Punctuated Equilibrium theory (I have no idea if it's still valid) states that evolution goes through stages of acceleration... So I guess it's possible that we're in a stage of slowness relative to the stages of speed...

The Red Queen theory (named for the Red Queen from Alice in Wonderland) says that a creature needs to continue to evolve constantly in order to simply maintain it's position in the food chain, the way that Alice had to keep running as fast as she possibly could in order to remain in the same place.

That doesn't really explain creatures who have evolved very little for long periods of time, like sharks, but as long as a single one with a mutation who does exactly as well as each other shark in it's generation will not encourage evolution, I guess there's no reason for them to change.

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