Final answer:
Vestigial structures are remnants of features from an organism's ancestors that have lost most or all of their original function through evolution. Examples include the wings of flightless birds and the human appendix. These structures provide evidence of evolution and the common ancestry of species.
Step-by-step explanation:
The structures that are remnants of features that served important functions in the organism's ancestors are known as vestigial structures. These biological structures have lost most or all of their original function through the process of evolution. For example, the wings of flightless birds, such as the ostrich, have become vestigial as these birds no longer fly. Similarly, the human appendix, a tiny remnant of a once larger organ, is also classified as a vestigial structure because it no longer has a significant role in digesting food as it did in our distant ancestors.
Vestigial structures provide evidence of evolution, indicating how organisms have evolved from their ancestors and how certain traits have become obsolete due to changes in the organism's way of life or environment. When structures that are no longer used shrink in size, it may be because there would be an energy and developmental cost to maintaining them. A large, unused structure could potentially reduce an organism's fitness by being cumbersome or by using resources that could be allocated to more useful developments.