The process where two unrelated structures in unrelated organisms evolve to perform similar functions. (The structures are called "analogous structures.") For example, the wings of bats, birds, and insects evolved separately from each other but all are used to perform the function of flying. For another example, the complex eyes of vertebrates, cephalopods (squid and octopus), cubozoan jellyfish, and arthropods (insects, spiders, crustaceans) evolved separately, but all perform the function of vision.