Alternative explanations for the major shifts

Several alternative explanations have been proposed to account for the formation and maintenance of GC-rich isochores (see Table 12.4). 

Natural selection thermodynamic stability hypothesis (Bernardi Bernardi, 1986a) 

From a general point of view, all the alternative hypotheses of Table 12.4 do not answer one or more of the following crucial questions which are answered by natural selection  

Some specific comments on the alternative hypotheses of Table 12.4 follow  

It has been repeatedly stressed that the depletion hypothesis (still considered as a pos- 

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