达尔文提出生物进化的内因是遗传变异，下列哪个过程为自然选择提供了这一基础

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='20848' and id < 1817992 and id<>'1817992' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='20848' and id > 1817992 and id<>'1817992' order by id asc limit 5