Although the Brodies discovered and studied the arms race between the rough-skinned newt and common garter snake, several other species of newts and garter snakes can also live in the same areas. After some searching and a tip-off by Jens Vindum and student Chris Feldman, the Brodies discovered a population in the southern Sierra Mountains where a different species of garter snake (the Sierra garter snake) and a different species of newt (the California newt) were also engaged in an arms race! The common garter snake also lives in this area — but it is not very resistant to TTX.
Why didn’t the common garter snake join this arms race, when it had joined in so many others? One possibility is that when the common garter snake arrived on the scene, the other newts and snakes had already evolved toxicity and toxicity-resistance — the fight had already escalated so far that the common garter snake simply couldn’t compete. Imagine this population of newly arrived common garter snakes. The population has some variation in TTX resistance — some snakes can handle a little bit of TTX and some cannot handle any — but they face a population of heavily armed, TTX-loaded newts. Even though some snakes can handle more TTX than others, none in the population are resistant enough to eat the already highly toxic newts; instead, they would have to eat other non-toxic organisms. Any common garter snake that tried to eat a newt would die and would not pass on its genes to its offspring. So in this situation, being mildly resistant offers no advantage and is not favored by natural selection. The common garter snake joined the fray with too little variation too late. The first combatants involved (and evolved!) in an arms race may outgun all other species, excluding them from the battle.