Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before.Atavisms can occur in several ways. number of traits can vary as a result of shortening of the fetal development of a trait (neoteny) or by prolongation of the same. In such a case, a shift in the time a trait is allowed to develop before it is fixed can bring forth an ancestral phenotype.