Now, it is true that genes occasionally move between species; the genome of many species appears to be somewhat more fluid than scientists used to think. Yet for reasons we don’t completely understand, distinct species do exist in nature, and they exhibit a certain genetic integrity—sex between them, when it does occur, doesn’t produce fertile offspring. Nature presumably has some reason for erecting these walls, even if they are permeable on occasion. Perhaps, as some biologists believe, the purpose of keeping species separate is to put barriers in the path of pathogens, to contain their
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