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Homologous structures are structures with simliar construction but not necessarily the same function. However, these structures indicate common ancestry. When one examines a bird wing, a frog limb and a human arm, one can see each bone although they may be modified due to natural selection for a particular use. The bird wing upon examination has the radius and ulna bones as seen in the frog limb and the human forearm. The bones of the wrist, hand and fingers are all there, although modified for flight, picking up something or for jumping. Analogous structures on the other hand, evolved along different evolutionary pathways to the same end. For example, if you observe a bat wing and a butterfly wing, they both serve the same purpose--flight. But, upon close scrutiny, the bat wing has muscles, bones, joints, skin, hair, etc. However, the wing of a butterfly is membranous, attached to an exoskeleton with a completely different design. These analagous structures do not suggest common ancestry.
Homoiogy:-Similarity of the structure, physiology, or development of different species of organisms based on their descent from a common evolutionary ancestor. Analogy, by contrast, is a functional similarity of structure that is based not on common evolutionary origins but on mere similarity of use. The forelimbs of such widely differing mammals as humans, bats, and deer are homologous; the form of construction and the number of bones in each are practically identical and represent adaptive modifications of the forelimb structure of their shared ancestor. The wings of birds and insects, on the other hand, are merely analogous; they are used for flight in both types of organisms but do not share a common ancestral origin.
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