全文获取类型
收费全文 | 190篇 |
免费 | 0篇 |
国内免费 | 2篇 |
专业分类
系统科学 | 21篇 |
教育与普及 | 3篇 |
理论与方法论 | 2篇 |
现状及发展 | 13篇 |
研究方法 | 31篇 |
综合类 | 117篇 |
自然研究 | 5篇 |
出版年
2019年 | 1篇 |
2018年 | 2篇 |
2015年 | 2篇 |
2014年 | 2篇 |
2012年 | 14篇 |
2011年 | 22篇 |
2010年 | 9篇 |
2009年 | 4篇 |
2008年 | 13篇 |
2007年 | 14篇 |
2006年 | 16篇 |
2005年 | 21篇 |
2004年 | 18篇 |
2003年 | 24篇 |
2002年 | 18篇 |
2001年 | 1篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 2篇 |
排序方式: 共有192条查询结果,搜索用时 296 毫秒
181.
If a common ancestor of all living humans is defined as an individual who is a genealogical ancestor of all present-day people, the most recent common ancestor (MRCA) for a randomly mating population would have lived in the very recent past. However, the random mating model ignores essential aspects of population substructure, such as the tendency of individuals to choose mates from the same social group, and the relative isolation of geographically separated groups. Here we show that recent common ancestors also emerge from two models incorporating substantial population substructure. One model, designed for simplicity and theoretical insight, yields explicit mathematical results through a probabilistic analysis. A more elaborate second model, designed to capture historical population dynamics in a more realistic way, is analysed computationally through Monte Carlo simulations. These analyses suggest that the genealogies of all living humans overlap in remarkable ways in the recent past. In particular, the MRCA of all present-day humans lived just a few thousand years ago in these models. Moreover, among all individuals living more than just a few thousand years earlier than the MRCA, each present-day human has exactly the same set of genealogical ancestors. 相似文献
182.
Smallwood S 《Population trends》2004,(117):20-26
This article explores the characteristics of live births where no father was present on the birth certificate (sole registrations) using registration data. It then uses data from the ONS Longitudinal Study to examine some characteristics of mothers who have ever experienced a sole registered live birth. It shows that as a proportion of all births, sole registrations have remained fairly constant over the last two decades, although since 1998 there is some evidence of a fall in the proportion. For mothers born between 1955 and 1962 around nine per cent experienced a sole registration. Those who ever experienced a sole registration were around four years younger when they began their childbearing. These women also had larger families and were more likely to come from a lower social class background. 相似文献
183.
Mutations in CLCN2 encoding a voltage-gated chloride channel are associated with idiopathic generalized epilepsies 总被引:24,自引:0,他引:24
Haug K Warnstedt M Alekov AK Sander T Ramírez A Poser B Maljevic S Hebeisen S Kubisch C Rebstock J Horvath S Hallmann K Dullinger JS Rau B Haverkamp F Beyenburg S Schulz H Janz D Giese B Müller-Newen G Propping P Elger CE Fahlke C Lerche H Heils A 《Nature genetics》2003,33(4):527-532
Idiopathic generalized epilepsy (IGE) is an inherited neurological disorder affecting about 0.4% of the world's population. Mutations in ten genes causing distinct forms of idiopathic epilepsy have been identified so far, but the genetic basis of many IGE subtypes is still unknown. Here we report a gene associated with the four most common IGE subtypes: childhood and juvenile absence epilepsy (CAE and JAE), juvenile myoclonic epilepsy (JME), and epilepsy with grand mal seizures on awakening (EGMA; ref. 8). We identified three different heterozygous mutations in the chloride-channel gene CLCN2 in three unrelated families with IGE. These mutations result in (i) a premature stop codon (M200fsX231), (ii) an atypical splicing (del74-117) and (iii) a single amino-acid substitution (G715E). All mutations produce functional alterations that provide distinct explanations for their pathogenic phenotypes. M200fsX231 and del74-117 cause a loss of function of ClC-2 channels and are expected to lower the transmembrane chloride gradient essential for GABAergic inhibition. G715E alters voltage-dependent gating, which may cause membrane depolarization and hyperexcitability. 相似文献
184.
185.
The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes 总被引:2,自引:0,他引:2
Skaletsky H Kuroda-Kawaguchi T Minx PJ Cordum HS Hillier L Brown LG Repping S Pyntikova T Ali J Bieri T Chinwalla A Delehaunty A Delehaunty K Du H Fewell G Fulton L Fulton R Graves T Hou SF Latrielle P Leonard S Mardis E Maupin R McPherson J Miner T Nash W Nguyen C Ozersky P Pepin K Rock S Rohlfing T Scott K Schultz B Strong C Tin-Wollam A Yang SP Waterston RH Wilson RK Rozen S Page DC 《Nature》2003,423(6942):825-837
186.
187.
At temperatures up to about 80 degrees C, petroleum in subsurface reservoirs is often biologically degraded, over geological timescales, by microorganisms that destroy hydrocarbons and other components to produce altered, denser 'heavy oils'. This temperature threshold for hydrocarbon biodegradation might represent the maximum temperature boundary for life in the deep nutrient-depleted Earth. Most of the world's oil was biodegraded under anaerobic conditions, with methane, a valuable commodity, often being a major by-product, which suggests alternative approaches to recovering the world's vast heavy oil resource that otherwise will remain largely unproduced. 相似文献
188.
189.
190.