evidence for biological evolution from palaeontology (fossil record, transitional fossils), biogeography, comparative anatomy (homologous and analogous structures, vestigial organs) and molecular biology (DNA, protein sequence comparisons, molecular clocks)

What this dot point is asking

VCAA wants the four major lines of evidence for evolution (palaeontology, biogeography, comparative anatomy, molecular biology) with specific examples and the logic of why each supports descent with modification.

The answer

Multiple independent lines of evidence support evolution. The strongest case is that they all point to the same pattern: species have changed over time and share common ancestors.

Palaeontology: the fossil record

Fossils are preserved remains or impressions of organisms in rock. The fossil record shows that:

• Older rocks contain simpler or extinct life forms; younger rocks contain forms more like those alive today.
• New groups appear suddenly (in geological terms) and then diversify.
• Most species that ever lived are now extinct.

Transitional fossils link major groups. Examples include:

• Archaeopteryx (about 150 million years old): feathered dinosaur with teeth, claws on its wings and a bony tail, linking theropod dinosaurs to modern birds.
• Tiktaalik (about 375 million years old): a fish with a flat skull, wrist bones and a neck, linking lobe-finned fish to early tetrapods.
• Ambulocetus and other walking whale ancestors linking land mammals to modern cetaceans.

Limitations of the fossil record. Soft-bodied organisms rarely fossilise; many environments preserve poorly; the record is incomplete. Despite this, every fossil ever found falls into the predicted age order: no rabbits in Cambrian rocks.

Biogeography

Biogeography is the geographic distribution of species. Two patterns emerge:

• Closely related species cluster geographically. Marsupials dominate Australia because they evolved there in isolation after Gondwana broke up. Galapagos finches are most similar to mainland South American finches because they descended from a single colonising population.
• Similar habitats on different continents have unrelated species. Cacti in American deserts and euphorbias in African deserts look similar but are unrelated. Each lineage independently evolved succulent stems and spines (convergent evolution).

Biogeography is best explained by descent with modification combined with continental drift and isolation. A creation by separate design would not predict marsupial dominance on one island continent.

Comparative anatomy

Anatomical comparison between species reveals patterns that only descent with modification explains.

Homologous structures. Structures with the same underlying anatomical plan but different functions. The pentadactyl limb of mammals, reptiles, birds and amphibians all has the same bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges), used for walking, flying, swimming or grasping. Homology supports descent from a common ancestor.

Analogous structures. Structures with the same function but different underlying anatomy. The wing of a bird (with arm bones and feathers) and the wing of an insect (with chitin membranes) both produce flight but evolved independently. Analogy is evidence of convergent evolution, not common descent.

Vestigial structures. Reduced, non-functional remnants of structures that were useful in ancestors. Examples include the human appendix and tailbone, whale pelvis bones, and the eyes of cave fish. Vestigial structures make sense only if the species descended from an ancestor in which the structure was functional.

Embryology. Vertebrate embryos pass through similar stages (pharyngeal arches, post-anal tail), reflecting shared developmental genes inherited from a common ancestor.

Molecular biology

DNA and protein sequencing have provided overwhelming evidence for evolution:

Universal genetic code. All known organisms use essentially the same DNA codons for the same amino acids. This is strong evidence that all life shares a common origin.

Sequence similarity reflects relatedness. The more recently two species shared a common ancestor, the more similar their DNA and protein sequences. Human and chimpanzee DNA differs by about 1 to 2 per cent. Human and mouse DNA differs by about 15 per cent. Human and yeast cytochrome c proteins differ by about 45 amino acids.

Molecular clocks. Some sequences accumulate mutations at a roughly constant rate. By counting differences between species and calibrating against the fossil record, scientists can estimate when two lineages diverged. Cytochrome c, mitochondrial DNA and ribosomal RNA are commonly used.

Pseudogenes and shared mutations. Humans and other great apes share the same broken gene for vitamin C synthesis at the same point in the sequence. The only sensible explanation is inheritance of the defect from a common ancestor.

Endogenous retroviruses. Viral DNA fragments inserted at the same locations in the genomes of humans and chimpanzees show common ancestry, because the chance of independent insertion at identical sites is negligible.

Why multiple lines matter

Each line of evidence can be questioned in isolation, but they converge on the same pattern of relationships. Fossils, anatomy, biogeography and molecules independently produce nearly identical evolutionary trees. The mutual support of independent evidence is the hallmark of a robust scientific theory.

Worked example

Whale evolution is one of the most thoroughly documented examples:

• Fossils. Transitional forms (Pakicetus, Ambulocetus, Rodhocetus, Dorudon) show a gradual change from terrestrial mammals with four legs to fully aquatic ancestors over about 10 million years.
• Biogeography. The oldest fossils come from what is now Pakistan, where the shallow Tethys Sea separated Asia and Africa.
• Comparative anatomy. Modern whales retain vestigial pelvic bones and (in some embryos) hind-limb buds.
• Molecular biology. Whale DNA shows their closest living relatives are hippopotamuses, not other marine mammals.

All four lines independently point to the same story: whales evolved from land mammals around 50 million years ago.

Common traps

Saying "evolution is just a theory". In science, a theory is a well-supported explanation for a wide range of observations. Evolution is supported by all four lines of evidence above.

Confusing homologous and analogous structures. Homologous = same plan, different function = common ancestry. Analogous = same function, different plan = convergent evolution.

Saying the fossil record has "gaps", so evolution is wrong. Gaps are expected because fossilisation is rare. Transitional fossils are continually found in predicted positions.

Treating molecular clocks as exact. They give estimates, not precise dates. Different genes evolve at different rates and must be calibrated against fossils.

Saying humans evolved from chimpanzees. Humans and chimpanzees share a common ancestor that lived about 6 to 7 million years ago. Neither species descended from the other.

In one sentence

Evidence for evolution comes from palaeontology (transitional fossils such as Archaeopteryx and Tiktaalik in their predicted ages), biogeography (the marsupial radiation in Australia and Galapagos finches), comparative anatomy (homologous pentadactyl limbs, vestigial whale pelvises) and molecular biology (universal genetic code, sequence similarity, molecular clocks, shared pseudogenes), and these independent lines converge on the same tree of common descent with modification.