首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1125篇
  免费   123篇
  国内免费   212篇
  1460篇
  2024年   11篇
  2023年   40篇
  2022年   17篇
  2021年   30篇
  2020年   49篇
  2019年   50篇
  2018年   31篇
  2017年   36篇
  2016年   42篇
  2015年   43篇
  2014年   41篇
  2013年   60篇
  2012年   50篇
  2011年   45篇
  2010年   59篇
  2009年   56篇
  2008年   71篇
  2007年   98篇
  2006年   74篇
  2005年   63篇
  2004年   54篇
  2003年   44篇
  2002年   55篇
  2001年   41篇
  2000年   47篇
  1999年   40篇
  1998年   21篇
  1997年   32篇
  1996年   34篇
  1995年   13篇
  1994年   20篇
  1993年   23篇
  1992年   12篇
  1991年   8篇
  1990年   6篇
  1989年   7篇
  1988年   4篇
  1987年   4篇
  1986年   5篇
  1985年   7篇
  1984年   3篇
  1983年   1篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
  1979年   1篇
  1978年   2篇
  1977年   4篇
  1975年   1篇
  1958年   2篇
排序方式: 共有1460条查询结果,搜索用时 15 毫秒
1.
2.
A widespread trend in animals is the evolution of morphological ornaments and behaviours that are involved in aggressive and courtship displays. These display traits are important from the standpoint of communication, sexual selection, and speciation. Previous authors have suggested that the evolution of display morphology and display behaviour should be closely linked. In this study, I tested for this association with behavioural and morphological data for 59 taxa of phrynosomatid lizards using phylogenetic comparative methods (Mad-dison's concentrated changes test and Felsenstein's independent contrasts). The results showed little significant association between features of display morphology and behaviour, suggesting that the evolution of these traits is not tightly coupled. This decoupling is particularly evident in the genus Sceloporus , in which several species have lost the display coloration but retain unmodified display behaviour. The results also suggest that display morphology is more evolutionarily labile than display behaviour in this group.  相似文献   
3.
This research tested the utility of two classes of skin secretion compounds to the phylogeny of the Bufo crucifer group. Skin secretions from specimens of nine populations of B. crucifer group were obtained and submitted to qualitative analysis. We observed a clear difference in the composition of the skin secretion molecules obtained from the species of Bufo studied. Fifty-nine molecules, 16 indolealkylamines and 43 proteins, were used as characters, and 39 of these were parsimonious informative. The tree topology of the skin secretion combined data showed areas of congruence and conflict when compared to an mtDNA phylogeny of the B. crucifer group. We used the Templeton test to evaluate the heterogeneity between the skin secretion and mtDNA data. Although not recommended, we performed a combined analysis with the two partitions. The skin secretion characters from the species of Bufo studied have phylogenetic signal. These data are indicative, at least as a preliminary study, of the phylogenetic relationships among the B. crucifer group taxa.  相似文献   
4.
There is a striking difference in body size of jungle cats ( Felis chaus ) in the west and the east of their distribution, with Israeli cats being 43% heavier than Indian cats. We tested the hypothesis that increasing competition from other small felids towards the east is responsible for the difference in body size. We measured jungle cat skulls for eight cranial and dental variables and related these to independent variables such as species richness (local and regional), latitude, longitude, temperature, and precipitation. Data from a narrow band between latitudes 24.0°N and 33.9°N, where Bergmann's rule was largely not observed, showed that the western population (≤ 50.0°E longitude) of jungle cats is larger than the eastern (> 60.0°E longitude) population with the size difference being most evident in the upper carnassials (P4L). Species richness at the regional level showed a significant negative relation to P4L. An even spacing in condylobasal length for a small-cat guild from India through null model analysis indicated the occurrence of character displacement. The results support the hypothesis that competition is responsible for geographical variation in jungle cat body size in the region where Bergmann's rule does not apply. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 163–172.  相似文献   
5.
We hypothesize that the evolution of an ecologically important character, the host associations of specialized phytophagous insects, has been influenced by limitations on genetic variation. Using as a historical framework a phylogenetic reconstruction of the history of host associations in the beetle genus Ophraella (Chrysomelidae), we have employed quantitative-genetic methods to screen four species for genetic variation in larval survival, oviposition (in one species only), and feeding responses to their congeners' host plants, in the Asteraceae. We here report results of studies of one species and evaluate the results from all four. Analysis of half-sib/full-sib families and of progenies of wild females of O. notulata, a specialist on Iva (Ambrosiinae), provided evidence of genetic variation in larval consumption of five of six test plants and in adult consumption of four of six. Larval mortality was complete on five plants; only on Ambrosia, a close relative of the natural host, was there appreciable, and genetically variable, survival. Oviposition on Ambrosia showed marginally significant evidence of genetic variation; a more distantly related plant elicited no oviposition at all. In compiling results from four Ophraella species, reported in this and two other papers, we found no evidence of genetic variation in 18 of 39 tests of feeding responses and 14 of 16 tests of larval survival on congeners' hosts. This result is consistent with the hypothesis that absence or paucity of genetic variation may constrain or at least bias the evolution of host associations. The lower incidence of genetic variation in survival than in feeding behavior may imply, according to recent models, that avoidance is a more common evolutionary response to novel plants than adaptation. The usually great disparity between mean performance on congeners' hosts and the species' natural hosts, and an almost complete lack of evidence for negative genetic correlations, argue against the likelihood that speciation has occurred by sympatric host shift. The presence versus apparent absence of genetic variation in consumption was correlated with the propinquity of relationship between the beetle species tested and the species that normally feeds on the test plant, suggesting that the history of host shifts in Ophraella has been guided in part by restrictions on genetic variation. It was also correlated with the propinquity of relationship between a test plant and the beetle's natural host. The contributions of plant relationships and insect relationships, themselves correlated in part, to the pattern of genetic variation, are not readily distinguishable, but together accord with phylogenetic evidence that these and other phytophagous insects adapt most readily to related plants. In this instance, therefore, the macroevolution of an ecologically important character appears to have been influenced by genetic constraints. We hypothesize that absence of the structural prerequisites for genetic variation in complex characters may affect genetic variation and the trajectory of evolution.  相似文献   
6.
The remarkable diversity of fruits inValerianella andFedia is revealed by a comparative morphological and anatomical analysis. Characters are evaluated in regard to a new systematic grouping. The knowledge on dispersal biology is summarized and supplemented. Finally, salient features of fruit differentiation are discussed.
3. Teil der Publikationsserie Beiträge zur Systematik und Evolution vonValerianella undFedia (Valerianaceae) (vgl.Ernet 1977a, b).  相似文献   
7.
三台核桃结实特性研究   总被引:3,自引:0,他引:3  
刘德良  张琴 《广西植物》2001,21(1):67-71
选择标准株 70株 ,其中有草果核桃 ( cv.Zaogu)、大油笼核桃 ( cv.Dayoulong)、早熟核桃 ( cv.Zaoshu)、小油笼核桃 ( cv. Xiaoyoulong)、荚绵核桃 ( cv. Jiamian) 5个品种。据对不同品种、海拔、年龄的结实特性研究 ,结果表明 :草果核桃是优良的栽培品种 ,早熟核桃结实性状表现最差 ,果枝粗是影响座果率最重要的因素。  相似文献   
8.
顾海峰  孙军  邹迎麟  方琦  蓝东兆 《生态学报》2006,26(4):1021-1027
在2003年厦门港发生的一次裸甲藻赤潮中分离得到了纤小裸甲藻(Takayama pulchellum),并对其形态学进行了光镜和电镜的观察,该株藻细胞个体较小,长约20~23μm,宽约14~20μm.藻细胞外表卵圆形,细胞上壳有一条明显的S形顶沟,顶沟是特异性的反S形.与模式种相比,这一株裸甲藻细胞核更靠上部,大部分细胞横沟都没有偏移.纤小裸甲藻在盐度为28时比生长率最大,达到0.42,随着盐度下降,比生长率也跟着下降,当盐度下降到16时,生长率为0.盐度范围在8~16时,超过80%的藻类能存活48h;当盐度低于4时,生长率和存活率都为0.24~27℃是纤小裸甲藻的最适生长温度,比生长率超过0.50,当温度升到30℃时,生长率急剧下降到约0.35.毒素测定结果显示该株裸甲藻不含麻痹性贝毒和神经性贝毒.本株纤小裸甲藻大亚基D1~D2区序列长度为721bp,与基因库中该种的一株相似种同源性超过99%.对18株裸甲藻转录间隔区(ITS)序列建立的系统进化树显示,纤小裸甲藻和Karlodinium micrum的距离最近,通过ITS序列建立的系统发育树大致能够把Akashiwo属、Karenia属、Karlodinium属和Takayama属与Gymnodinium属区分开来.  相似文献   
9.
1. Impacts of global change on the distribution, abundance, and phenology of species have been widely documented. In particular, recent climate change has led to widespread changes in animal and plant seasonality, leading to debate about its potential to cause phenological mismatches among interacting taxa. 2. In mountainous regions, populations of many species show pronounced phenological gradients over short geographic distances, presenting the opportunity to test for effects of climate on phenology, independent of variation in confounding factors such as photoperiod. 3. Here we show for 32 butterfly species sampled for five years over a 1700 m gradient (560–2260 m) in a Mediterranean mountain range that, on average, annual flight period is delayed with elevation by 15–22 days per kilometre. Species mainly occurring at low elevations in the region, and to some extent those flying earlier in the year, showed phenological delays of 23–36 days per kilometre, whereas the flight periods of species that occupy high elevations, or fly in late summer, were consistently more synchronised over the elevation gradient. 4. Elevational patterns in phenology appear to reflect a narrowing phenological window of opportunity for larval and adult butterfly activity of high elevation and late‐flying species. 5. Here, we speculate as to the causes of these patterns, and the consequences for our ability to predict species responses to climate change. Our results raise questions about the use of space–time substitutions in predicting phenological responses to climate change, since traits relating to flight period and environmental associations may influence the capacity of species to adapt to changing climates.  相似文献   
10.
薜荔和爱玉及其传粉昆虫繁殖特性   总被引:16,自引:0,他引:16       下载免费PDF全文
薜荔(Ficus pumila L.var.pumila)隶属桑科榕属,爱玉(F.pumila L.var.awkeotsanmg Corner)为其变种,它们的花是单性的,雌雄异株。雌花序中着生雌花,雄花序中有瘿花和雄花,每个花序中花的数量极多,达4000~6000朵。薜荔榕小蜂是唯一能进入薜荔和爱玉的隐头花序中产卵或传粉的共生昆虫,自然状态下雌花的结实率分别为82%、83.52%;瘿花的成虫瘿率分别为58.71%、51.32%,因此可形成大量的果实和虫瘿。物候观察表明薜荔和爱玉花期不遇,它们花序中的榕小蜂种群已经生殖隔离。人为的放蜂实验表明,生活于爱玉花序中的榕小蜂,已无法在薜荔花序中繁殖,生殖隔离进一步得到证实;实验同时表明爱玉的花粉亦不能使薜荔雌花结实,宿主两变种间生理上已不亲和。本文从共生双方协同进化的角度出发,探讨了榕树2变种间与传粉昆虫繁殖特性的差异,以及变种产生的主要原因。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号