Q.What is a vestigial organ?

Comparing external structure (morphology) and internal structure (anatomy) across species reveals three evolutionarily informative categories of organ. Homologous organs are structurally similar but functionally different — such as the forelimb of the lizard, bird, bat, horse, whale and human, all built on the same humerus-radius-ulna-carpal-metacarpal-phalange plan but reshaped for walking, flying, swimming or grasping, or the thorns/tendrils of Bougainvillea and Cucurbita in plants — and their structural similarity points to a shared common ancestry, while their functional differences illustrate divergent evolution/adaptive radiation. Analogous organs are structurally dissimilar but functionally similar, such as the wings of a butterfly and a bird, the eyes of an octopus and a mammal, or the flippers of a penguin and a dolphin; because they arise independently through convergent evolution in response to similar functional demands, they carry no evidence of common ancestry and play no role in tracing evolutionary relationships. Vestigial (rudimentary) organs are imperfectly developed, non-functional, degenerate structures that remain functional in related organisms or ancestors — in humans these include the vestigial nictitating membrane (plica semilunaris), the wisdom teeth, the reduced coccyx (tailbone, since an erect po …