1,720,979 research outputs found
Tracing the Evolution of the Galápagos Iguanas: A Molecular Approach
No full textRassmann K, Markmann M, Tautz D, Trillmich F. Tracing the Evolution of the Galápagos Iguanas: A Molecular Approach. In: Alberts AC, Carter RL, Hayes WK, Martins EP, eds. Iguanas. Biology and Conservation. Berkeley: University of California Press; 2004: 71-83
Hybridization between the Galapagos land and marine iguana (Conolophus subcristatus and Amblyrhynchus cristatus) on Plaza Sur
No full textRassmann K, Trillmich F, Tautz D. Hybridization between the Galapagos land and marine iguana (Conolophus subcristatus and Amblyrhynchus cristatus) on Plaza Sur. JOURNAL OF ZOOLOGY. 1997;242(4):729-739.Hybridization plays an important role in the evolution of some of the vertebrate taxa on the Galapagos Islands, such as the Darwin finches. Conversely, only a single possible hybrid between the Galapagos marine iguana (Amblyrhynchus) and the land iguana (Conolophus) has been reported from the island Plaza Sur. In this paper, the hybrid status of a morphologically unusual iguana from this island is confirmed, using restriction fragment length polymorphism (RFLP) analyses of the nuclear ribosomal DNA (rDNA). Sequencing of the hybrid's mitochondrial cytochrome b gene revealed that it was the offspring of a female land iguana and a male marine iguana. Preliminary molecular analyses of morphologically typical marine and land iguanas from Plaza Sur did not detect introgression of nuclear or mitochondrial markers between species. Thr potential significance of hybridization for the evolution of the Plaza Sur iguana populations is discussed
The microevolution of the Galapagos marine iguana Amblyrhynchus cristatus assessed by nuclear and mitochondrial genetic analyses
No full textRassmann K, Tautz D, Trillmich F, Gliddon C. The microevolution of the Galapagos marine iguana Amblyrhynchus cristatus assessed by nuclear and mitochondrial genetic analyses. MOLECULAR ECOLOGY. 1997;6(5):437-452.Marine iguanas may have inhabited the Galapagos archipelago and its former, now sunken islands for more than 10 million years (Myr). It is therefore surprising that morphological and immunological data indicate little evolutionary divergence within the genus. We utilized mitochondrial DNA (mtDNA) sequence analyses and nuclear DNA fingerprinting to re-evaluate the level and pattern of genetic differentiation among 22 marine iguana populations from throughout the archipelago. Both genetic marker systems detect a low level of within-genus divergence, but they show contrasting levels of geographical subdivision among the populations. The mitochondrial gene pools of populations from different regions of the archipelago are isolated, and the mtDNA pattern appears to follow the sequence in which the islands were colonized by marine iguanas. Conversely, the nuclear DNA study indicates substantial interpopulational gene exchange, and the geographical distribution of the nuclear markers seems to be determined by isolation by distance among the populations. The natural history of marine iguanas suggests that the contrasting nuclear and mitochondrial DNA patterns result from an asymmetric migration behaviour of the two sexes, with higher (active and passive) interisland dispersal for males than females. Separate genetic analyses for the sexes appear to support this hypophesis. Based on these findings, a scenario is proposed that explains the marine iguanas' low genetic divergence, notwithstanding their long evolutionary history in the Galapagos archipelago
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Are marine iguanas endanged on islands with introduced predators?
Get PDFCayot LJ, Rassmann K, Trillmich F. Are marine iguanas endanged on islands with introduced predators? Noticias de Galápagos. 1994;53:13-15
- …
