For one thing, we now know that only one mutation caused birds to lose the ability to produce teeth. (Beaks are an advantage in some situations, but a disadvantage in others).
Even more cool is a species of deep water worm. These worms eat bones - they eat the skeletons of whales and bony fish that fall to the bottom of the ocean. And in almost all of them the males have shrunk. Females keep harems and the males are permanently attached to them.
Almost all. They just found a new species in which the sexes are the same size - and they're saying it's an evolutionary reversal - a return to an older state. This doesn't happen very often because unused genes tend to slowly atrophy or disappear (just like the genes that would make teeth in birds). And these males are, of course, fully mobile.
Turning stuff back on? Evolution doesn't do that very often.
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