The big idea: Homologous structures have the same basic plan but may do different jobs. The shared plan was inherited from a common ancestor.
The points to remember
- Homologous structures: the same basic structure, often with different functions.
- The plan was inherited from a common ancestor, then changed in each line.
- The guide's example is the pentadactyl limb: five-digit limb of amphibians, reptiles, birds and mammals.
- Its bones: humerus; radius and ulna; carpals; metacarpals and phalanges.
- The bones are in the same order even when they look very different.
Remember it as: Same bones, different jobs.
Real example: a mole and a seal: A mole digs with a broad, spade-like forelimb; a seal swims with a flipper. Inside, both have one upper arm bone, two forearm bones, wrist bones and five digits, just like your arm.
Watch the test you use:
Plan, not looks: Two structures are homologous because their plan matches, not because they look alike or do the same job. A whale flipper looks nothing like a bat wing, yet the two are homologous.
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The big idea: Homologous structures come from divergent evolution: one ancestral plan, adapted to different jobs as populations met different selection pressures.
The points to remember
- Divergent evolution: one ancestral form gives rise to several forms with different jobs.
- It happens by natural selection under different selection pressures.
- Vestigial structures are homologies too: whales have small hip bones but no back legs.
- Vertebrate embryos look alike at an early stage, with the same basic body plan.
- Fossils of early four-limbed animals show the same limb plan, long before mammals.
Divergent evolution
- One ancestor, many forms
- Different selection pressures
- Produces homologous structures
Convergent evolution
- Unrelated groups, similar forms
- Similar selection pressures
- Produces analogous structures
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How it is tested: Paper 1A: what type of evidence a drawing of limb bones shows, or which items are evidence for evolution. Higher Level Paper 2: describe the evidence from homologous structures, in four marks.
IB-style questionOutline[2 marks]
The forelimb of a frog and the wing of a bird contain the same arrangement of bones. Outline why this is evidence for evolution.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Plan, not job: Homologous structures are matched by their plan of bones, not by what they do.