Homologous structures are anatomical features in different species that share the same underlying structural plan because they were inherited from a common ancestor. The pentadactyl limb is a key example and provides evidence for evolution at both SL and HL.
In vertebrates, the pentadactyl limb contains the same basic arrangement of bones: one upper limb bone, two lower limb bones, a group of wrist or ankle bones, and five digits. These correspond to the humerus, radius and ulna, carpals, metacarpals and phalanges in a human arm.
The same basic plan occurs in limbs with very different functions:
| Organism and limb | Main function |
|---|---|
| Human arm | Grasping and manipulation |
| Bat wing | Flight |
| Whale flipper | Swimming |
| Horse forelimb | Running |
This pattern is explained by common ancestry. An ancestral vertebrate possessed the basic limb structure, which was inherited by descendant populations. Different environments created different selection pressures, so natural selection favoured modifications suited to functions such as flying, swimming or running. This is divergent evolution: related groups become increasingly different while retaining evidence of their shared origin.
Homologous structures must be distinguished from analogous structures:
| Homologous structures | Analogous structures |
|---|---|
| Same evolutionary origin and underlying structure | Different evolutionary origins |
| Functions may differ | Functions are similar |
| Example: human arm and whale flipper | Example: bird wing and insect wing |
A common misconception is that homologous structures must perform the same function. They do not: homology is determined by shared evolutionary origin and structural pattern, not by function.
Exam technique: This links directly to A4.1 Evolution and speciation, particularly evidence for evolution from homologous structures. For an “explain” question, identify the shared pentadactyl bone plan, state that it was inherited from a common ancestor, and explain how different selection pressures produced modifications for different functions.
