different GC* levels and by fairly homogeneous base compositions (at least in the 3–300 kbase range); and (ii) seem to correspond to the DNA segments present in

作者: GIORGIO BERNARDI , GIA COMO BERNARDI

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摘要: The evolution of living organisms is caused primarily by mutations that may subsequently be eliminated or become fixed in the genome. While it is generally agreed that elimination affects deleterious mutations and occurs by negative selection, fixation has been visualized as due either (i) to positive Darwinian selection acting on advantageous mutations or (ii) to random genetic drift acting on selectively neutral (ie selectively equivalent) mutations. Since both advan-tageous and neutral mutations definitely can be fixed in evolution, the issue is of a quantitative and not of a qualitative nature and concerns the predominance of deterministic or stochastic events in genome evolution. That the issue is a difficult one is proved by the fact that this debate has gone on, in the form just stated, for almost twenty years.We will summarize here a new approach to the problem (see [1–3] for recent reports). The question we have …

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