Explanation

Atavisms Example living whales with legs, newborn babies with tails. Anatomical atavisms are closely related conceptually to vestigial structures. An atavism is the reappearance of a lost character specific to a remote evolutionary ancestor and not observed in the parents or recent ancestors of the organism displaying the atavistic character. Atavisms have several essential features (i) presence in adult stages of life, (ii) absence in parents or recent ancestors and (iii) extremely rare in a population. For developmental reasons, the occasional occurrence of atavisms is expected under common descent if structures of functions are gradually lost between ancestor and descendant lineages