Lesson 3 of 6 / Evolution, species and phylogeny
Use independent evidence of ancestry
Why is agreement between molecules, fossils and geography powerful?
In this lesson: Explain molecular/anatomical homology, fossils and Wallace's biogeography.
About 7 min
The key ideaIndependent patterns that agree with common ancestry strengthen an evolutionary explanation more than superficial similarity alone.
Explore the idea
Ask what each pattern supports
Purple: upper-arm bone (humerus). Green: paired forearm bones (radius and ulna). Gold: distal wrist and digit region. These are simplified arrangements; individual small bones and joint details are omitted, and neither limb is drawn to scale.
What does this pattern support, and what does it leave uncertain?
Agreement across independent anatomical, molecular, fossil and geographical patterns strengthens common-ancestry explanations. One resemblance or one tiny dataset is not sufficient by itself. Diagrams and example sequences are original.
Explanation
Homologous features share an evolutionary origin, even if present functions differ. Similar underlying limb arrangements in vertebrates can reflect common ancestry. Analogous features can serve similar functions through independent evolution, so functional resemblance alone does not establish close relationship.
Molecular homologies include comparable DNA or protein sequences. Shared sequence features, especially across many independently informative regions, support relatedness and can reveal connections obscured by external appearance. Similarity must be evaluated in homologous sequences with suitable alignment.
Fossils record organisms at different times, allowing anatomical changes and transitional combinations to be compared. The record is incomplete and preservation is biased, but consistent sequences of forms provide historical evidence unavailable from living organisms alone.
Biogeography asks why organisms occur where they do. Wallace's observations of contrasting faunas across Southeast Asian island regions support the importance of historical isolation, dispersal and common ancestry. Nearby islands can differ sharply because present distance is not the only influence; geological history and barriers matter.
When molecular, fossil and geographical evidence converge, they corroborate descent with modification through independent predictions. Disagreement can motivate revised phylogenies or better sampling rather than being hidden. A strong answer explains the inference and its limits, not merely lists evidence types.
Step by step
- 1
Identify the evidence
Sequence, anatomical pattern, date or distribution?
- 2
State the predicted ancestry pattern
Explain why common descent would generate it.
- 3
Consider alternatives and limits
Convergence, sampling and preservation can complicate inference.
Worked example
Work through the evidence
A whale and a fish have streamlined bodies, while a whale forelimb shares detailed bone arrangement with other mammals. Which clue better supports ancestry?
One way to explain it
The homologous forelimb arrangement supports mammalian ancestry; streamlined form can arise independently under similar swimming selection.
Why this answer works
- Similar function can cause convergence.
- Detailed shared structural organisation provides a different kind of evidence.
Is this true? "Any two organisms with the same function must share a recent common ancestor."
Convergent evolution can produce analogous functions in distantly related lineages.