What are Vestigial Organs?
Vestigial organs are the structures which have no apparent function in a particular organism perhaps once they did. They appear to be a residual part of a past ancestor. For example, as in snakes, they have pelvic bones although they do not have legs because they are descendants of reptiles which possess legs.
Human vermiform appendix is another example of vestigial organs and this is the most cited example to support human evolution. These unused structures present in an organism are called vestigial structures.
Vestigial Structures and Evolution
Vestigial structures in one species are homologous to other similar functional structures in other species. Therefore, these structures can be considered an evidence for evolution. The existence of vestigial structures can be accredited to change in environment and behavioral patterns of the particular organism. But if the presence of the trait is no more useful for the organism, it is most likely that it will not be inherited in future generations or it will be gradually decreased.
But if there is no selection pressure which could lower the fitness of the organism, it will persist in future generations until it is eliminated by genetic drift or other such events.
Although in most cases the vestigial structures are of no direct harm to the organism but they require extra energy for development and maintenance. There is also some risk of the disease such as cancer. So, the selection pressure forces to remove those parts.
Some vestigial structures persist in organisms due to some limitations. In some cases, complete removal of the organ would lead to a drastic change in the developmental pattern of the organism which might pose seriously negative outcomes.
The vestigial structures of an organism can be compared with original and functioning structures in other organisms in order to determine the homology of that particular structure. Homologous structures present in organisms indicate that those organisms shared a common ancestry.
Examples of Vestigial Organs
The presence of tail bone or coccyx which is attached to the bottom of the sacrum.
Arrector pili muscles in humans.
The presence of eyes in cave-dwelling animals such as cave-dwelling tetra fish (Astyanax mexicanus). Wings in Ostriches, Emus, and other flightless birds.