Homologous structures share a common evolutionary origin and basic structural plan, although their functions may differ. Analogous structures perform similar functions but evolved independently from different ancestral structures.
This distinction is part of IB Biology subtopic A4.1, Evolution and speciation, and applies at both SL and HL.
| Feature | Homologous structures | Analogous structures |
|---|---|---|
| Evolutionary origin | Inherited from a common ancestor | Evolved independently from different ancestral structures |
| Structure | Same underlying structural plan | Different underlying structural plan |
| Function | May be similar or different | Similar |
| Evolutionary process | Often associated with divergent evolution | Produced by convergent evolution |
| Example | Pentadactyl limbs of humans, whales and bats | Wings of birds and insects |
The pentadactyl limbs of humans, whales and bats contain the same basic arrangement of bones, including the humerus, radius, ulna and digits. This shared plan indicates inheritance from a common ancestor. However, natural selection modified the limbs for different functions: grasping in humans, swimming in whales and flying in bats.
Bird and insect wings both enable flight, but they have different structures and evolutionary origins. Bird wings are modified vertebrate forelimbs supported by bones, whereas insect wings are extensions of the exoskeleton. Similar selection pressures favoured flight in both groups, producing similar functions through convergent evolution.
A common misconception is that structures are homologous whenever they perform the same function. Function alone does not establish homology: the key evidence is a shared structural plan resulting from common ancestry.
Exam technique: For the command term distinguish, state a linked difference referring to both structures: “Homologous structures share an evolutionary origin but may have different functions, whereas analogous structures have different evolutionary origins but similar functions.” Support the distinction with one accurate example. Avoid saying that homologous structures must look identical or perform the same function.
