Evolution
Discover how Charles Darwin’s theory of evolution by natural selection explains the remarkable diversity of life on Earth — including Australia’s unique wildlife.
Darwin and Natural Selection
Charles Darwin (1809–1882) proposed the theory of evolution by natural selection after his famous voyage on HMS Beagle, during which he observed diverse species on the Galapagos Islands. He published his ideas in On the Origin of Species (1859).
The Four Key Observations of Natural Selection
Overproduction
Organisms produce more offspring than can possibly survive. A female cod can release millions of eggs per year.
Variation
Individuals within a population show variation in their traits. Some variations are heritable (passed to offspring via genes).
Survival of the fittest (“struggle for existence”)
Individuals with favourable variations are more likely to survive and reproduce in their environment.
Inheritance and change over time
Favourable traits are passed on, so they become more common in the population over many generations — the population evolves.
Australian Example: Peppered Moth
During the Industrial Revolution, pollution turned tree bark dark with soot. Light-coloured moths became visible to predators, while dark moths survived and reproduced more. Over time, the proportion of dark moths increased — a direct example of natural selection changing a population.
Variation and Its Sources
Variation is essential for natural selection to act. Without differences between individuals, no individual would have an advantage. Variation arises from:
Genetic variation
- Mutations: random changes to DNA sequences
- Sexual reproduction: mixing of parental genes produces unique combinations
- Crossing over: exchange of DNA between chromosomes during meiosis
Environmental variation
- Diet and nutrition affect body size
- Sun exposure affects skin tone
- Exercise affects muscle development
- Environmental variation is NOT inherited by offspring
Evidence for Evolution
Evolution is one of the most well-supported theories in all of science. Multiple independent lines of evidence all point to the same conclusion:
Fossil Record
Fossils show a progression of life forms from simple to complex over billions of years. Transitional fossils (e.g., Archaeopteryx, showing features of both dinosaurs and birds) document intermediate stages. Australia has rich fossil sites including the Ediacaran fossils in the Flinders Ranges, South Australia.
Comparative Anatomy
Homologous structures are anatomically similar body parts in different species that have evolved from a common ancestor (e.g., the forelimb bones of humans, bats, whales and dogs). Analogous structures look similar but evolved independently (e.g., wings of birds and insects).
Molecular Evidence (DNA)
Species that are closely related share more similarities in their DNA sequences. Comparing DNA across species allows scientists to construct evolutionary family trees (phylogenies) and estimate when species diverged.
Biogeography
The geographic distribution of species reflects evolutionary history. Australia’s isolation produced unique marsupials (e.g., kangaroos, koalas, wombats) that evolved separately from placental mammals on other continents after the breakup of Gondwana.
Key Vocabulary
| Term | Definition |
|---|---|
| Natural selection | The process by which individuals with heritable traits better suited to their environment tend to survive and reproduce more successfully. |
| Adaptation | A heritable trait that increases an organism's fitness (ability to survive and reproduce) in its environment. |
| Variation | Differences in the characteristics of individuals within a population, which can arise from genetic mutations, sexual reproduction, or environmental factors. |
| Speciation | The evolutionary process by which populations accumulate so many differences that they become distinct species unable to interbreed. |
Worked Examples
Explaining antibiotic resistance using natural selection.
Scenario: A population of bacteria is treated with an antibiotic. Most bacteria die, but some survive and reproduce.
Step 1 (Variation): The bacterial population has genetic variation — some bacteria randomly have mutations giving them slight resistance to the antibiotic.
Step 2 (Selection pressure): The antibiotic kills non-resistant bacteria. Resistant bacteria survive (“survival of the fittest”).
Step 3 (Inheritance): Resistant bacteria reproduce rapidly, passing on the resistance gene. Over generations, the entire population becomes resistant.
Answer: This is natural selection in action. It is why we must complete antibiotic courses and why antibiotic resistance is a major health concern in Australia and worldwide.
Identifying homologous and analogous structures.
Question: Are the wings of a bat and the wings of a butterfly homologous or analogous structures?
Analysis: Both are used for flight. However, a bat wing is a modified forelimb (same bones: humerus, radius, ulna, digits) as a human arm. A butterfly wing has a completely different internal structure made of chitin.
Answer: They are analogous structures — similar function, different evolutionary origin. A bat wing and a human arm are homologous — same structure, common ancestor.
Explaining why Australian marsupials are so diverse.
Context: Australia separated from other landmasses around 50 million years ago, when marsupials were present but placental mammals were not.
Explanation: In isolation, marsupials faced different environments (forests, grasslands, deserts) and experienced different selection pressures. Over millions of years, they adapted to fill ecological niches, resulting in the diverse species we see today — from the tree-dwelling koala to the burrowing wombat and the grazing kangaroo.
Answer: Geographic isolation led to independent evolution. Different environments created different selection pressures, leading to the radiation of diverse marsupial species through natural selection.
Knowledge Check
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Key Concepts Summary
- •Natural selection acts on heritable variation: individuals with favourable traits survive and reproduce more, passing traits to offspring.
- •Variation arises from mutations and sexual reproduction; only genetic (inherited) variation is subject to natural selection.
- •Evidence for evolution includes the fossil record, comparative anatomy, DNA comparisons, and biogeography.
- •Homologous structures indicate common ancestry; analogous structures indicate convergent evolution (different ancestry, same function).
- •Australia’s unique marsupials evolved in geographic isolation after the continent separated from Gondwana ~50 million years ago.