9 min readConcept sheet1-mark MCQ or assertion-reason, a 2 or 3-mark question with examples, and a spotting item in the practical; also a NEET favourite

Comparative anatomy: homologous and analogous organs

Same basic structure, different jobs: homologous organs (divergent evolution). Different structure, same job: analogous organs (convergent evolution).

Homologous organsSame basic structure and origin, different functions; show divergent evolution and common ancestry

Analogous organsDifferent structure and origin, similar function; show convergent evolution

Mammal forelimb bonesHumerus, radius and ulna, carpals, metacarpals, phalanges

Homologous vs analogous

Homologous organsThe forelimbs of whale, bat, cheetah and human do different jobs (swimming, flying, running, holding) but have the same bones. The same structure changed in different directions to suit different needs. This is divergent evolution, and it shows common ancestry.

Analogous organsThe wings of a butterfly and a bird both help in flying but are built quite differently. Different structures evolved for the same function. This is convergent evolution; it does not show close common ancestry.

Examples to learn

Type	Example	Why
Homologous	Forelimbs of whale, bat, cheetah, human	Same bones, different uses
Homologous	Hearts and brains of vertebrates	Same basic plan in all vertebrates
Homologous (plants)	Thorn of Bougainvillea and tendril of Cucurbita	Both are modified stems (axillary in position); one protects, one climbs
Analogous	Wings of butterfly and bird	Both fly; very different structure
Analogous	Eye of octopus and eye of a mammal	Both see; they develop differently
Analogous	Flippers of penguin and dolphin	Both swim; a bird and a mammal
Analogous (plants)	Sweet potato and potato	Both store food; sweet potato is a root, potato is a stem

How to draw homologous forelimbs

Draw four forelimbs side by side: human arm, cheetah leg, whale flipper, bat wing.
In each, draw one long upper bone and label it humerus.
Below it draw two bones side by side; label radius and ulna.
Draw a group of small wrist bones (carpals), then palm bones (metacarpals), then finger bones (phalanges).
Make the bat's finger bones very long with a skin flap between them, and the whale's short and broad inside a paddle.
Shade the same bone the same way in all four, and title it: Homologous organs, forelimbs of vertebrates.

Activity: spotting homologous and analogous organs (flash cards or models)

1
Look at the pair of organs on the flash card or model, for example a bat's wing and a whale's flipper.
2
Write the function of each organ. Note whether the functions are the same or different.
3
Look at the internal structure (bones, position, origin). Note whether it is the same plan or a different one.
4★
Decide: same structure but different function means homologous; different structure but same function means analogous.
5
Record your answer with the kind of evolution it shows: divergent for homologous, convergent for analogous.
6
Comment: for example, "Bat wing and whale flipper: homologous; same forelimb bones adapted for flying and swimming; divergent evolution."

★ key step

In real life

In an Indian kitchen, sweet potato (shakarkandi) and potato (aloo) both store starch. Yet one is a root and the other a stem: an everyday example of analogous organs.

Exam angle

2 marks: “Differentiate between homologous and analogous organs with one example of each.” Homologous: same structure, different function, divergent evolution (forelimbs of whale and bat). Analogous: different structure, same function, convergent evolution (wings of butterfly and bird).

3 marks: “The thorns of Bougainvillea and the tendrils of Cucurbita are homologous, while sweet potato and potato are analogous. Explain both.” Thorn and tendril are both modified stems with different jobs (protection, climbing): divergent evolution. Sweet potato (root) and potato (stem) both store food: different origin, same function, convergent evolution.

Remember it

Homologous = Have the same bones. Analogous = Act the same.