eukaryotes, the genetic material is locked inside the protected vault of the nucleus. That material is released only under the most stringent conditions, using rules less promiscuous and more orderly than those of prokaryotes, and these rules affect how eukaryotic cells evolve. When eukaryotes produce germ cells—eggs and sperm, the cells from which their offspring will be formed—they don’t just copy their DNA. They stir it around first. They swap some of their genetic material with another individual of their species so that the offspring of the two parents gets a random selection of genes,
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