Take the snake example: imagine the dose delivered is ingested by most (if not all) the population. Now imagine that some snakes have a mutation that render thi

Take the snake example: imagine the dose delivered is ingested by most (if not all) the population. Now imagine that some snakes have a mutation that render this dose non-lethal, either making it completely harmless or just sub-lethal. The snakes carrying the mutation would survive. The key part here (and in evolutionary theory) is that there is a pre-existing mutation to select for. If there isn't one, then we should see a local extinction.