Which one of the following pairs may indicate common ancestry?

The correct answer is Option (3) → Forelimbs of man - forelimbs of whale

Analogous organs do the same job but differ in structure and origin. They arise by convergent evolution, because similar habitats select similar features, so they do not show common ancestry.

Option (1) Sweet potato - potato — Incorrect. Both store food, but sweet potato is a modified root and potato is a modified stem. They are analogous.

Option (2) Octopus eye - human eye — Incorrect. Both eyes see, but they developed separately in molluscs and vertebrates. They are analogous.

Option (3) Forelimbs of man - forelimbs of whale — Correct. A human arm grasps and a whale flipper swims. Yet both have the same bones: humerus, radius, ulna, carpals, metacarpals and phalanges. They are homologous and show a common ancestor.

Option (4) Wings of butterfly - wings of birds — Incorrect. Both wings fly, but an insect wing is a fold of the body wall while a bird wing is a modified forelimb. They are analogous.

NCERT Extract:
What Are the Evidences for Evolution? – “… Comparative anatomy and morphology shows similarities and differences among organisms of today and those that existed years ago. Such similarities can be interpreted to understand whether common ancestors were shared or not. For example whales, bats, Cheetah and human (all mammals) share similarities in the pattern of bones of forelimbs (Figure 6.3b). Though these forelimbs perform different functions in these animals, they have similar anatomical structure – all of them have humerus, radius, ulna, carpals, metacarpals and phalanges in their forelimbs. Hence, in these animals, the same structure developed along different directions due to adaptations to different needs. This is divergent evolution and these structures are homologous. Homology indicates common ancestry.”

Figure 6.3 Example of homologous organs in (a) Plants and (b) Animals