Answer:
Rough-skinned newts and common garter snakes are in an "evolutionary arms race." In this phenomenon, two species continually evolve adaptations against one another to increase their own chance of survival. Rough-skinned newts produce tetrodotoxin, a type of poison, as a defense mechanism. Most common garter snakes have some resistance to the toxin. In areas where common garter snakes and rough-skinned newts coexist, rough-skinned newts express very high levels of tetrodotoxin, and common garter snakes have evolved a very high degree of resistance to tetrodotoxin.