Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on comparison with homologous features in related species. The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful, but if the lack of the feature provides no advantage, and its presence provides no disadvantage, the feature may not be phased out by natural selection and persist across species.
The Darwin-tubercle (left) is a vestigial form of the ear tip (right) in the mammalian ancestors of humans—here shown in a crab-eating macaque.
Vestigial hindlegs (spurs) in a boa constrictor
The blind mole rat (Spalax typhlus) has tiny eyes completely covered by a layer of skin.
The uniform of the German Feldgendarmerie during WW2, complete with gorget.
The vomeronasal organ (VNO), or Jacobson's organ, is the paired auxiliary olfactory (smell) sense organ located in the soft tissue of the nasal septum, in the nasal cavity just above the roof of the mouth in various tetrapods. The name is derived from the fact that it lies adjacent to the unpaired vomer bone in the nasal septum. It is present and functional in all snakes and lizards, and in many mammals, including cats, dogs, cattle, pigs, and some primates. Some humans may have physical remnants of a VNO, but it is vestigial and non-functional.
Sagittal section of the vomeronasal organ of garter snake
Stallion exhibiting the flehmen response