DragonflyIssuesInEvolution13 Wiki

Hindlimbs that are rarely found in dolphins are an atavistic trait.

Atavisms are traits or features that have reappeared in a species after having disappeared due to evolution. Even when a species has evolved to have a different trait, often the genes for the lost trait remain in the genome; they are only very rarely expressed in the phenotype.

Not vestigial structures[]

Atavisms are the 'throwback' traits and different from vestigial structures. Our tailbone is a vestigial structure, not entirely useless but a leftover from a useful tail. The atavistic version of this would be a human that is actually born with a tail. An atavism is a very rare expression of a trait that has not been seen for several generations. It is also a trait that did exist in ancestors of that species. Some other examples of atavisms include hindlimbs in whales and dolphines, an extra toe in horses, teeth in birds, and more developed thigh muscles in birds. (Theobald, 2004; Tomic & Meyer-Rochow, 2011)

Atavisms add mystery[]

The implications of atavisms are especially intriguing. Some have seen them as controversial to the theory of evolution because of Doss's law, which states that evolved organisms are unlikely to return to a previous state. Atavisms add another layer to the mysteries that still surround the mechanisms of evolution, especially in terms of genetics. The presence of atavisms imply that certain traits are basically lying dormant in our genome. These traits are usually repressed but rare mutations allow them to develop and be expressed. Atavisms  "point to a 'deep homology' that predates morphological specialization and is indeed indicative of the common origin of life on Earth" (Tomic and Meyer-Rochow 2011, p. 347). Atavisms raise questions about how long a trait will persist in the genome even though it is no longer expressed and seems to have no genetic value.
Theobald, D. (2004). 29+ evidences for macroevolution: Part 2: Past History: Atavisms. Retrieved from http://www.talkorigins.org/faqs/comdesc/section2.html#atavisms on Sept 12, 2013.
Tomic, N. & Meyer-Rochow, V.B. (2011). Atavisms: Medical, genetic, and evolutionary implications. Perspectives in Biology & Medicine, 54 (3), 332-353.

Image from www.talkorigins.org