全文获取类型
收费全文 | 8340篇 |
免费 | 903篇 |
国内免费 | 389篇 |
出版年
2023年 | 122篇 |
2022年 | 109篇 |
2021年 | 259篇 |
2020年 | 277篇 |
2019年 | 352篇 |
2018年 | 315篇 |
2017年 | 309篇 |
2016年 | 349篇 |
2015年 | 371篇 |
2014年 | 431篇 |
2013年 | 532篇 |
2012年 | 433篇 |
2011年 | 362篇 |
2010年 | 312篇 |
2009年 | 479篇 |
2008年 | 426篇 |
2007年 | 419篇 |
2006年 | 379篇 |
2005年 | 359篇 |
2004年 | 280篇 |
2003年 | 243篇 |
2002年 | 233篇 |
2001年 | 198篇 |
2000年 | 203篇 |
1999年 | 186篇 |
1998年 | 154篇 |
1997年 | 149篇 |
1996年 | 143篇 |
1995年 | 108篇 |
1994年 | 125篇 |
1993年 | 106篇 |
1992年 | 94篇 |
1991年 | 93篇 |
1990年 | 74篇 |
1989年 | 76篇 |
1988年 | 78篇 |
1987年 | 67篇 |
1986年 | 43篇 |
1985年 | 42篇 |
1984年 | 59篇 |
1983年 | 32篇 |
1982年 | 39篇 |
1981年 | 47篇 |
1980年 | 35篇 |
1979年 | 32篇 |
1978年 | 22篇 |
1977年 | 16篇 |
1976年 | 19篇 |
1973年 | 13篇 |
1972年 | 8篇 |
排序方式: 共有9632条查询结果,搜索用时 14 毫秒
1.
2.
Timothy F. Murphy 《Bioethics》2015,29(5):369-377
Most advocates of biogenetic modification hope to amplify existing human traits in humans in order to increase the value of such traits as intelligence and resistance to disease. These advocates defend such enhancements as beneficial for the affected parties. By contrast, some commentators recommend certain biogenetic modifications to serve social goals. As Ingmar Persson and Julian Savulescu see things, human moral psychology is deficient relative to the most important risks facing humanity as a whole, including the prospect of Ultimate Harm, the point at which worthwhile life is forever impossible on the planet. These risks can be mitigated, they say, by enhancing moral psychology in novel ways. Persson and Savulescu argue that some parents should modify the underlying biogenetics of their children's moral psychology, if such measures were safe and effective, but they admit these interventions may not decouple humanity from Ultimate Harm. Neither are these modifications the only options, they concede, for addressing risks to humanity. Even with these concessions, saving humanity from itself is a fairly poor reason to modify the moral psychology of children. In most ways, adults would be better candidates, morally speaking, for modifications of psychology. Even then, there is no direct link between morally enhanced human beings and the hoped‐for effect of better protection from Ultimate Harm. Asserting a general duty of all to contribute to the avoidance of Ultimate Harm is a better moral strategy than intervening in the moral psychology of some, even though meeting that duty may involve substantial interference with the free exercise of one's interests. 相似文献
3.
The copy frequency distribution of a transposable element family in a Drosophila melanogaster natural population is generally characterised by the values of the Charlesworths' model parameters α and β (Charlesworth & Charlesworth, 1983). The estimation of these parameters is made using the observed distribution of the occupied sites in a population sample. Several results have been interpreted as due either to the influence of stochastic factors or to deterministic factors (transposition, excision, selection…). The accuracy of this method was tested by estimations performed on samples from simulated populations. The results show that with the sample size usually used for natural population studies, the confidence intervals are too large to reasonably deduce either the element copy number distribution or the values of transposition and excision rate and selective coefficients. 相似文献
4.
Adrián Melo-Carrillo Jacob C. Dunn Liliana Cortés-Ortiz 《American journal of primatology》2020,82(8):e23160
Genetic diversity provides populations with the possibility to persist in ever-changing environments, where selective regimes change over time. Therefore, the long-term survival of a population may be affected by its level of genetic diversity. The Mexican howler monkey (Alouatta palliata mexicana) is a critically endangered primate restricted to southeast Mexico. Here, we evaluate the genetic diversity and population structure of this subspecies based on 83 individuals from 31 groups sampled across the distribution range of the subspecies, using 29 microsatellite loci. Our results revealed extremely low genetic diversity (HO = 0.21, HE = 0.29) compared to studies of other A. palliata populations and to other Alouatta species. Principal component analysis, a Bayesian clustering method, and analyses of molecular variance did not detect strong signatures of genetic differentiation among geographic populations of this subspecies. Although we detect small but significant FST values between populations, they can be explained by a pattern of isolation by distance. These results and the presence of unique alleles in different populations highlight the importance of implementing conservation efforts in multiple populations across the distribution range of A. p. mexicana to preserve its already low genetic diversity. This is especially important given current levels of population isolation due to the extreme habitat fragmentation across the distribution range of this primate. 相似文献
5.
6.
7.
8.
A partial swine cDNA which encodes the functional domain of PIT-1 was isolated by the polymerse chain reaction (PCR). The swine PIT-1 cDNA clone is 95% identical at the protein level to the rat Pit-1 gene. Thus, Pit-l's known function in control of rat growth hormone and prolactin expression is likely to be conserved in swine. This swine cDNA clone was used to investigate genetic variability at PIT-1 in several American and Chinese breeds. Polymorphic BamIII fragments were found in pure-bred Meishan animals (n= 13), but only monomorphic fragments in five American breeds (n= 36). 相似文献
9.
We isolated five polymorphic microsatellite loci from the drepanosiphid aphid Tuberculatus quercicola (Matsumura) that is associated with the Daimyo oak, Quercus dentata Thunberg, using the magnetic particles method. The isolated loci were polymorphic, with four to 16 alleles in 40 aphids. Expected heterozygosities ranged from 0.4 to 0.82. These loci can be used to quantify seasonal changes in clonal diversity in the metapopulation and the extent of clonal mixing in the colonies. 相似文献
10.
1. Genetic polymorphisms of flowering plants can influence pollinator foraging but it is not known whether heritable foraging polymorphisms of pollinators influence their pollination efficacies. Honey bees Apis mellifera L. visit cranberry flowers for nectar but rarely for pollen when alternative preferred flowers grow nearby. 2. Cranberry flowers visited once by pollen‐foraging honey bees received four‐fold more stigmatic pollen than flowers visited by mere nectar‐foragers (excluding nectar thieves). Manual greenhouse pollinations with fixed numbers of pollen tetrads (0, 2, 4, 8, 16, 32) achieved maximal fruit set with just eight pollen tetrads. Pollen‐foraging honey bees yielded a calculated 63% more berries than equal numbers of non‐thieving nectar‐foragers, even though both classes of forager made stigmatic contact. 3. Colonies headed by queens of a pollen‐hoarding genotype fielded significantly more pollen‐foraging trips than standard commercial genotypes, as did hives fitted with permanently engaged pollen traps or colonies containing more larvae. Pollen‐hoarding colonies together brought back twice as many cranberry pollen loads as control colonies, which was marginally significant despite marked daily variation in the proportion of collected pollen that was cranberry. 4. Caloric supplementation of matched, paired colonies failed to enhance pollen foraging despite the meagre nectar yields of individual cranberry flowers. 5. Heritable behavioural polymorphisms of the honey bee, such as pollen‐hoarding, can enhance fruit and seed set by a floral host (e.g. cranberry), but only if more preferred pollen hosts are absent or rare. Otherwise, honey bees' broad polylecty, flight range, and daily idiosyncrasies in floral fidelity will obscure specific pollen‐foraging differences at a given floral host, even among paired colonies in a seemingly uniform agricultural setting. 相似文献