Nature is the ultimate selective breeder. Every generation, more offspring are born than can possibly survive. The individuals best suited to their environment tend to live longer and reproduce more — and over time, populations change. This is natural selection.
Giraffes have very long necks. Two explanations have been proposed throughout history:
Explanation A (Lamarck): Giraffes stretched their necks to reach higher leaves, and this acquired longer neck was passed to their offspring.
Explanation B (Darwin): Some giraffes were born with naturally longer necks due to variation. These individuals survived and reproduced more because they could reach more food.
Which explanation is supported by modern genetics? Why?
Natural selection is not a single event. It is a continuous process that requires five conditions to operate in a population. If any one condition is missing, natural selection cannot produce evolutionary change.
1. Variation exists within the population. Individuals must differ in their traits. This variation arises from genetic differences — different alleles, mutations and recombination during sexual reproduction. Without variation, every individual is identical and there is nothing for selection to act upon.
2. Overproduction of offspring. Most species produce far more offspring than can possibly survive to adulthood. A single female cod can release millions of eggs. A pair of rabbits can theoretically produce thousands of descendants in a year. This overproduction creates a surplus that cannot all be supported.
3. Competition for limited resources. Because there are too many individuals and not enough food, water, shelter or mates, organisms must compete. This competition may be direct (fighting) or indirect (simply needing the same limited resource).
4. Differential survival and reproduction. Individuals with traits better suited to the environment are more likely to survive and reproduce. This is the "selection" part of natural selection. It is not random — it depends on how well an organism's traits match its environment.
5. Heritability of advantageous traits. The traits that provide an advantage must be heritable — encoded in DNA and passable to offspring. If a trait is not genetic (e.g., a scar, learned skill), it cannot spread through a population via natural selection.
One of the most famous and well-documented examples of natural selection happened in England during the Industrial Revolution — and it demonstrates every principle we have just learned.
Before the 1800s, the peppered moth (Biston betularia) was mostly light-coloured with dark speckles. This camouflage helped it blend against light-coloured tree bark and lichen, hiding it from predatory birds. A rare dark (melanic) form existed but was uncommon because it stood out against the pale trees.
During the Industrial Revolution, coal pollution killed lichen and darkened tree bark with soot. Suddenly, the light moths were conspicuous against the dark background, while the dark moths were camouflaged. Birds preyed heavily on the light moths. The dark moths survived and reproduced more. Within 50 years, the dark form rose from less than 1% to over 90% of the population in polluted areas.
When clean air laws were introduced in the mid-20th century, lichen recovered and tree bark lightened. The trend reversed — light moths became common again. This is natural selection in action: the environment changed, the selective pressure changed, and the population adapted.
The peppered moth story was first studied by Henry Bernard Kettlewell in the 1950s. He released moths in polluted and unpolluted woodlands and recaptured them. His recapture rates showed dark moths survived better in polluted areas and light moths in clean areas. While some details of his methods were later criticised, the overall pattern of industrial melanism is supported by extensive modern data including bird predation experiments and DNA analysis.
Wrong: "The moths changed colour to match the trees."
Right: Individual moths did not change colour. The proportion of dark moths in the population increased because dark moths survived and reproduced more. This is a crucial distinction.
Wrong: "Evolution is random."
Right: Mutations are random, but natural selection is not. Selection favours traits that improve survival and reproduction in a specific environment. It is a filtering process, not a lottery.
Australia's unique and often harsh environments have driven remarkable adaptations through natural selection. Our isolation from other continents has produced a living laboratory of evolutionary change.
Kangaroo hopping. Kangaroos use elastic energy stored in their long leg tendons to hop efficiently at high speeds. This adaptation reduces energy cost compared to running — a huge advantage in Australia's arid interior where food and water are scarce. Individuals with slightly better tendon elasticity survived droughts better and passed those genes on.
Echidna electroreception. The short-beaked echidna has electroreceptors on its bill — it can detect the tiny electrical signals given off by ants and termites. This remarkable adaptation evolved because echidnas that could locate prey underground survived better than those that could not.
Koala diet specialisation. Koalas eat almost exclusively eucalyptus leaves — a food source that is toxic to most mammals. Koalas evolved specialised liver enzymes to detoxify eucalyptus oils. This dietary niche meant less competition with other herbivores, but also tied koalas closely to the distribution of their food trees.
The thorny devil (Moloch horridus) is a small lizard found in Australia's arid interior. Its skin is covered in microscopic channels that capture dew and direct it toward its mouth by capillary action — essentially drinking through its skin. This adaptation did not happen by choice. Over thousands of generations, lizards with slightly better skin channels survived droughts better and left more offspring. Today, the thorny devil is a masterpiece of natural selection sculpting a solution to desert life.
1 Variation: What two colour forms existed in the peppered moth population?
2 Overproduction and competition: Why did moths need to avoid birds?
3 Differential survival: Which colour moth survived better in polluted areas? Why?
4 Heritability: Why was it important that colour was genetically controlled?
5 What happened when clean air laws were introduced? Explain using natural selection.
1 Choose your animal and adaptation (e.g., echidna electroreception, koala eucalyptus detox, dingo sand-resistant paws).
2 Describe the selection pressure in the animal's environment.
3 Write a short paragraph explaining how natural selection produced this adaptation over many generations.
1. Which of the following is required for natural selection to occur?
2. In the peppered moth example, what was the selective pressure?
3. Why does overproduction of offspring matter for natural selection?
4. Which statement about natural selection is correct?
5. After a severe drought in outback Australia, a lizard population shows a higher proportion of dark-skinned individuals. Which explanation is most consistent with natural selection?
6. List and briefly describe the five conditions required for natural selection. 3 MARKS
7. Explain how industrial melanism in peppered moths demonstrates all five principles of natural selection. 4 MARKS
8. Choose an Australian animal and explain how one of its adaptations could have evolved through natural selection. Refer to variation, selection pressure and heritability. 5 MARKS
Go back to your Think First responses at the top of the lesson.
1. Variation: Light-coloured (speckled) and dark (melanic) forms existed in the population [1 mark].
2. Overproduction and competition: Moths produced more offspring than could survive, and birds preyed on those that were visible against tree bark [1 mark].
3. Differential survival: In polluted areas, dark moths were camouflaged against soot-darkened trees and survived better; light moths were eaten [1 mark]. In clean areas, the reverse occurred.
4. Heritability: Colour was genetically controlled, so surviving moths passed their colour genes to offspring [1 mark].
5. Clean air laws: Tree bark lightened, light moths became camouflaged again, and their proportion increased — another reversal driven by changed selection pressure [1 mark].
3. Example answer (thorny devil water channels): In a population of ancestral lizards, variation existed in skin texture [1 mark]. In arid Australia, drought was a strong selection pressure [1 mark]. Lizards with slightly better skin channels could capture more dew and survive longer [1 mark]. This trait was heritable, so over generations the population shifted toward more efficient water-capturing skin [1 mark]. Today the thorny devil is a highly specialised desert survivor [1 mark].
1. C — Heritable variation is essential. Without it, there is nothing for selection to act upon. Option A would prevent selection. Option B is irrelevant. Option D describes Lamarckism, which is incorrect.
2. B — Birds ate moths they could see. Pollution changed the background, changing which moths were visible. Option A confuses mutation with selection. Option C implies conscious choice. Option D is backwards.
3. A — Overproduction means too many offspring for the environment to support, creating competition. Option B is false. Option C contradicts variation. Option D is false.
4. D — Natural selection acts on existing variation over generations. Option A confuses mutation (random) with selection. Option B describes individual change, not evolution. Option C is false — selection is non-random.
5. B — Only heritable traits that improve survival can spread. Option A describes acquired characteristics (Lamarckism). Option C ignores selection. Option D describes directed mutation, which does not occur.
Q6 (3 marks): The five conditions are: (1) Variation — individuals must differ in traits [0.5 mark]; (2) Overproduction — more offspring are produced than can survive [0.5 mark]; (3) Competition — limited resources create a struggle for survival [0.5 mark]; (4) Differential survival — individuals with advantageous traits survive and reproduce more [0.5 mark]; (5) Heritability — advantageous traits must be passed to offspring [0.5 mark]. All five must be present for natural selection to produce evolutionary change [0.5 mark].
Q7 (4 marks): Industrial melanism demonstrates natural selection as follows: (1) Variation existed — both light and dark moths were present [1 mark]. (2) Overproduction and competition meant moths competed to avoid bird predation [0.5 mark]. (3) Selection pressure changed when pollution darkened tree bark [0.5 mark]. (4) Differential survival — dark moths survived better in polluted areas because they were camouflaged [1 mark]. (5) Heritability — colour was genetic, so dark moths passed the trait on [1 mark]. When air quality improved, the trend reversed, confirming that selection depends on environment.
Q8 (5 marks): Animal: Red kangaroo. Adaptation: Efficient hopping using elastic tendon energy [1 mark]. Variation: Some ancestral kangaroos had slightly more elastic tendons due to genetic differences [1 mark]. Selection pressure: Arid Australia has scarce food and water; individuals that could travel further using less energy survived droughts better [1 mark]. Heritability: Tendon elasticity is genetically controlled, so efficient hoppers passed the trait to offspring [1 mark]. Result: Over thousands of generations, the population became dominated by efficient hoppers — the characteristic kangaroo hop is an adaptation to Australian aridity [1 mark].
Test your knowledge of natural selection, variation and adaptation in this fast-paced quiz battle. Correct answers power your attacks!
Climb platforms using your knowledge of variation, competition and survival. Pool: Lesson 12.
Tick when you have finished all activities and checked your answers.