In the Pacific Northwest, a quiet evolutionary war has been raging for millions of years between the rough-skinned newt (Taricha granulosa) and the common garter snake (Thamnophis sirtalis). The newt defends itself using tetrodotoxin (TTX), a potent neurotoxin that blocks sodium channels in nerve cells, leading to paralysis and death. While TTX is lethal to almost any creature that ingests it, certain populations of garter snakes have developed genetic mutations that grant them immunity to the poison.
As these resistant snakes predominated, they exerted intense natural selection on the newts, favoring individuals with increasingly extreme levels of TTX. In response, snakes evolved even higher resistance, creating a coevolutionary arms race. Today, some newts possess enough toxin in their skin to kill 25 adult humans, a level of lethality vastly disproportionate to any other predator in their ecosystem.
This extreme toxicity is unnecessary against birds or mammals, serving solely to deter the highly resistant garter snakes sharing their habitat. Scientists studying this dynamic have uncovered how precise amino acid substitutions in the snake's sodium channel proteins prevent TTX from binding, showcasing one of the clearest examples of reciprocal natural selection in nature.