The term atavism (derived from the Latin atavus, a great-grandfather's grandfather and, thus, more generally, an ancestor) denotes the tendency to revert to ancestral type. An atavism is a real or supposed evolutionary throwback, such as traits reappearing which had disappeared generations ago. Atavisms occur because genes for previously existing phenotypical features are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them.
Examples observed include:
hind legs on whales
hind fins on dolphins
extra toes on horses, as in archaic horses
reemergence of sexual reproduction in Hieracium pilosella and CrotoniidaeNorton, R. (2007), Proceedings of the National Academy of Sciences, April 24, cited in Science News, vol. 171, p. 302
Atavisms have been observed in humans as well. For example, babies have been born with a vestigial tail, called "coccygeal process", "coccygeal projection", and "caudal appendage". It can also