全文获取类型
收费全文 | 6307篇 |
免费 | 660篇 |
国内免费 | 517篇 |
专业分类
7484篇 |
出版年
2024年 | 22篇 |
2023年 | 98篇 |
2022年 | 100篇 |
2021年 | 135篇 |
2020年 | 197篇 |
2019年 | 245篇 |
2018年 | 215篇 |
2017年 | 271篇 |
2016年 | 220篇 |
2015年 | 290篇 |
2014年 | 325篇 |
2013年 | 451篇 |
2012年 | 299篇 |
2011年 | 335篇 |
2010年 | 259篇 |
2009年 | 312篇 |
2008年 | 357篇 |
2007年 | 338篇 |
2006年 | 314篇 |
2005年 | 282篇 |
2004年 | 255篇 |
2003年 | 255篇 |
2002年 | 191篇 |
2001年 | 181篇 |
2000年 | 167篇 |
1999年 | 176篇 |
1998年 | 147篇 |
1997年 | 116篇 |
1996年 | 115篇 |
1995年 | 102篇 |
1994年 | 94篇 |
1993年 | 76篇 |
1992年 | 79篇 |
1991年 | 77篇 |
1990年 | 45篇 |
1989年 | 40篇 |
1988年 | 41篇 |
1987年 | 41篇 |
1986年 | 29篇 |
1985年 | 34篇 |
1984年 | 35篇 |
1983年 | 20篇 |
1982年 | 24篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1976年 | 3篇 |
1974年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有7484条查询结果,搜索用时 15 毫秒
61.
The relative roles of gene flow and natural selection in maintaining species differentiation have been a subject of debate for some time. The traditional view is that gene flow constrains adaptive divergence and maintains species cohesiveness. Alternatively, ecological speciation posits that the reverse is true: that adaptive ecological differentiation constrains gene flow. In this study, we examine gene flow and population differentiation among populations of two species of the Hawaiian silversword alliance, Dubautia arborea and D. ciliolata. We compare divergence in putatively neutral microsatellite markers with divergence in leaf morphometric traits, which may be selectively important or physiologically linked to selectively important traits. Gene flow between populations was found to be significant in only one of the two species, D. arborea. Leaf morphometric differentiation between species was significant, though not among populations within species. No evidence of effective genetic introgression was observed between apparently 'pure' populations of these species. Gene flow as measured by microsatellites was not correlated with geographic distance between populations, but was correlated with the linear placement of the widest part of the leaf. Because these two species are interfertile, as demonstrated by the presence of active hybrid zone, the lack of genetic introgression and the maintenance of species boundaries may be associated with natural selection on differential habitat. 相似文献
62.
Aims: To study the efficacy of the lytic phage φS1 in eliminating Pseudomonas fluorescens in the early stage of biofilm formation, using an in situ and real time methodology for cell quantification.
Methods and Results: Cell adhesion and phage infection studies were carried out in a parallel plate flow chamber under laminar conditions. Cells were allowed to adhere until reaching 1·7–1·8 × 106 cells cm−2 and phage infection was performed with two different phage concentrations (2 × 109 PFU ml−1 and 1 × 1010 PFU ml−1 ). Phage concentration clearly affects the speed of infection. The less concentrated phage solution promoted a three times slower rate of cell removal but did not affect the overall percentage of cell removal. In fact, after a longer infection period the less concentrated phage solution reached the same 93% cell removal value.
Conclusions: Phages are efficient in the eradication of bacterial cells at the early stage of biofilm formation and their presence at the surface did not allow bacterial recolonization of the surface.
Significance and Impact of the Study: To date, no published studies have been made concerning in situ and real time quantification of cell removal from surfaces due to phage action. 相似文献
Methods and Results: Cell adhesion and phage infection studies were carried out in a parallel plate flow chamber under laminar conditions. Cells were allowed to adhere until reaching 1·7–1·8 × 10
Conclusions: Phages are efficient in the eradication of bacterial cells at the early stage of biofilm formation and their presence at the surface did not allow bacterial recolonization of the surface.
Significance and Impact of the Study: To date, no published studies have been made concerning in situ and real time quantification of cell removal from surfaces due to phage action. 相似文献
63.
W. Rossi B. Corrias P. Arduino R. Cianchi L. Bullini 《Plant Systematics and Evolution》1992,179(1-2):43-58
Data are presented on genetic variation at 27 enzyme loci of the Green-Winged orchid,Orchis morio, in 18 population samples from Italy. The existence in Italy of two subspecies, i.e. subspp.morio andpicta, is not supported by allozyme data. No genetic heterogeneity was found betweenmorio-like andpicta-like samples and specimens. Moreover, morphological transition between the two forms was observed in different Italian populations. The parameters of genetic variability estimated forO. morio populations are consistent with those found among monocotyledon plants, and among those outcrossing, animal-pollinated and with wind-dispersed seeds. Genetic diversity of ItalianO. morio is mostly within populations. Correspondingly, low values of interpopulational genetic distance were found. This appears to be due to high levels of gene flow, which were estimated with different methods. The lack ofO. longicornu from Italian samples, as well as of any hybrid withO. morio (F1, backcrossed or recombinant individuals) is demonstrated on the basis of genetic data. It is concluded that recurrent reports ofO. longicornu from Italy are due to confusion withO. morio or with otherOrchis species. 相似文献
64.
Carl M. Way Albert J. Burky Juliana M. Harding Skippy Hau William K.L.C. Puleloa 《Environmental Biology of Fishes》1998,51(1):53-65
Constant pressure in Hawai'i to use limited freshwater resources has resulted in increasing concern for the future of the
native stream fauna. Hawaiian freshwater gobies have an amphidromous life cycle with a marine larva period and require streams
which flow continuously to the ocean for the critical reproductive periods and during recruitment. As such, the stream fauna
is particularly sensitive to any anthropogenic perturbations which disrupt the continuity of stream flows. The objective of
this 2-year study was to compare the life cycles of the goby, Lentipes concolor, from a heavily diverted stream on Moloka'i
and a relatively undisturbed stream on Maui. In Makamaka'ole Stream, Maui, the population of L. concolor was reproductively
active all year with females potentially spawning 2–3 times annually. The timing of spawning did not occur consistently during
the wet or dry season but coincided with high stream flow conditions regardless of time-of-year. In Waikolu Stream, Moloka'i,
the reproductive pattern was more variable with the number of reproductively active females ranging from 0% to 100%. In general
the number of eggs was greater and egg size smaller for female L. concolor in Waikolu Stream than in Makamaka'ole Stream.
However, female reproductive condition of L. concolor from Maui was consistently higher than from fish on Moloka'i. Reproduction
of L. concolor in Makamaka'ole Stream was correlated with the seasonal pattern of flow rates with peaks in female reproductive
condition associated with periods of elevated discharge. No correlation between reproduction and discharge occurred in Waikolu
Stream. There were considerable differences between the magnitude of discharge in the two streams. Waikolu Stream experienced
prolonged periods of extremely low flows which have become common since the Moloka'i Irrigation System began diverting water
from the stream in 1960. In Makamaka'ole Stream, L. concolor was capable of reproducing throughout the year and adjusting
fecundity in response to stream flow conditions. In contrast, the population in Waikolu Stream appeared to have a ‘boom or
bust’ reproductive pattern; the population had reduced or no reproduction when stream flow conditions reached extreme low
levels, but the population succesfully reproduced during higher flow months. The diversion structure in Waikolu Stream has
dampened the natural seasonal discharge cycle, exacerbated natural low flow conditions, and increased the likelihood of prolonged
periods of extremely low flow. Stream management practices in the Hawaiian Islands must take into account the complex life
cycles and sensitivity to variable stream flow conditions of the native fauna.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
65.
Two brown trout Salmo trutta stocks of different origin (wild Polish, domestic commercial) came into secondary contact after deliberate releases conducted in virgin rivers systems of the Subantarctic Kerguelen Islands (70 degrees E 49 degrees S). Samples obtained in 2001-2003 and a historical sample from 1993 were analysed for genetic variation at seven microsatellite loci and one allozyme locus (LDH-C1*). Bayesian clustering analysis demonstrated that rapid genetic differentiation formed separate genetic units in neighbouring rivers in less than 20 years. These genetic units were characterized by a large proportion of Polish genotypes mixed with some genomes of domestic origin (up to 30%). A different colonization strategy of the naturalized stocks, likely related with differential performance, was identified as a cause of rapid population differentiation in this area. 相似文献
66.
PM2.5和PM10已成为我国大部分城市空气的首要污染物.本文通过分析南昌市2013—2015年的空气PM2.5和PM10质量浓度、气象因素、交通流量的监测数据,探讨了空气颗粒物污染的时空动态规律以及气象、交通对颗粒物浓度变化的影响.结果表明: 2013、2014、2015年,南昌市PM2.5浓度(70.92 μg·m-3>53.70 μg·m-3>43.65 μg·m-3)、PM10浓度(119.72 μg·m-3>86.11 μg·m-3>73.32 μg·m-3)逐年降低,并呈现出夏季低(PM2.5和PM10平均浓度分别为36.74、69.20 μg·m-3)、冬季高(PM2.5和PM10平均浓度分别为74.29、111.64 μg·m-3)的季节动态和由城市中心向郊区递减的城乡梯度变化; PM2.5/PM10值(0.595>0.584>0.557)逐年降低,并且表现出城市中心高、城市边缘低的空间分布格局;PM2.5、PM10浓度受到多种气象因素的影响,与气压、温度、相对湿度、风速、降水量、日照时数显著相关,各种气象因子对PM2.5、PM10浓度的影响存在差异;车流量会显著提高周边PM2.5浓度,但对PM10浓度影响不明显. 相似文献
67.
Pia Gruber Filipe Carvalho Marco P. C. Marques Brian O'Sullivan Fabiana Subrizi Dragana Dobrijevic John Ward Helen C. Hailes Pedro Fernandes Roland Wohlgemuth Frank Baganz Nicolas Szita 《Biotechnology and bioengineering》2018,115(3):586-596
68.
69.
Altered growth kinetics precede calcium-induced differentiation in mouse epidermal cells 总被引:1,自引:0,他引:1
K. Elgjo H. Hennings O. P. F. Clausen 《In vitro cellular & developmental biology. Plant》1986,22(6):332-336
Summary Primary cultures of newborn mouse epidermal cells proliferate rapidly and with a high growth fraction for several months when
grown in medium with low calcium (0.02 to 0.1 mM). Addition of calcium to levels generally used in culture medium (1.2 mM) was followed by rapid changes in the pattern of proliferation. By using a combination of technics (a stathmokinetic method,
autoradiography, [3H]thymidine incorporation into DNA, DNA flow cytometry) it was found that cell flux was blocked for 5 to 6 h, followed by
a short rise in the mitotic rate at 10 h, and a gradual fall in all growth parameters until about 32 h after the calcium switch.
There was no accumulation of cells in any particular cell cycle phase. The results indicate that the calcium switch is followed
by a strong reduction in cell flux from G1 whereas the majority of the cells that had left G1 at the time of the switch completed one cell division before cessation of all proliferative activity. Both before and after
the switch the primary epidermal cultures consisted of one diploid and one tetraploid G1 DNA stemline that seemed to react in the same way to calcium.
This work reported in this paper was undertaken during the tenure of an American Cancer Society-Eleanor Roosevelt-International
Cancer Fellowship awarded by the International Union Against Cancer (K. E.). The project was supported by funds partly provided
by the International Cancer Research Data Bank Program of the National Cancer Institute, National Institutes of Health, Bethesda,
MD, under contract N01-C0-65341 (International Cancer Research Technology Transfer) and partly by the International Union
Against Cancer (O.P.F.C.). 相似文献
70.
A reassessment of explanations for discordant introgressions of mitochondrial and nuclear genomes 下载免费PDF全文
《Evolution; international journal of organic evolution》2017,71(9):2140-2158
Hybridization is increasingly recognized as a significant evolutionary process, in particular because it can lead to introgression of genes from one species to another. A striking pattern of discordance in the amount of introgression between mitochondrial and nuclear markers exists such that substantial mitochondrial introgression is often found in combination with no or little nuclear introgression. Multiple mechanisms have been proposed to explain this discordance, including positive selection for introgressing mitochondrial variants, several types of sex‐biases, drift, negative selection against introgression in the nuclear genome, and spatial expansion. Most of these hypotheses are verbal, and have not been quantitatively evaluated so far. We use individual‐based, multilocus, computer simulations of secondary contact under a wide range of demographic and genetic scenarios to evaluate the ability of the different mechanisms to produce discordant introgression. Sex‐biases and spatial expansions fail to produce substantial mito‐nuclear discordance. Drift and nuclear selection can produce strong discordance, but only under a limited range of conditions. In contrast, selection on the mitochondrial genome produces strong discordance, particularly when dispersal rates are low. However, commonly used statistical tests have little power to detect this selection. Altogether, these results dismiss several popular hypotheses, and provide support for adaptive mitochondrial introgression. 相似文献