What are Analogous Structures?
Analogous structures are those structures in different species which perform the same function, have similar appearance but are not evolved together; therefore do not share a common ancestor. Analogous structures show how different species have evolved to become similar to each other. The analogy is not just limited to appearance but it can be behavioral as well.
Carolus Linnaeus first began the classification of species with taxonomy. He started grouping all similar looking species together. This led to the wrong classification of species because all similar looking structures are not evolved at the same time. Just because species look alike does not mean they are evolutionarily related. One analogous structure may have evolved centuries ago while its analogous match may have evolved in the recent past, which means they are genetically unrelated regardless of the fact that they have evolved similar structures.
Wings of a butterfly and wings of a bird are superficially similar in structure and perform the same function. But they have independently evolved for adaptation to the same function, i.e. flying. Analogous structures evolve due to convergent evolution when different organisms adapt to the same environment. For example wings of an insect and wings of a bird, both do the same thing: flying, but are evolved separately.
Another example is the fins of a fish and a penguin. In both, they help them to navigate through their natural environments. As one is a bird and other is not, it clearly suggests the development of fin in both species is an adaptation to a similar environment they inhabit.
Other examples of analogous structures include limbs of humans, horses and dolphins, food storage strategy of potato (underground root) and sweet potato (underground stem), and behavioral similarity of egg-laying between Duck (bird) and Duckbill platypus (mammal).
It does not depend only on appearance to determine which species are closely related, it takes more than just looks. However, analogous structures act as evidence that the theory of natural selection persists and the adaptations accumulate over time.