In evolutionary biology, predator-prey species pairs can be observed participating in evolutionary arms races between adaptations and counter-adaptations. For example, as a prey becomes more adept at avoiding capture, its predator becomes a more adept hunter. The rough-skinned newt (Taricha granulosa) produces a toxin that protects it from virtually all predators, except one. That one predator is the common garter snake (Thamnophis sirtalis), which has evolved resistance to this toxin. This predator-prey pair is seemingly engaged in a perpetual battle for higher toxicity and better resistance. While both adaptations come with costs, the coexistence of newt and garter snake imposes reciprocal selective pressure that drives this arms race.

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