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.

Examples in context

Example 1. Australian marsupial biogeography. Australia's marsupial fauna provides classic biogeographic evidence for evolution. Marsupials (kangaroos, koalas, possums) dominate Australia, having diversified after the continent separated from Antarctica 50 million years ago. Placental mammals had not arrived (apart from bats and rodents) so marsupials occupied the niches placentals fill elsewhere - the marsupial mole, marsupial wolf (thylacine) and sugar glider show convergent evolution with placental moles, wolves and flying squirrels. These analogous structures arose independently from different ancestors. By contrast, the kangaroo's pouch is homologous across all marsupials, indicating descent from a common pouched ancestor. Museum Victoria's marsupial collection makes this visible to VCE students.

Example 2. Molecular clocks at Australian National University. Researchers at ANU sequence DNA from extinct Australian megafauna preserved in dry caves at Naracoorte. They calibrate molecular clocks by counting differences in mitochondrial DNA between known-age fossils and modern species. The clock for mammalian cytochrome b ticks at about 2 percent divergence per million years. Using this clock, ANU showed that Thylacoleo (marsupial lion) diverged from wombats around 30 million years ago, matching skeletal evidence. Where fossil and molecular dates agree, the evolutionary tree is considered well-supported. Australian Synchrotron crystallography of ancient proteins can extend the clock to amino-acid sequence comparisons.

Try this

Q1. Identify three categories of evidence for evolution and give one Australian example for each. [3 marks]

• Cue. Palaeontology (Naracoorte fossil megafauna); biogeography (marsupial radiation in Australia); comparative anatomy (homologous bones in tetrapod limbs); molecular biology (mtDNA divergence). Any three.

Q2. Two species of Australian wallaby share 98.5 percent of their mtDNA. (a) Estimate divergence time using a clock of 2 percent per million years. (b) Identify two assumptions of the molecular-clock calculation. [3 marks]

• Cue. (a) 1.5 percent divergence at 2 percent per Myr = 0.75 million years ago. (b) Mutation rate is constant; neutral substitutions; no selection.

Q3. Refer to homologous and analogous structures. (a) Distinguish the two terms. (b) Classify the marsupial-mole-and-placental-mole comparison. (c) Explain what analogous structures reveal about evolutionary process. [2+2+2 marks]

• Cue. (a) Homologous: same origin, possibly different function. Analogous: different origin, similar function. (b) Analogous (independent origin, similar burrowing function). (c) Convergent evolution: similar selection pressures produce similar adaptations from unrelated ancestors.