Evidence for Evolution
- Interpret the fossil record and transitional forms as a timeline of change
- Distinguish homologous, analogous, and vestigial structures and what each implies about ancestry
- Use molecular and biogeographical evidence to infer evolutionary relationships
The fossil record: change written in rock
Fossils form in sedimentary layers, and deeper layers are older. Reading the column from bottom to top gives a timeline of life, and it shows organisms changing over time rather than appearing all at once. Transitional fossils — like Tiktaalik, which has both fish gills and limb-like fins, or Archaeopteryx, with reptilian teeth and feathered wings — capture intermediate stages between major groups. Radiometric dating pins absolute ages to these layers, so the sequence is not just relative but calibrated in millions of years.
Anatomy: three kinds of structural clue
Homologous structures share the same underlying architecture because they were inherited from a common ancestor, even when they now do different jobs — the human arm, whale flipper, bat wing, and cat leg all contain the same one-bone/two-bone/wrist/digit pattern. This is evidence of divergent evolution from a shared ancestor. Analogous structures perform the same function and may look alike but have different underlying anatomy and independent origins — the wing of an insect and the wing of a bird. These arise by convergent evolution, where unrelated lineages independently evolve similar solutions to similar environments; analogous structures do not indicate close ancestry. Vestigial structures are reduced remnants that no longer serve their original function — the human tailbone, whale pelvic bones — and make sense only as leftovers from ancestors in which they were functional.
Molecules and geography
Evidence also comes from molecular biology: all life shares DNA and RNA, the same genetic code, and core molecules like ribosomes and ATP, pointing to a single common ancestor. The more similar two species’ DNA or protein sequences are, the more recently they shared an ancestor — cytochrome c and ribosomal RNA are widely used molecular "clocks." Biogeography adds another layer: species on isolated islands typically resemble mainland species nearby rather than ecologically similar species elsewhere, and marsupials dominate Australia because the continent split off before placental mammals spread — patterns that only make sense if species descend, with modification, from ancestors in the same region.
A bird’s wing and a butterfly’s wing both power flight and look superficially similar, yet the bird wing is built from an internal bony skeleton (the same bones as a human arm) while the butterfly wing is a thin membrane with no bones at all. Are these homologous or analogous, and what does that tell you about their ancestry?
- 1.Compare underlying structure, not just function: the two wings share a function (flight) but have completely different internal anatomy — bone versus membrane.
- 2.Shared function with different underlying architecture and independent origins defines analogous structures, produced by convergent evolution.
- 3.Because analogous structures evolved independently, they do not indicate a recent common ancestor — birds and insects are only distantly related and evolved flight separately.
The forelimbs of a human, a whale, and a bat all contain the same arrangement of bones — one upper bone, two lower bones, wrist bones, and digits — yet are used for very different tasks. This is best described as:
Do not equate "looks similar" with "closely related." Analogous structures can be strikingly alike yet reflect convergent evolution in distant lineages. Only homologous structures — shared underlying anatomy — are evidence of common ancestry. Always compare the internal structure, not the outward function.
A biologist compares a cytochrome c protein sequence across four species and finds species A differs from species B by 2 amino acids, from species C by 14, and from species D by 45. Which species most likely shares the most recent common ancestor with species A?
The AP exam treats evidence for evolution as convergent lines pointing to one conclusion: fossils, homologies, molecular sequences, and biogeography independently agree on the same tree of life. In free response, cite more than one line of evidence and state what each independently demonstrates.
Answer the 2 checkpoints as you read.
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