Coevolution · The Red Queen
Two species evolve against each other at the same time. When predators get faster, slow prey are eaten and the prey speed up — which starves the slow predators, so they speed up too. Neither side ever wins: they run faster and faster just to stay in the same place. That is the Red Queen. Change the terrain to switch which weapon the race is fought with.
In a normal selection story, a species adapts to a fixed environment. Here the environment fights back: each species is the other's environment, and both evolve at once. This is coevolution, and a predator-prey arms race is its clearest form.
Watch the chart. Both lines climb — prey and predators keep getting faster (or stealthier) generation after generation. But the gap between them barely changes. A prey that is "fast" is only fast relative to today's predators; tomorrow's predators have caught up. This is the Red Queen hypothesis: like the character in Alice who runs to stay still, each species must keep evolving just to maintain the same chance of survival.
Notice that a trait only escalates when the terrain rewards it. On open plains the race is about speed; in dense cover it shifts to camouflage versus sharp eyesight. The unused weapon stops improving, because carrying it costs energy for no benefit. Selection always acts on what matters here and now.