AP Syllabus focus:
‘Morphological homologies, including vestigial structures, indicate common ancestry among different species.’
Morphological evidence supports evolution by showing how body structures are shared, modified, or reduced across species. Understanding homologous and vestigial structures helps you infer common ancestry and distinguish similarity due to shared history from similarity due to similar function.
Morphological homologies as evidence for common ancestry
What “homology” means in anatomy
Homologous structures: anatomical features in different species that share an underlying structural plan because they were inherited from a common ancestor, even if they now perform different functions.
Homologies matter because evolution often modifies existing structures rather than building entirely new ones. If two species share the same basic arrangement of bones, tissues, or organs, the simplest explanation is inheritance from an ancestor that also had that arrangement.
Key features of morphological homologies
Shared structural plan: parts correspond positionally (e.g., same sequence of bones), not just superficially.
Divergent function is allowed: the same structure can be adapted for different uses (e.g., grasping vs swimming).
Comparative forelimb anatomy across vertebrates (e.g., human, whale, horse, bat, bird) shows the same underlying set of bones arranged in similar relative positions. The figure highlights how evolution modifies a shared ancestral structure for different functions such as flying, swimming, and running—evidence of common ancestry rather than mere similarity in function. Source
Best interpreted comparatively: multiple species comparisons strengthen inferences about ancestry.
Homology vs analogy (common misconception)
Analogous structures: features that perform similar functions and may look similar but evolved independently (not from a recent common ancestral structure), often due to similar selective pressures.
Similar function alone does not demonstrate common ancestry. Homology focuses on structure and relative position; analogy focuses on function and can result from independent evolution. Correctly identifying homology prevents over-claiming relatedness based on look-alikes.
How homologies support evolutionary relationships
When several species share a complex structural pattern, it is unlikely to have arisen multiple times in exactly the same way.
Homologies can be used to infer branching relationships: species sharing more derived modifications of a shared structure are often more closely related.
Homologies provide evidence even when species occupy very different environments, because ancestry can persist through major ecological change.
Vestigial structures as evidence for common ancestry
What “vestigial” means
Vestigial structure: a reduced or remnant anatomical feature that has lost most or all of its ancestral function in a species, but persists because of inheritance and development.
Vestigial structures are powerful evidence because they are often non-optimal for a current lifestyle yet make sense as leftovers from ancestors with different needs.
Photograph of a blue whale pelvic bone, a reduced remnant of the pelvic girdle found in terrestrial mammals. Its presence in a fully aquatic animal illustrates descent with modification: the structure persists from ancestors in which a larger pelvis supported hind limbs, even though modern whales no longer walk on land. Source
Their presence supports the specification idea that morphological homologies, including vestigial structures, indicate common ancestry among different species.
Characteristics of vestigial structures
Reduced size or complexity compared with the ancestral state.
Limited or altered function: may be nonfunctional, or repurposed for a minor/new role.
Consistency within a lineage: often found across related species, matching patterns expected from descent.
Why vestigial structures persist
If the structure does not strongly reduce fitness, natural selection may not eliminate it quickly.
Developmental pathways can constrain change: reducing a structure completely may require multiple coordinated genetic/developmental changes.
Some vestigial traits are byproducts of building other features, so they remain “attached” to necessary developmental programs.
Using morphology carefully in evolutionary claims
What strengthens a morphology-based claim
Multiple independent homologies supporting the same relationship.
Clear correspondence in position, connections, and detailed anatomy.
Vestigial features that fit a coherent ancestral scenario (a functional structure in ancestors, reduced in descendants).
Common pitfalls to avoid
Assuming “looks similar” implies homology without checking structural correspondence.
Treating vestigial as “useless”; many are better described as reduced or repurposed.
Over-relying on a single trait; robust inference typically requires several anatomical lines of evidence.
FAQ
They look for evidence of an ancestral primary function that is largely lost now, plus signs of reduction inconsistent with current optimisation.
Useful lines of support include comparative anatomy across relatives and developmental/functional studies.
Yes, if selection favours increased use and variation exists to build on.
Re-expansion is more likely when underlying developmental pathways remain intact rather than fully lost.
Detailed correspondence in:
relative position within the body plan
connections to other structures (attachments, articulations)
arrangement of subparts (e.g., consistent component order)
Loss can occur at different rates due to differences in selection, drift, or developmental constraints.
A lineage may also evolve alternative developmental routes that remove the remnant entirely.
Shared early developmental patterns can reveal the same underlying structure before it is modified in adults.
This can clarify homology when adult forms are highly specialised and superficially dissimilar.
Practice Questions
Explain how morphological homologies provide evidence for common ancestry among different species. (1–3 marks)
States that homologous structures share an underlying structural plan inherited from a common ancestor (1).
Notes that functions may differ due to modification over time (divergence) (1).
Links the shared structural pattern to descent with modification/common ancestry rather than coincidence (1).
A biologist claims a reduced pelvic bone in a whale species is evidence of evolution. Describe what makes a structure vestigial and explain how vestigial structures support common ancestry. Include two features a scientist might examine to justify calling the pelvic bone vestigial. (4–6 marks)
Defines a vestigial structure as a reduced/remnant feature inherited from ancestors with diminished ancestral function (1).
Explains that vestigial structures indicate ancestors possessed a more functional version, supporting common ancestry (1).
Connects vestigiality to descent with modification over time (1).
Feature 1: reduced size/complexity relative to ancestral/related species condition (1).
Feature 2: loss of primary original function (e.g., not used for hindlimb locomotion), possibly minor/repurposed role (1).
Uses comparative evidence across related species or fossil/ancestral reconstruction as justification (1).
