift structure. These precise measurements are otherwise rare and hard to obtain and will broaden our understanding of atmospheric conditions, flight aerodynamics, and bird flight strategies. With an increasing number of GPS‐tracked animals, we may soon be able to use birds to inform us about the atmosphere they are flying through and thus improve future ecological and environmental studies. 相似文献
64.
Single-copy nuclear DNAs (scnDNAs) of eight species of arvicoline and six
species of murine rodents were compared using DNA-DNA hybridization. The
branching pattern derived from the DNA comparisons is congruent with the
fossil evidence and supported by comparative biochemical, chromosomal, and
morphological studies. The recently improved fossil record for these
lineages provides seven approximate divergence dates, which were used to
calibrate the DNA-hybridization data. The average rate of scnDNA divergence
was estimated as 2.5%/Myr. This is approximately 10 times the rate in the
hominoid primates. These results agree with previous reports of accelerated
DNA evolution in muroid rodents and extend the DNA-DNA hybridization data
set of Brownell.
相似文献
65.
Small-subunit ribosomal RNA sequence from Naegleria gruberi supports the polyphyletic origin of amoebas 总被引:2,自引:0,他引:2
We have sequenced the small-subunit ribosomal RNA gene of the amoebo-
flagellate protozoan Naegleria gruberi. Comparison of this sequence with
the rRNA sequences of other eukaryotes resulted in a phylogenetic tree that
supports the suggested polyphyletic origin of amoebas and suggests a
flagellate ancestry for Naegleria.
相似文献
66.
MC Chiale D Montalti MA Flamini P Fernández E Gimeno CG Barbeito 《Biotechnic & histochemistry》2016,91(1):30-37
The uropygial glands of birds are sebaceous organs that contribute to the water-repellent properties of the feather coat. We studied the histological and histochemical characteristics of the uropygial gland of chimango caracara using hematoxylin and eosin (H & E), Gomori´s trichrome, orcein, Gomori´s reticulin, periodic acid-Schiff (PAS), Alcian blue (AB) and a variety of lectins. The gland is composed of two lobes and a papilla with 20 downy feathers. It is surrounded by a capsule of dense connective tissue that contains elastic, reticular and smooth muscle fibers. The papilla is delicate and has two excretory ducts. The gland mass relative to body mass was 0.143%. Both adenomer cells and their secretions were stained with Sudan IV, PAS and AB, and were positive for numerous lectins that indicated the presence of lipids and carbohydrates. Immunohistochemical techniques to detect PCNA confirmed cell proliferation in the basal stratum of the adenomer cells. The lipids and glycoconjugates secreted by the uropygial gland serve numerous functions including protection against microorganisms. 相似文献
67.
Evolutionary rates for tuf genes in endosymbionts of aphids 总被引:5,自引:1,他引:4
The gene encoding elongation factor Tu (tuf) in aphid endosymbionts (genus
Buchnera) evolves at rates of 1.3 x 10(-10) to 2.5 x 10(-10) nonsynonymous
substitutions and 3.9 x 10(-9) to 8.0 x 10(-9) synonymous substitutions per
position per year. These rates, which are at present among the most
reliable substitution rates for protein-coding genes of bacteria, have been
obtained by calibrating the nodes in the phylogenetic tree produced from
the Buchnera EF-Tu sequences using divergence times for the corresponding
ancestral aphid hosts. We also present data suggesting that the rates of
nonsynonymous substitutions are significantly higher in the endosymbiont
lineages than in the closely related free-living bacteria Escherichia coli
and Salmonella typhimurium. Synonymous substitution rates for Buchnera
approximate estimated mutation rates for E. coli and S. typhimurium, as
expected if synonymous changes act as neutral mutations in Buchnera. We
relate the observed differences in substitution frequencies to the absence
of selective codon preferences in Buchnera and to the influence of Muller's
ratchet on small asexual populations.
相似文献
68.
Ay N Flack J Krakauer DC 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2007,362(1479):441-447
In animal communication, signals are frequently emitted using different channels (e.g. frequencies in a vocalization) and different modalities (e.g. gestures can accompany vocalizations). We explore two explanations that have been provided for multimodality: (i) selection for high information transfer through dedicated channels and (ii) increasing fault tolerance or robustness through multichannel signals. Robustness relates to an accurate decoding of a signal when parts of a signal are occluded. We show analytically in simple feed-forward neural networks that while a multichannel signal can solve the robustness problem, a multimodal signal does so more effectively because it can maximize the contribution made by each channel while minimizing the effects of exclusion. Multimodality refers to sets of channels where within each set information is highly correlated. We show that the robustness property ensures correlations among channels producing complex, associative networks as a by-product. We refer to this as the principle of robust overdesign. We discuss the biological implications of this for the evolution of combinatorial signalling systems; in particular, how robustness promotes enough redundancy to allow for a subsequent specialization of redundant components into novel signals. 相似文献
69.
A Bell OA Rodríguez LA de Castro e Paula MB Padua J Hernández-Cerón CG Gutiérrez A De Vries PJ Hansen 《BMC veterinary research》2008,4(1):22
Background
Results regarding the use of bovine somatotropin for enhancing fertility in dairy cattle are variable. Here, the hypothesis was tested that a single injection of a sustained-release preparation of bovine somatotropin (bST) during the preovulatory period would improve pregnancy success of lactating dairy cows at first service.Results
The first experiment was conducted in a temperate region of Mexico. Cows inseminated following natural estrus or timed artificial insemination were given a single injection of bST or a placebo injection at insemination (n = 100 cows per group). There was no significant difference between bST and control groups in the proportion of inseminated cows diagnosed pregnant (29 vs 31% pregnant). The second experiment was performed during heat stress in Florida. Cows were subjected to an ovulation synchronization regimen for first insemination. Cows treated with bST received a single injection at 3 days before insemination. Controls received no additional treatment. As expected, bST did not increase vaginal temperature. Treatment with bST did not significantly increase the proportion of inseminated cows diagnosed pregnant although it was numerically greater for the bST group (24.2% vs 17.8%, 124–132 cows per group). There was a tendency (p = 0.10) for a smaller percent of control cows to have high plasma progesterone concentrations (≥ 1 ng/ml) at Day 7 after insemination than for bST-treated cows (72.6 vs 81.1%). When only cows that were successfully synchronized were considered, the magnitude of the absolute difference in the percentage of inseminated cows that were diagnosed pregnant between bST and control cows was reduced (24.8 vs 22.4% pregnant for bST and control).Conclusion
Results failed to indicate a beneficial effect of bST treatment on fertility of lactating dairy cows.70.