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171.
Wild ruminants are susceptible to infection from generalist helminth species, which can also infect domestic ruminants. A better understanding is required of the conditions under which wild ruminants can act as a source of helminths (including anthelmintic-resistant genotypes) for domestic ruminants, and vice versa, with the added possibility that wildlife could act as refugia for drug-susceptible genotypes and hence buffer the spread and development of resistance. Helminth infections cause significant productivity losses in domestic ruminants and a growing resistance to all classes of anthelmintic drug escalates concerns around helminth infection in the livestock industry. Previous research demonstrates that drug-resistant strains of the pathogenic nematode Haemonchus contortus can be transmitted between wild and domestic ruminants, and that gastro-intestinal nematode infections are more intense in wild ruminants within areas of high livestock density. In this article, the factors likely to influence the role of wild ruminants in helminth infections and anthelmintic resistance in livestock are considered, including host population movement across heterogeneous landscapes, and the effects of climate and environment on parasite dynamics. Methods of predicting and validating suspected drivers of helminth transmission in this context are considered based on advances in predictive modelling and molecular tools. 相似文献
172.
173.
Cultural and genetic evolution in mountain white-crowned sparrows: song dialects are associated with population structure 总被引:6,自引:0,他引:6
MacDougall-Shackleton EA MacDougall-Shackleton SA 《Evolution; international journal of organic evolution》2001,55(12):2568-2575
Bird song often varies geographically within a species; when this geographic variation has distinct boundaries, the shared song types are referred to as song dialects. How dialects are produced and their adaptive significance are longstanding problems in biology, with implications for the role of culture in the evolution and ecology of diverse organisms, including humans. Here we test the hypothesis that song dialect, a culturally transmitted trait, is related to the population genetic structure of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha). To address this, we compared microsatellite allele frequencies from 18 sample sites representing eight dialect regions in the Sierra Nevada. Pairwise genetic distances were not significantly correlated with geographic distances either within or between dialects, nor did dialect groups form distinct genetic groups according to neighbor-joining or UPGMA analysis, and most variation in allele frequencies occurred among individuals rather than at higher levels. However, most of the remaining variation was attributable to differences among, rather than within, dialect regions, and this among-dialect component of variance was statistically significant. Moreover, when controlling for the effect of geographic distance, song dissimilarity and genetic distance between site pairs were significantly correlated. Thus, song dialects appear to be associated with reductions in, but not strict barriers to, gene flow among dialect regions. 相似文献
174.
Wolbachia bacteria are obligatory intracellular parasites of arthropods and have been detected in about 70 species of parasitic wasps
and three parasitoid flies. Wolbachia are transmitted cytoplasmically (maternally) and modify host reproduction in different ways to enhance their own transmission:
parthenogenesis induction (PI), cytoplasmic incompatibility (CI), or feminization (F) of genetic males. Only PI and CI are
known in parasitoids. PI-Wolbachia cause thelytoky in otherwise arrhenotokous parasitoids by generating diploid (rather than haploid) unfertilized wasp eggs.
CI-Wolbachia cause incompatibility of crosses between infected males and uninfected females because the paternally derived chromosomes
fail to decondense and are destroyed after syngamy. More complex situations arise when hosts harbor multiple infections, which
can lead to bidirectional incompatibility and may be involved in parasitoid speciation. The relative fitness of infected and
uninfected hosts is important to the population dynamics of Wolbachia, and more data are needed. Evolutionary conflict should be common between host genes, Wolbachia genes, and other "selfish" genetic elements. Wolbachia-specific PCR primers are now available for several genes with different rates of evolution. These primers will permit rapid
screening in future studies of spatial and temporal patterns of single and multiple infection. Molecular phylogenies show
that CI- and PI-Wolbachia do not form discrete clades. In combination with experimental transfection data, this result suggests that host reproductive
alterations depend on the interaction between attributes of both Wolbachia and host. Moreover, Wolbachia isolates from closely related hosts do not usually cluster together, and phylogenies suggest that Wolbachia may have radiated after their arthropod hosts. Both results support considerable horizontal transmission of Wolbachia between host species over evolutionary time. Natural horizontal transmisson between parasitoids and their hosts, or with
entomoparasitic nematodes or ectoparasitic mites, remains a tantalizing but equivocal possibility.
Received: November 27, 1998 / Accepted: January 15, 1999 相似文献
175.
The reflective properties of different types of dinoflagellate eyespots were investigated using confocal laser scanning microscopy in the epireflection contrast mode. Although the eyespots studied differed with respect to localization (cytosol or plastid) and organization of the globule layer(s), all types effectively absorbed and reflected blue-green laser light (principal lines of 488/514 nm). The relative orientation of the eyespot surface towards the light source strongly influenced the reflective properties. Maximal reflection occurred when the eyespot surface was approximately perpendicular to the light source and rapidly decreased at increasing angles of light incidence. Horizontal and vertical optical sectioning of live and fixed cells resolved differences in the reflection patterns. Focusing of reflected light on the basal portion of the longitudinal flagellum was observed for the cytosolic eyespot of Glenodinium sp. and the triple membrane-bounded eyespot of Peridinium foliaceum, presumably a vestige of a host plastid. This flagellum is thought to be mainly involved in mediating orientational movement responses. In contrast, the reflection patterns obtained from the eyespot of Woloszynskia pascheri, which represents the third and most commonly observed dinoflagellate eyespot type within a plastid, point to only minor focusing. Reflection signals could be followed a considerable distance into the sulcus in all cases, indicating that in dinoflagellate eyespots, irrespective of the presumed receptor location (plasma membrane overlying the eyespot and/or the basal part of the longitudinal flagellum), back reflection of non-absorbed light can enhance the excitation probability of the photoreceptor(s). Such a combined reflection/absorption screen allows maximal contrast modulation and will, in conjunction with the specialized geometry of the dinoflagellate eyespots, increase the directionality of these eyespot aparatuses considerably. 相似文献
176.
Sequences in current databases show that a number of proteins involved in respiratory processes are homologous in archaeal
and bacterial species. In particular, terminal oxidases belonging to oxygen, nitrate, sulfate, and sulfur respiratory pathways
have been sequenced in members of both domains. They include cytochrome oxidase, nitrate reductase, adenylylsulfate reductase,
sulfite reductase, and polysulfide reductase. These proteins can be assigned to the last common ancestor of living organisms
assuming that the deepest split of the three domains of life occurred between Archaea and Bacteria and that they were not
acquired through lateral gene transfer by one of these domains. These molecular data indicate that several of the most important
respiratory pathways arose early in evolution and that the last common ancestor of living organisms was not a simple organism
in its energetic metabolism. Rather, it may have been able to gain energy by means of at least four electron transport chains,
and therefore it may have been prepared to face a wide range of environmental conditions. 相似文献
177.
The morphology and ultrastructure of the larval spiracle system of three phlebotomine sandfly species, Phlebotomus perniciosus, P. perfiliewi and P. papatasi, were examined by scanning (SEM) and transmission (TEM) electron microscopy and by confocal scanning laser microscopy (CSLM). During larval development, thoracic and abdominal spiracles show considerable modifications. In fourth instar larvae, the spiracles consist of a plate with a sclerotized central portion and a peripheral circle of papillae. The latter is distinctive in the larvae of P. papatasi, which are readily distinguished from the other species. Opening clefts across the papillae communicate with an internal chamber that encircles an electrondense plug. Many cylindrical projections cross the chamber, uniting the central plug with the larval body, forming an air filter. Spiracular development in successive larval instars has both a taxonomic and adaptive value. 相似文献
178.
179.
180.
H.?Namba T.?NagayamaEmail author 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2004,190(8):615-622
Nonspiking local interneurones are the important premotor elements in arthropod motor control systems. We have analyzed the synaptic interactions between nonspiking interneurones in the crayfish terminal (6th) abdominal ganglion using simultaneous intracellular recordings. Only 15% of nonspiking interneurones formed bi-directional excitatory connections. In 77% of connections, however, the nonspiking interneurones showed a one-way inhibitory interaction. In these cases, the presynaptic nonspiking interneurones received excitatory synaptic inputs from the sensory afferents innervating hairs on the surface of the uropods and the postsynaptic nonspiking interneurones received inhibitory synaptic inputs that were partly mediated by the inputs to the presynaptic nonspiking interneurones. The membrane hyperpolarization of the postsynaptic nonspiking interneurones mediated by the presynaptic nonspiking interneurones was reduced in amplitude when the hyperpolarizing current was injected into the postsynaptic interneurones, or when the external bathing solution was replaced with one containing low calcium and high magnesium concentrations. The role of these interactions in the circuits controlling the movements of the terminal appendages is discussed.Abbreviations AL antero-lateral - epsp excitatory postsynaptic potential - ipsp inhibitory postsynaptic potential - PL postero-lateral 相似文献