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21.
M. J. D. White 《Australian Journal of Entomology》1970,9(1):1-6
i
The essence of speciation lies in the origin of genetic isolating mechanisms. Argument has raged as to whether the primary isolating mechanisms which initiate specific divergence operate at the pre-mating or post-mating level and whether complete geographic isolation necessarily precedes divergence. Speciation is essentially a cytogenetic process and the role of chromosomal rearrangements as primary isolating factors has probably been underestimated in the past. But speciation depends on many factors such as population size, vagility and type of life cycle, so it is unlikely that any universal model is applicable. 相似文献
The essence of speciation lies in the origin of genetic isolating mechanisms. Argument has raged as to whether the primary isolating mechanisms which initiate specific divergence operate at the pre-mating or post-mating level and whether complete geographic isolation necessarily precedes divergence. Speciation is essentially a cytogenetic process and the role of chromosomal rearrangements as primary isolating factors has probably been underestimated in the past. But speciation depends on many factors such as population size, vagility and type of life cycle, so it is unlikely that any universal model is applicable. 相似文献
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P. White 《BMJ (Clinical research ed.)》1989,298(6682):1199-1200
25.
M. D. Furones † D. J. Alderman D. Bucke T. C. Fletcher ‡§ D. Knox ‡ A. White ‡§ 《Journal of fish biology》1992,41(6):1037-1041
Rainbow trout fed diets containing 7, 86 or 806 mg vitamin E kg−1 for 22 weeks were exposed to virulent Yersinia ruckeri by bath or injection. Mortalities were always least among those fed the highest concentration of vitamin E but serum antibody production was not affected by vitamin E levels. 相似文献
26.
In this paper we report the observation of a rapidly developed vertebrate symbiosis involving ectoparasite cleaning by a native corvid of northern Australia, the Torresian crow Corvus orru , on a recently introduced bovid ungulate, the banteng Bos javanicus . Facultative symbioses benefiting both participants (mutualisms) between birds and mammals appear to be rare, despite the apparent advantages obtained by the participants (ecto-parasite removal from the host and food provision to the cleaner). On three separate dates we observed a total of four C. orru individuals eliciting facilitation behaviours by a total of ten female banteng to assist in the removal of ectoparasites. Our observations document the first-known incidence of facultative cleaning behaviour by a native bird species on a non-native, wild vertebrate that has developed in approximately 150 years since the banteng's introduction to Australia. 相似文献
27.
Fungal sporocarps are described from Triassic silicified peat deposits from Antarctica. Sporocarps possess a two-layered wall and contain a single spore. The outer layer is mycelial; the inner layer, noncellular. The combination of primitive and advanced features suggests that this fungus is intermediate in complexity between the lower and evolutionarily more advanced fungi. The Antarctic fungus and morphologically similar fossils resemble extant members of the Endogonaceae, but appear to have been saprophytes rather than mycorrhizal symbionts. 相似文献
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29.
Variation of lumbar spine stiffness with load 总被引:1,自引:0,他引:1
W T Edwards W C Hayes I Posner A A White R W Mann 《Journal of biomechanical engineering》1987,109(1):35-42
Mechanical studies of the Functional Spinal Unit (FSU) in-vitro have shown that the slopes of the load-displacement curves increase with load. This nonlinearity implies that the stiffness of the FSU is not constant over the range of physiologic loads, and that measurements obtained for FSU specimens through the application of individual loads cannot be summed to predict the response of the specimens to combined loads. Both experimental and analytical methods were developed in the present study to better quantify the nonlinear FSU load-displacement response and to calculate the coupled stiffness of FSU specimens at combined states of load reflecting in-vivo conditions. Results referenced to the center of the vertebral body indicate that lumbar FSU specimens are stiffer in flexion than in extension, and that FSU specimens loaded in flexion are stiffer at high loads than at low loads. The importance of combined load testing and a nonlinear interpretation of load-displacement data is demonstrated. 相似文献
30.
Manganese cycling in an acidic Adirondack lake 总被引:4,自引:4,他引:0
There is considerable interest in the chemistry of Mn in acidic waters because of its role in the generation of acid neutralizing capacity during reduction processes, as an adsorbent in element cycling, and as a potential toxicant to aquatic organisms. Temporal and spatial variations in the concentration of Mn were evident in acidic Dart's Lake (1.0–2.3 mol l–1), located in the Adirondack Region of New York. Seasonal changes in pH and dissolved oxygen concentration had subtle effects on the chemistry and transport of Mn. Despite oversaturation with respect to the solubility of manganite during periods of stratification, vertical deposition of Mn was minimal. The conservative nature of Mn appears to be due to the acidic conditions in Dart's Lake. 相似文献