Natural Selection, The Mechanism
In 2022, Australian hospitals reported 11,000+ infections by multi-drug-resistant bacteria, the same mechanism Charles Darwin described in 1859 running at deadly speed.
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Q1 · Antibiotic-resistant "superbugs" are increasing worldwide. Before reading this lesson, explain how you think bacteria might become resistant to antibiotics.
Think about what happens when bacteria are exposed to antibiotics over many generations.
Q2 · The peppered moth population in England changed from mostly light to mostly dark during the Industrial Revolution. What environmental change do you think caused this shift?
Consider how pollution from factories might have affected the moths' camouflage.
● Know
- The five conditions required for natural selection: variation, overproduction, competition, differential survival, heritability
- Definitions of adaptation, fitness and population
- That natural selection is a mechanism of evolution, not evolution itself
● Understand
- How natural selection leads to adaptation in populations over generations
- Why acquired characteristics are not inherited
- The difference between natural selection and selective breeding
● Can do
- Apply the steps of natural selection to explain a real-world example
- Distinguish between Lamarckian and Darwinian explanations
- Use Australian animal examples to illustrate natural selection
Place 100 bacteria in a dish and add an antibiotic: 99 die within hours, but 1 survives, divides, and within 24 hours the dish is full again, of bacteria that resist the drug. That is natural selection running in a Petri dish. Natural selection is the mechanism that drives adaptive evolution. It was proposed by Charles Darwin and Alfred Russel Wallace in 1858, and it rests on three simple observations. First, individuals in any population vary in their traits. Second, many of these traits are heritable, they can be passed to offspring. Third, more offspring are produced than can possibly survive, so there is a struggle for existence. The result is that individuals with traits better suited to their environment tend to survive and reproduce more successfully.
Over many generations, this differential survival and reproduction changes the genetic makeup of the population. Advantageous alleles become more common; disadvantageous alleles become rarer. The population becomes better adapted to its environment, but only because the environment selected which variants survived. Natural selection has no foresight, no goal and no plan. It is simply the unavoidable consequence of variation, heredity and limited resources.
Antibiotic resistance is natural selection in fast-forward. When you take antibiotics, most susceptible bacteria are killed. But a few may carry a mutation that makes them resistant. These survivors reproduce, and soon the population is dominated by resistant strains. The antibiotic did not create the resistance, the variation already existed. The antibiotic selected for it. This is why finishing your full course of antibiotics matters: incomplete treatment leaves more survivors to repopulate.
Australian health: The Australian Commission on Safety and Quality in Health Care tracks antibiotic resistance nationwide. 'Superbugs' such as MRSA (methicillin-resistant Staphylococcus aureus) are a growing threat in Australian hospitals. Understanding natural selection helps clinicians prescribe antibiotics wisely to slow the evolution of resistance.
One of the most persistent misconceptions is that species evolve 'in order to' survive, that evolution has a goal. This is teleological thinking, and it is wrong. Bacteria do not evolve resistance 'in order to' survive antibiotics. Rather, resistant bacteria happen to survive, and their descendants inherit the trait. Evolution is a filter, not a designer. It acts on whatever variation happens to exist, without planning for future challenges.
Click each stage of natural selection.
Variation exists
Individuals in a population differ in their traits due to mutation, recombination and other sources of genetic variation.
Selection pressure
The environment presents a challenge: predators, climate, disease, or limited resources. Not all individuals survive equally well.
Differential survival
Individuals with traits better suited to the pressure are more likely to survive and reproduce.
Inheritance
Advantageous alleles are passed to offspring. Over generations, they become more common in the population.
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 in polluted areas where trees were darkened by soot.
Wrong: 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.
Right: Correct, natural selection changes the proportion of traits in a population over generations. Individual organisms do not change genetically to match their environment.
Wrong: "Evolution is random."
Right: Mutations are random, but natural selection is not. Selection systematically favours traits that improve survival and reproduction in a specific environment.
Wrong: 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.
Right: Correct, mutations are random, but natural selection is a non-random filtering process. The environment determines which traits are advantageous, not chance.
Natural selection does not produce perfection, it produces what works well enough, given the constraints of history and genetics. An organism cannot evolve a trait unless the genetic variation for that trait exists in the population. Evolution is also constrained by past adaptations: vertebrate eyes have a blind spot because the retina evolved backwards, with nerves and blood vessels in front of the light-sensitive cells. A designer would not make this choice, but evolution cannot easily rewind and start over.
Selection acts on populations, not individuals. An individual does not evolve during its lifetime; it is born with its genes, and those genes either help it survive and reproduce or they do not. The population evolves as allele frequencies shift across generations. And evolution is not progressive in the sense of aiming toward complexity or intelligence. Parasites often evolve toward simplicity, losing structures their ancestors had because they no longer need them.
Cave fish have evolved to lose their eyes and pigmentation. In the darkness of a cave, eyes are useless and costly to build. Fish born with mutations that reduce eye development are not at a disadvantage, in fact, they may save energy. Over thousands of generations, natural selection favours eyeless, pale fish because the resources saved can be used for other functions. Evolution has no preference for complexity; it favours efficiency in the current environment.
Australian cave biology: The Nullarbor Plain in southern Australia contains extensive limestone cave systems inhabited by blind cave beetles and fish. Researchers from the Western Australian Museum study these species to understand how natural selection drives the loss of traits when they are no longer useful. The caves provide a natural laboratory for watching evolution in action.
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.
Trace the Steps, Peppered Moth
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.
Design an Australian Adaptation
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.
Copy Into Your Book
▼The Five Principles
- Variation = differences in traits exist
- Overproduction = more offspring than can survive
- Competition = limited resources
- Differential survival = best-adapted survive
- Heritability = advantageous traits passed on
Key Distinctions
- Natural selection acts on populations, not individuals
- Evolution happens over generations, not lifetimes
- Acquired characteristics are not inherited
- Mutations are random; selection is non-random
Peppered Moth Summary
- Pollution darkened trees
- Dark moths were camouflaged
- Light moths were eaten more
- Dark form increased in population
- Trend reversed when air cleaned up
Australian Examples
- Kangaroo hopping = energy efficiency
- Echidna bill = electroreception
- Koala liver = eucalyptus detox
- Thorny devil skin = water capture
At the start of this lesson you were told that Australian hospitals reported over 11,000 cases of multi-drug-resistant bacteria in a single year, bacteria that evolved resistance because natural selection favoured the survivors of antibiotic treatment. That example was deliberately chosen to show you that Darwin's mechanism, described in 1859, is still running in fast-forward right now.
Now that you understand all four conditions for natural selection, variation, heritability, competition and differential survival, explain precisely how those hospital bacteria evolved their resistance. What part of natural selection's mechanism surprised you most, or changed how you see evolution?
Q1. List and briefly describe the five conditions required for natural selection. 3 MARKS
Q2. Explain how industrial melanism in peppered moths demonstrates all five principles of natural selection. 4 MARKS
Q3. 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
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly identify that Lamarck's explanation (acquired characteristics are inherited) is not supported by modern genetics?
- Did you recognise that natural selection acts on existing variation in populations, not on individuals changing during their lifetime?
- Write one sentence summarising the most important new concept you learned about the mechanism of natural selection.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Trace the Steps
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].
Activity 2, Design an Australian Adaptation
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].
Multiple Choice
1. CHeritable 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. BBirds 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. AOverproduction means too many offspring for the environment to support, creating competition. Option B is false. Option C contradicts variation. Option D is false.
4. DNatural 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. BOnly 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.
Short Answer Model Answers
Q6 (3 marks): The five conditions are: (1) Variationindividuals must differ in traits [0.5 mark]; (2) Overproductionmore offspring are produced than can survive [0.5 mark]; (3) Competitionlimited resources create a struggle for survival [0.5 mark]; (4) Differential survivalindividuals with advantageous traits survive and reproduce more [0.5 mark]; (5) Heritabilityadvantageous 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 survivaldark moths survived better in polluted areas because they were camouflaged [1 mark]. (5) Heritabilitycolour 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].
Jump Through Natural Selection!
Climb platforms using your knowledge of variation, competition and survival. Pool: Lesson 12.