Evidence for evolution: WACE Year 12 Biology
“Evaluate the evidence for evolution from fossils, comparative anatomy, biochemistry and biogeography”
Multiple independent lines of evidence support evolution: fossils show change over time and transitional forms; comparative anatomy reveals homologous structures inherited from a common ancestor; biochemistry shows that related species share more similar DNA and protein sequences; and biogeography explains why isolated regions such as Australia have unique organisms. Because these separate lines agree, the case for common descent is strong.
What this dot point is asking
SCSA wants you to describe each line of evidence, explain what it shows, and recognise that the strength of the theory comes from many independent lines pointing to the same conclusion. A strong answer links each type of evidence to common descent.
The fossil record
Fossils are the preserved remains or traces of past organisms. Arranged by the age of the rock layers in which they are found, they show that life has changed over time, with simpler forms in older rocks and more complex or modern forms in younger rocks. Transitional fossils, which show features intermediate between groups, document the gradual change from one form to another. The fossil record is incomplete because fossilisation is rare, but the patterns it does show are consistent with evolution.
Comparative anatomy
Comparing the body structures of different species reveals telling patterns.
- Homologous structures have the same basic underlying structure but different functions, such as the bone pattern shared by a human arm, a whale flipper and a bat wing. Their shared plan points to a common ancestor.
- Vestigial structures are reduced remnants of organs that were functional in ancestors, such as the pelvic bones of whales.
Homologous structures share a common ancestor and an underlying plan even if their functions differ. Analogous structures (such as the wings of insects and birds) do similar jobs but have different underlying structures, evolving separately through convergent evolution. Only homologous structures are evidence of common descent.
Biochemical and molecular evidence
All living things use DNA, the same genetic code, and many of the same proteins, which itself points to a common origin. More precisely, the more closely related two species are, the more similar their DNA and protein sequences. By comparing sequences such as cytochrome c or DNA itself, biologists can measure relatedness and build evolutionary trees. This molecular evidence often confirms relationships first suggested by anatomy.
Biogeography
Biogeography is the study of where organisms live and why. Isolated regions tend to have unique organisms found nowhere else, which makes sense if those organisms evolved in isolation from a few ancestral colonisers.
Read the biogeographic distribution
Australia separated from other landmasses long ago and remained isolated. As a result it has many unique organisms, including marsupials such as kangaroos and the monotreme platypus, which evolved in isolation rather than appearing independently elsewhere. The distribution of these animals, found in Australia but not on other continents, is exactly what evolution by common descent in an isolated region predicts.
Why multiple lines matter
No single line of evidence proves evolution on its own; the strength of the theory comes from the agreement of many independent lines. Fossils show change over time, anatomy shows shared ancestry, biochemistry quantifies relatedness, and biogeography explains distribution. When evidence collected by different methods converges on the same conclusion of common descent, the explanation becomes very well supported.
Homologous structure: a structure with a shared underlying plan inherited from a common ancestor. Vestigial structure: a reduced remnant of an ancestral organ. Transitional fossil: a fossil with features intermediate between groups. Biogeography: the study of the geographic distribution of organisms.
Linking evidence to the mechanisms
The evidence for evolution and the mechanisms of evolution fit together. Natural selection, drift, gene flow and speciation explain how change happens; fossils, anatomy, biochemistry and biogeography are the record of that change having happened. Understanding both means you can explain not only that species share ancestry but how and why their gene pools diverged over time.
- Treating analogous structures as evidence of common ancestry
- Only homologous structures, with a shared underlying plan, show common descent.
- Saying the fossil record is complete
- It is patchy because fossilisation is rare, but the patterns it shows still support evolution.
- Relying on one line of evidence
- The strength of the theory is the agreement of many independent lines.
- Calling evolution just a guess
- It is a well-supported scientific theory backed by multiple converging lines of evidence.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksExplain how homologous structures and molecular (biochemical) evidence each support the theory of common descent. Use an example for each.Show worked answer →
A 6 mark explain answer needs both lines defined, linked to common descent, with examples.
- Homologous structures
- These share the same underlying structural plan but may perform different functions. The pentadactyl (five-boned) limb of a human arm, a whale flipper and a bat wing has the same bone arrangement despite different uses. The shared plan is best explained by inheritance from a common ancestor whose descendants modified the limb for different roles.
- Molecular evidence
- All organisms use DNA and the same genetic code, and the more closely related two species are, the more similar their DNA and protein sequences. Comparing a conserved protein such as cytochrome c shows fewer differences between closely related species, allowing relatedness to be quantified and evolutionary trees to be built.
- Link
- Both lines point independently to common descent: shared structure and shared sequence are inherited from shared ancestors.
Markers reward a correct example for each line and an explicit link to inheritance from a common ancestor.
WACE 2023-style5 marksAustralia has many unique organisms, such as marsupials and the platypus, that are found nowhere else. Explain how biogeography provides evidence for evolution, and why evidence from several independent fields strengthens the theory.Show worked answer →
A 5 mark answer needs the biogeography reasoning plus the multiple-lines point.
Biogeography. Australia separated from other landmasses and remained isolated for a long time. Organisms there evolved from ancestral colonisers without gene flow from other continents, so they diverged into unique forms (marsupials, monotremes) found only in Australia. This distribution is exactly what evolution in geographic isolation predicts, and is hard to explain if species were unchanging.
Multiple lines. No single line proves evolution alone, but fossils, comparative anatomy, biochemistry and biogeography are independent methods that converge on the same conclusion of common descent. Agreement between independent lines makes the theory very well supported.
Markers reward the isolation-and-divergence reasoning and the point that converging independent evidence strengthens the theory.