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   189篇
  免费   17篇
  2016年   10篇
  2015年   12篇
  2014年   10篇
  2013年   15篇
  2012年   15篇
  2011年   9篇
  2009年   1篇
  2008年   2篇
  2005年   1篇
  2004年   6篇
  2003年   7篇
  2002年   20篇
  2001年   30篇
  2000年   25篇
  1999年   21篇
  1998年   21篇
  1993年   1篇
排序方式: 共有206条查询结果,搜索用时 296 毫秒
121.
The holotype of the lower-middle Pliocene hyaenid Lycyaenops rhomboideae is redescribed and compared with contemporaneous hyaenids of the genera Lycyaena, Hyaenictis, Chasmaporthetes and Pliocmcuta. These comparisons show that the material represents a valid and distinct genus and species. The genus Lycyaenops is referred to the Chasmaporthetes lineage on the basis of its two-cusped m 1 talonid with reduced entoconid, reduced posterolingual cingulum cusp on p4 and premolar accessory cusps set in a straight line. It is distinguished from all other genera of that lineage by its smaller premolar accessory cusps, broad premolars and squared-off and very broad posterior premolar shelves. The species L. silberbergi, previously assigned to Chasmaporthetes, is also referred to Lycyaenops. It differs from L. rhomboideae in its greater development of the premolar accessory cusps and less developed posterior premolar shelves, but shares the broad, squared-off premolars. The interrelationships of Hyaenictis, Chasmaporthetes and Lycyaenops are at present best described by an unresolved trichotomy, with Lycyaena as its sister taxon.  相似文献   
122.
Melanthiaceae (Liliales) comprise 17 genera of rhizomatous or bulbous perennials and are distributed across the Northern Hemisphere. The relationships among the five tribes in this family have been evaluated in many molecular and morphological studies. In this study, we performed a phylogenetic analysis of the 17 genera, including 106 species of Melanthiaceae sensu APG III and nine related species as outgroups, based on sequences of five plastid regions (atpB, rbcL, matK, ndhF and trnL‐F). Support values for the monophyly of the family (BSMP = 96%, BSML = 100%, PPBI = 1.00) and each tribe were improved in comparison with previous studies. Among the tribes, Melanthieae were sister to the remainder of the family and sister relationships between Xerophylleae and Parideae (BSMP = 96%, BSML = 100%, PPBI = 1.00) and Chionographideae and Heloniadeae (BSMP = 96%, BSML = 100%, PPBI = 1.00) were confirmed. Notably, the generic concept of Veratrum s.l. including Melanthium was not supported in the present study and these genera should be treated as distinct. In the case of Parideae, the relationship of Trillium govanianum to the other species remains uncertain and requires further studies. Finally, we mapped seven representative morphological characters onto the molecular phylogenetic tree for Melanthiaceae.  相似文献   
123.
Morphometric comparison of three taxa of the Senecio nemorensis group, namely S. germanicus Wallr., S. nemorensis L. and a new species S. dacicus Hodálová & Marhold is presented. S. dacicus, representing plants with eight ligules from the Romanian Carpathians, previously identified as S. nemorensis L., is shown to be clearly distinct from the Siberian plants of S. nemorensis, which include the lectotype of this Linnaean name. Detailed data on the distribution of S. dacicus are presented. Numerical methods used include principal components analysis, cluster analysis, and canonical and non-parametric classificatory discriminant analysis.  相似文献   
124.
Uroleucon is one of only two genera of Aphidinae to have undergone significant speciation in South America, with a group of about 14 endemic, morphologically similar species feeding on native Compositae, especially Baccharis . Multivariate morphometrics (canonical variates analysis, CVA) was used to discriminate between these species and compare them with the North American subgenus Lambersius , in which most of them have hitherto been placed. It is concluded that they probably form a separate monophyletic group. Four new species are described in this group— brevisiphon, essigi, petrohuense and pseudomuennosum— and five names are placed in synonymy. Of the other species of Uroleucon in South America, U. lizerianum is widespread and feeds on many species of native and introduced Compositae; it is morphometrically inseparable from the North American species U. ambrosiae and is probably an introduced form of that species, but has consistently fewer secondary rhinaria. A key is provided to the 19 available species of Uroleucon recorded from South America.  相似文献   
125.
The quantification of lobule shape for Radula spp. shows that there is overlap in lobule shape space occupied by subgenera, such that lobule shape does not always reflect relationships. Morphological convergence caused by lineages repeatedly traversing shared regions of morphospace appears commonplace in Radula, and means that many pairs of relatively unrelated species have similar lobule shapes. When observed over time, as in comparisons between fossil and extant species, this may give the impression of stasis if fossil species resemble modern species by chance, independent of their relatedness. This poses a challenge to relating fossils of known age to extant lineages, particularly when fossils are sterile. Significant rate variation between lineages was identified by Adams' Q‐mode analysis, with the fastest subgenus evolving 23 times more quickly than the slowest. Species of subgenus Volutoradula and subgenus Metaradula are apparently over‐dispersed throughout lobule morphospace according to Sidlauskas' method; morphometric branch lengths and hypervolumes in other subgenera can be explained by a stochastic process. In contrast, Bayesian analysis of macroevolutionary mixtures (BAMM) identified a single evolutionary rate as having the highest posterior probability. Consideration of the three independent accessions into auriculate lobule morphospace by Cladoradula and Radula, wherein convergent lobule shapes result from convergent lobule ontogenies and are correlated with bipinnately branched shoot systems and robust primary stems, leads to an ontogenetic hypothesis driven by structural requirements for light interception, under which auriculate lobules are a spandrel. It is speculated that lobules themselves, however, may be a key innovation facilitating radiation into microsites devoid of or depauperate in fungal endophytes. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 178 , 222–242.  相似文献   
126.
127.
The pseudotympanum, a reduction of the hypaxialis muscle in the body wall lateral to the anterior portion of the swim bladder, is unique to some members of the Otophysi, the largest clade of primarily freshwater fishes. Prior studies documented the presence of the pseudotympanum in a number of groups within the Characiformes, to a lesser extent in subunits of the Siluriformes, and in a few taxa of the Cypriniformes, but with only one record of the aperture in the Gymnotiformes – the Neotropical electric fishes. Surveys across the five families and c. two‐thirds of the genera in the Gymnotiformes revealed the occurrence of a pseudotympanum in all examined taxa. Pronounced variation in the expanse and details of the morphology of the opening and its relationship to the anterior elements of the axial skeleton was documented amongst Neotropical electric fish. Details of pseudotympanal morphology in the different families of the Gymnotiformes are described and the variation evaluated within the context of the two alternative hypotheses of higher‐level phylogenetic relationships within the order. © 2015 The Linnean Society of London  相似文献   
128.
Cutler suggested almost 30 years ago that there was convergent evolution between African and Australian Restionaceae in the distinctive culm anatomical features of Restionaceae. This was based on his interpretation of the homologies of the anatomical features, and these are here tested against a 'supertrec' phylogeny, based on three separate phvlogenies. The first is based on morphology and includes all genera; the other two are based on molecular sequences from the chloroplast genome-, one covers the African genera, and the other tin-Australian genera. This analysis corroborates Cutler's interpretation of convergent evolution between African and Australian Restionaceae. However, it indicates that for the Australian genera, the evolutionary pathway of the culm anatomy is much more complex than originally thought. In the most likely scenario, the ancestral Restionaceae have protective cells derived from the chlorenchyma. These persist in African Restionaceae, but are soon lost in Australian Restionaceae. Pillar cells and sclerenchyma ribs evolve early in the diversification of Australian Restionaceae, but are secondarily lost numerous times. In some of the reduction cases, the result is a very simple culm anatomy, which Cutler had interpreted as a primitively simple culm type, while in other cases it appears as if the functions of the ribs and pillars may have been taken over by a new structure, protective cells developed from epidermal, rather than chlorenchyma, cells. Cutler suggested that this convergent evolution might have been in response to Tertiary climatic deterioration, but this study finds no strong corroborating evidence for this.  相似文献   
129.
Phylogeny of the Platyhelminthes and the evolution of parasitism   总被引:4,自引:0,他引:4  
Robust phylogenies provide the basis for interpreting biological variation in the light of evolution. Homologous features provide phylogenetically informative characters whereas homoplasious characters provide phylogenetic noise. Both provide evolutionary signal. We have constructed molecular and morphologically based phylogenies of the phylum Platyhelminthes using a recently revised morphological character matrix and complete 18S and two partial 28S rRNA gene sequences in order to evaluate the emergence and subsequent divergence of parasitic forms. In total we examine 65 morphological characters, 97 18S rDNA, 41 Dl domain 28S rDNA, and 49 D3-D6 domain 28S rDNA sequences. For the molecular data there were 748, 132 and 249 phylogenetically informative sites for the 18S, Dl and D3-D6 28S rDNA data sets respectively. Morphological and molecular phylogenetic solutions are incongruent but not incompatible, and using the principles of conditional combination (18S rDNA + morphology passing Templeton's test) they demonstrate: a single and relatively early origin for the parasitic Neodermata (including the cestodes, trematodes and monogeneans); sister-group status between the cestodes and monogeneans, and between these taxa and the trematodes (digeneans and aspidogastreans). The sister-group to the Neodermata is likely to be a large clade of neoophoran turbellarians, based on combined evidence, or a clade consisting of the Fecampiid + Urastomid turbellarians, based on morphological evidence alone. The combined evidence solution for the phylogeny of fiatworms based on 18S rDNA and morphology is used to interpret morphological and life-history data and to support a model for the evolution and radiation of neodermatan parasites in the group.  相似文献   
130.
The leaf anatomy of species representing all seven genera of the Penaeaceae was studied by light and scanning electron microscopy. Due to variability and inconsistency, leaf anatomical characters are not regarded as particularly useful for systematics within or among genera in this family. Across the family, a number of taxa exhibit a trend towards amphistomatous, isobilateral leaves, generally associated with increased leaf thickness and amount of palisade mesophyll. This trend is not apparent in closely related families, e.g. Alzateaceae, Cryp-teroniaceae, Oliniaceae and Rhynchocalycaceae. Most species are found in comparatively mesic habitats and it is difficult to postulate primary xeromorphic trends in leaf anatomy. The sclerophylly encountered across the family as a whole is likely to be related to paucity of soil nutrients rather than a response to water stress.  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