This National Geographic story showed up in another subreddit this morning. I was confused by an aspect of it, but unfortunately the comments on the submission are a catastrophe, so I'm asking here instead.
In the story as written — I looked for but did not find the referenced paper — it's said that a particular type of fish is sometimes found in nature to have a variation that affects the expression of a particular protein. This protein offers a resistance to the negative effects of a particular toxic substance.
Fish of this kind that live in a river with a high concentration of tha
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