The concept of “species” is commonly supposed to be secure. It isn’t secure. It’s especially insecure in the realm of bacteria and archaea, but it’s even a bit blurry when scientists try to distinguish one species of plant or one species of animal from another. The boundaries blur. The edges are as porous as Goretex—or, in some cases, as cheese cloth. One reason for the blur, one symptom of the porosity, is horizontal gene transfer—genes moving sideways instead of just downward from parent to offspring. If genes cross the boundary between one species of bacteria and another, then in what sense
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