For example, the insulin gene causes only one small step in the enormously complicated process of its own replication (that process being the whole life cycle of the organism). But the overwhelming majority of variants of that gene would not instruct cells to manufacture a chemical that could do the job of insulin. If the insulin genes in an individual organism’s cells were replaced by slightly different molecules, that organism would die (unless it were kept alive by other means), and would therefore fail to have offspring, and those molecules would not be copied. So whether copying takes
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