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An evolutionary arms race between a species of North American snake and the newts on which they prey has been decisively won by the reptiles, in a remarkable example of natural selection in action.

Though rough-skinned newts ward off predators with a toxin so deadly that a single animal can make enough to kill a roomful of people, some garter snakes have developed total resistance that allows them to eat the amphibians with impunity.

This extreme resistance has been caused by a mutation that has altered just a single DNA letter of the snakes’ genomes, illustrating clearly how tiny and random genetic copying mistakes can drive evolution forward if they confer a major survival advantage.

Natural selection would have greatly favoured super-resistant snakes, which would have had more and more offspring until their progeny came to dominate entire local populations.

The skin of the rough-skinned newt secretes a poison called terodotoxin (TTX), which is lethal to most animals in very small quantities. The same poison is also made by the puffer-fish, and is responsible for the infamous toxic qualities of the sushi delicacy fugu when it is not expertly prepared.

Some newts produce enough TTX to kill up to 20 people, or more than a thousand mice. “Some populations of these newts may very well represent the most toxic amphibians on the planet,” said Charles Hanifin, of Stanford University in California, who has led new research into their interaction with snakes.

Human deaths are rare, as the newts must be eaten for the poison to work. At least one death is known, of a 29-year-old American man who swallowed a rough-skinned newt for a dare. A second man survived eating five to win a bet, though he became too weak to walk before recovering.

On the West Coast of North America, the newts live alongside garter snakes, a non-venomous species that hunts insects, worms and small birds, rodents, amphibians and fish.

Over many generations, the snakes have evolved a resistance to the TTX poison, as those that were capable of eating newts without dying or being incapacitated had a major advantage, and consequently had more offspring.

The newts, however, struck back, evolving greater and greater toxicity in response to the snakes’ resistance. As partially resistant snakes ate more poisonous newts, they would have been killed or put off pursuing them.

Snakes, in turn, have evolved greater resistance, in what scientists often describe as an evolutionary arms race.

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