Clues about ancestry of species, traits once useful in an ancestor. A feature of a species that was an adaptation in its ancestors but has lost its usefulness or has been co-opted for new uses, no longer performs the function for which it evolved. Why some species lost ability to fly. Metabolically costly, w/ no predators on islands reduced wings are favoured. Vestigial pelvis and leg bones bc of descent from 4 legged ancestors. Appendix which serve as fermenting vessel to break cellulose into sugars for leaf-eating ancestors. Throwback traits produced by the occasional reawakening of ancestral genes that have been long silenced. Can sometimes be awakened when something goes wrong in development. Whales still have genetic info for making legs, info degraded bc of unuse. Horses w/ five toes like ancient ancestor. Tail gene in humans normally deactivated in fetuses. Birds evolved from tooth reptiles, have genes for teeth but missing protein, when protein supplied teeth form.