Fish and dolphins are vastly different animals with different underlying DNA sequences and nervous systems. Yet because they inhabit similar environmental conditions, they have developed many similar structures. The fin of a fish and the fin of a dolphin serve a common purpose, but they evolved quite differently. Dolphins, which are placental mammals, have a fin that is closely related to the human hand. Fish have no close relatives with hands, so their fins come from a very different source on a genetic level. This is an example of:

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