Same parts, different jobs: relatives. Same job, different parts: convergence.
Evolution isn't one clue. Fossils, anatomy, DNA, geography, and populations caught changing in real time come from different methods, yet they tell the same branching history. That agreement makes the case strong.
Key terms
- Fossil record — preserved remains of past life, stacked in rock layers. It shows what lived when, and how forms changed.
- Homologous structures — the same parts, inherited from a shared ancestor, even when they do different jobs: human arm, bat wing, whale flipper.
- Analogous structures — the same job done with different parts, evolved separately, like bat and moth wings. The result of convergent evolution.
- Vestigial structure — a shrunken leftover of a part that worked in an ancestor, like the tiny hip bones inside whales and pythons.
- Molecular clock — using sequence differences between species to estimate when they split. Each gene or region needs its own rate.
- Biogeography — where species live. Island species usually resemble those on the nearest mainland, because their ancestors came from there.
Reading each line of evidence
- Fossils: deeper layers are usually older, and radioactive isotopes in the rock give ages in years. Transitional fossils mix old and new features, like early whales with hind legs.
- Anatomy: compare how a structure is built, not what it does. Shared build → shared ancestry. Same job, different build → convergence.
- Molecules: compare the same gene across species. More differences → an earlier split. Vital genes change slowly and time ancient splits; fast-changing DNA times recent ones.
- Geography: distributions must fit the map's history. One freshwater species fossilized on continents now split by ocean shows those continents were once joined.
- Real time: when the environment shifts, allele frequencies follow. Dark moths declined as soot fell, and bacteria evolve antibiotic resistance within years.
Why it matters
Questions here hand you data (sequence differences, a fossil map, a trend over decades) and ask which conclusion it supports. Match the data to its inference, and reject answers that claim more than the data show. The strongest case combines lines: fossils, anatomy, and DNA come from different methods, so their agreement is hard to explain any other way.
Exam traps
| Trap | Actually |
|---|
| Bats and moths both have wings, so they're closely related. | Same job, different build: finger bones versus hollow tubes. That's convergence. Relatedness shows in shared construction, not shared use. |
| Individual moths lightened as the air cleaned, and passed that on. | A moth's wing color is set by its genes. Birds ate more of whichever moth stood out, so allele frequencies shifted over generations. |
| More DNA differences always means an older split. | Only within the same gene or region. Vital genes change slowly and much noncoding DNA changes fast, so compare like with like. |
| The species on the oldest island is the most evolved. | It has had the longest to diverge from its mainland ancestor. No species is 'more evolved': every living lineage has evolved equally long. |
Worked example
Fossils show that lizard species A and B split from a common ancestor 10 million years ago. Their versions of gene G differ at 20 sites. A third species, C, differs from A at 50 sites in gene G and at 160 sites in a stretch of noncoding DNA. Assuming gene G changes at a steady rate, about when did A and C split?
- Calibrate on the split you can date: 20 differences in 10 million years → 2 differences per million years in gene G.
- Apply that rate to the same gene. A and C differ at 50 sites in gene G, and 50 ÷ 2 = 25 million years.
- Set the noncoding count aside. You have no rate for that stretch, and noncoding DNA usually changes faster than a working gene.
About 25 million years ago.
A molecular clock needs a rate measured for that same stretch of DNA, anchored to an independent date (here, the fossils). The near miss is 160 ÷ 2 = 80 million years. It borrows gene G's rate for DNA that changes faster, which pushes the split far too deep.