An heir and a spare are reckoned a desirable outcome of a royal wedding. Something similar seems to have applied during the evolution of plants. Lots of them have multiple sets of chromosomes in their cells, the result of developmental errors in their ancient ancestors. Spare chromosomes mean spare genes. And that allows the spares to mutate and evolve to do new jobs while the existing jobs are covered by the unchanged "heir" genes. Fleshy fruits such as tomatoes are believed to have evolved this way, as are the root nodules in which many legumes give houseroom to bacteria that fix atmospheric nitrogen, thus making it available to turn into protein.
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