共查询到20条相似文献,搜索用时 31 毫秒
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H. James CleavesII 《Evolution》2012,5(3):342-360
How life on Earth began remains an unexplained scientific problem. This problem is nuanced in its practical details and the way attempted explanations feedback with questions and developments in other areas of science, including astronomy, biology, and planetary science. Prebiotic chemistry attempts to address this issue theoretically, experimentally, and observationally. The ease of formation of bioorganic compounds under plausible prebiotic conditions suggests that these molecules were present in the primitive terrestrial environment. In addition to synthesis in the Earth's primordial atmosphere and oceans, it is likely that the infall of comets, meteorites, and interplanetary dust particles, as well as submarine hydrothermal vent synthesis, may have contributed to prebiotic organic evolution. The primordial organic soup may have been quite complex, but it did not likely include all of the compounds found in modern organisms. Regardless of their origin, organic compounds would need to be concentrated and complexified by environmental mechanisms. While this review is by no means exhaustive, many of the issues central to the state of the art of prebiotic chemistry are reviewed here. 相似文献
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Tobias Heed 《Current biology : CB》2010,20(14):R604-R606
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《Current biology : CB》2020,30(10):R439-R441
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The average travelling speed increases in a nontrivial manner with the travel distance. This leads to scaling-like relations on quite extended spatial scales, for all mobility modes taken together and also for a given mobility mode in part. We offer a wide range of experimental results, investigating and quantifying this universal effect and its measurable causes. The increasing travelling speed with the travel distance arises from the combined effects of: choosing the most appropriate travelling mode; the structure of the travel networks; the travel times lost in the main hubs, starting or target cities; and the speed limit of roads and vehicles. 相似文献
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H C Pulley 《The Western journal of medicine》1968,108(3):241-242
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While Darwin (1862, 1877) showed that reproductive success in orchid populations depended on adaptive floral morphology coupled with pollinator visitation
a more recent review of the literature (Tremblay et al., 2005) confirmed that many out-breeding species are pollinator-limited because most orchid species showing low fecundity also lack
rewards. The absence of rewards depresses both pollinator fidelity and the frequency of pollinator visits to an orchid population
even though orchid flowers that lack rewards retain the same interlocking floral structures for precise pollinia removal and
deposition found in related species that offer rewards. Using the genus, Cypripedium, as a model lineage of non-rewarding flowers this study also shows that the correlation between low fruit set in a Cypripedium sp. and its specific pollinator(s) is insufficient to predict specific frequencies of low fecundity. Annual rates of fruit
set often vary broadly between populations of the same species and within the same population over several seasons. We speculate
that fruit-set rates also decline when orchid demography and additional biotic and abiotic factors interrupt rates of pollinator
activity (pre-zygotic) and fertilization/fruit maturation (post-zygotic). We suggest that that traditional field studies on
pollination ecology and breeding systems be combined with data sets recording genetic variation and orchid flower demography
in relation to seasonal variation in climate. We also propose that the same information be collected in regard to genetic
variation, demography and phenology of populations of known orchid pollinators and co-blooming angiosperm species native to
orchid habitats. 相似文献