In the frigid waters of the ocean surrounding Antarctica, fish have a special trait which allows them to survive the big chill. As scientists discovered in the 1960s, the fish have adapted by evolving a kind of antifreeze. It's composed of molecules called glycoproteins that circulate in the blood of the fishes, slightly lowering the temperature at which their body fluids would otherwise freeze and kill them. The glycoproteins surround tiny ice crystals and keep them from growing.
It's another of those ingenious evolutionary solutions that seem almost too clever to be true. But consider this: Nature did it not once, but at least twice. Fish at the other end of Earth, in the Arctic, also have antifreeze proteins. But those two populations of fish split long before they developed the antifreeze genes and proteins. And, researchers have found, the genes that produce the antifreeze proteins, north and south, are quite different. This is evidence that quite separate, independent episodes of molecular evolution occurred, with the same functional results.
This is a dramatic example of convergent evolution, when organisms that aren't closely related evolve similar traits as they both adapt to similar environments. There are a finite number of effective solutions to some challenges, and some of them emerge independently again and again.
Convergent evolution is responsible for the wings of the bat, the bird, and the pterodactyl. In each case, the forelimbs of these vertebrates morphed over time into wings, but they did so independently. Other examples are the different sorts of anteaters, found in Australia, Africa, and America. Though not closely related, they all evolved the "tools" necessary to subsist on an ant diet: a long, sticky tongue, few teeth, a rugged stomach, and large salivary glands. In each case, evolutionary adaptations allow them to exploit a food niche of ants and termites, but the developments occurred independently.