Rudiment and Atavism

Rudiments

Many organisms have features that are incomplete or no longer functional. This phenomenon, which is known in biology under the term rudiment, can affect organs as well as behavior and is an essential evidence for the theory of evolution. As living conditions changed during the course of evolution, some organs (e.g., human body hair) became obsolete and retreated, as they offered no evolutionary advantage. Some organs have even proved to be a disadvantage, such as the hind legs of the ancestors of the whale (see picture). The Whale's early ancestors were land-living, four-legged mammals. In the course of evolution certain factors have led to the ancestor of the whale shifting its habitat back into the water. The hind legs became superfluous and evolved over time and are today visible as a rudiment in the skeleton of the whale:

Further rudiments in humans are, for example:

 the coccyx, on which the typical monkey tail used to hang.
 Re-formed webbed hands and feet.

 Worm appendage of the cecum, which formerly had an important digestive function (conversion of the food of the human ancestor led to the regression of the cecum)

Atavism

An atavism is now understood to mean the accidental appearance of an anatomical feature that was already present in the course of tribal history, but was then somehow lost phenotypically in the course of evolution.

Atavisms indicate that the genes of earlier features are still present in the genotype, but must be blocked for certain reasons. The recurrence of these features can be triggered by a variety of factors. Mutations that can turn on / off certain genes or affect gene regulation may be as much a cause as bastardization of two related species, which could re-activate blocked genes.

Examples of atavisms are for example:

 Hypertrichosis (whole body hair, see picture on the right)

 Third nipple (indicates an earlier milk strip)

 Supernumerary hoof on the trotter leg of the horse

Summary

Rudiments are functionless or stunted organs.

Atavisms are accidentally recurrent features that have already disappeared in the genealogical history of each species.