Year 11 BiologyModule 3⏱ ~50 minPractice bank · 3 Short AnswerLesson 5 of 16
Darwin, Wallace and Natural Selection
Peter and Rosemary Grant began studying Geospiza fortis finches on Daphne Major, Galápagos, in 1973. When a severe drought struck in 1977, 85% of the finch population died. In the following year, the Grants measured that mean beak depth in the surviving population had increased by 0.5 mm, a measurable shift in a single generation. Their 35-year dataset, published across multiple papers from 1979 to 2012, measured the heritability of beak depth at 0.79, meaning 79% of beak-depth variation passes from parent to offspring. This long-term study transformed natural selection from a historical inference into a directly measured, quantified process.
Today's hook: Peter and Rosemary Grant's study of Geospiza fortis finches on Daphne Major, Galápagos, showed that after the catastrophic 1977 drought killed 85% of the population, mean beak depth increased by 0.5 mm in just one generation, heritability measured at 0.79. This is natural selection observed and quantified in real time. But when bacteria become antibiotic resistant in a hospital ward within months, is the same mechanism operating, or are the bacteria somehow "learning" to resist the drug after exposure?
0/5TASKS
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You are here
Orient and predict
See where this lesson is going, then commit to your own answer before you read.
Warm up first
Three quick questions from earlier lessons. Pulling old material back to mind before you learn something new makes the new material stick better, so this is not busywork.
Worksheets
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
1. When bacteria become antibiotic resistant, do individual bacteria somehow "learn" resistance after exposure, or is something else happening?
2. If a giraffe stretches its neck during its lifetime, can that acquired change be inherited by its offspring?
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Next up
Frame the learning
Set the intentions, load the key terms, and clear the misconceptions that trip most students.
Learning Intentions
goals
Know
The historical role of Darwin and Wallace in developing natural selection theory.
The four conditions required for natural selection.
The antibiotic-resistance example and why Lamarck was wrong.
Understand
Why populations evolve through changing allele frequencies, not individuals through effort.
How selection pressure changes which variants become more common over generations.
Why natural selection explains the evidence built in earlier lessons.
Can Do
State and apply the four conditions of natural selection.
Explain MRSA antibiotic resistance step by step without teleological wording.
Distinguish Darwinian natural selection from Lamarckian inheritance clearly.
Scan these before reading
vocab
Natural selectionDifferential survival and reproduction of individuals.
VariationDifferences in traits among individuals in a population.
OverproductionProducing more offspring than can survive to adulthood.
Struggle for existenceCompetition for limited resources in a population.
Differential reproductionSome individuals leaving more offspring than others.
FitnessThe reproductive success of an individual relative to others.
Cross-lesson links: L04 showed you Darwin's historical observations. L05 provides the modern experimental proof, the Grants' 35-year dataset gives you the actual numbers behind natural selection that exam questions will test.
Misconceptions To Fix
watch out
✗ Wrong: Natural selection acts on individual organisms over their lifetime, they become adapted.
✓ Right: Natural selection acts on populations over generations, not on individuals during their lifetimes. An individual's traits are fixed at birth by their genotype. Selection changes the frequency of alleles across generations by favouring individuals whose traits improve survival and reproduction.
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Next up
State the four conditions for natural selection
Get the historical idea and the four things that must all be true for selection to work.
Core Content
Key Point
Natural selection is not a vague idea that organisms "adapt because they need to." It is a process with specific conditions. If all four conditions are in place, allele frequencies can shift over generations and the population changes.
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Darwin, Wallace and the Four Conditions
+5 XP
The historical idea and the logic it depends on
In 1977, a severe drought hit Daphne Major in the Galápagos. Peter and Rosemary Grant of Princeton University had been measuring every beak on every Geospiza fortis finch since 1973. When 85% of the population died, they found that the survivors had systematically deeper beaks than average, because the drought had eliminated the small soft seeds, leaving only large, hard seeds that only deeper-beaked birds could crack. One year later, mean beak depth in the population had shifted by 0.5 mm. The Grants measured heritability at 0.79, proving the shift would be passed to offspring. This is natural selection operating through the four conditions that Darwin and Wallace independently identified.
The power of the theory is that it is not just a slogan. Natural selection requires a specific set of conditions. First, individuals in a population vary. Second, some of that variation is heritable. Third, individuals with certain variants survive or reproduce more successfully than others. Fourth, a selection pressure in the environment makes that difference matter. If all four conditions are in place, allele frequencies can shift over generations and the population changes.
Variation
Individuals differ in their traits.
Heritability
Some differences are passed to offspring.
Differential Success
Some variants leave more offspring.
Selection Pressure
The environment favours some variants over others.
Natural selection is a process with conditions, not a vague idea that organisms simply "adapt because they need to".
Darwin and Wallace independently developed natural selection; presented jointly 1858, expanded by Darwin in On the Origin of Species 1859. Four conditions: (1) variation in traits, (2) heritability, (3) differential survival and reproduction, (4) selection pressure. When all four apply, allele frequencies shift over generations, the population changes.
Exam framing
If a question asks for natural selection, name the conditions explicitly. Examiners usually want mechanism, not just the phrase "survival of the fittest".
Pause, copy the highlighted conditions summary into your book before the check below.
Which pair correctly identifies two of the required conditions for natural selection?
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Next up
Explain antibiotic resistance as selection
Run the four conditions through MRSA, and say it without any language of trying or needing.
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Antibiotic Resistance as a Worked Example
+5 XP
Why MRSA is selection, not intentional change
We just saw that natural selection depends on four specific conditions. That raises a question: how does this play out in a real, observable example? This card answers it → antibiotic resistance in MRSA is the clearest modern demonstration of natural selection in action.
The standard antibiotic-resistance example matters because it forces you to apply natural selection precisely rather than using sloppy wording.
In a bacterial population, random mutation may already have produced a resistant variant before any antibiotic is used. When the antibiotic is applied, susceptible bacteria are more likely to die, while resistant bacteria survive and reproduce. Over generations, the frequency of the resistance allele increases. The bacteria did not "develop" resistance because they wanted to survive. Resistance already existed in some individuals, and the environment selected for it.
Step
What Happens
Why It Matters
1. Variation exists
Some bacteria already carry a mutation for resistance
Selection cannot act unless variation is already present
2. Antibiotic applied
Drug kills more susceptible bacteria than resistant ones
Creates strong selection pressure
3. Survivors reproduce
Resistant bacteria leave more descendants
Differential reproduction shifts allele frequency
4. Population changes
Resistance becomes more common over generations
The population evolves
Resistant variants already existed due to random mutation. Antibiotic = selection pressure: kills susceptible bacteria, resistant bacteria survive and reproduce. Resistance allele frequency increases over generations, this is natural selection. Never use teleological language: bacteria did not "learn", "develop" or "try to adapt".
Common misconception
Antibiotics cause bacteria to invent resistance after exposure. The better explanation is that resistant variants already existed due to mutation, and antibiotic exposure selected them. Never say the bacteria "tried to adapt" or that "the antibiotic made them mutate on purpose".
Correct wording:
resistant variants already existed
antibiotic acted as selection pressure
resistant bacteria survived and reproduced
resistance allele frequency increased
Avoid:
"the bacteria tried to adapt"
"the antibiotic made them mutate on purpose"
Pause, copy the highlighted MRSA natural-selection sequence into your book before the check below.
What is the correct explanation for antibiotic resistance increasing in a bacterial population?
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Next up
Contrast Lamarck with Darwin
Pin down why acquired characteristics are not the mechanism, and what is.
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Lamarck vs Darwin
+5 XP
Why acquired characteristics are not the mechanism
We just saw that antibiotic resistance spreads through pre-existing variation, not organisms adapting on demand. That connects to a bigger question: how does Darwinian selection differ from the older idea of Lamarckian inheritance? This card answers it → the two theories make completely different claims about where heritable variation comes from.
Jean-Baptiste Lamarck proposed that organisms could pass on characteristics acquired during their lifetime. This idea is historically important, but it is not how evolution by natural selection works.
Darwinian natural selection acts on existing heritable variation already present in a population. A giraffe does not stretch its neck and then pass that acquired extra length to its offspring. Instead, some giraffes are born with slightly different neck lengths, those differences can be inherited, and if a longer neck improves feeding success in a particular environment, those individuals may leave more offspring. Over many generations, average neck length in the population can increase. Again, populations evolve; individuals do not rewrite their DNA because they need to.
Lamarck
Acquired characteristics passed to offspring.
Darwin
Selection acts on existing heritable variation.
Key Distinction
Need or effort does not create adaptive genetic change in individuals.
Idea
Lamarckian View
Darwinian / Natural Selection View
Source of change
Organism changes during lifetime because of need or use/disuse
Variation already exists within the population
Inheritance
Acquired traits passed to offspring
Heritable genetic variation passed to offspring
Population effect
Individuals direct the change
Selection shifts allele frequencies over generations
Lamarck: acquired traits (from use/disuse or environmental need) are inherited by offspring. Darwin: selection acts on heritable variation that already exists; individuals do not create adaptive changes by effort. Giraffe neck example, Lamarck says stretching is inherited; Darwin says longer-necked individuals already existed and left more offspring. Populations evolve; individual organisms do not change their genotype in response to need.
Assessment angle
If you are asked to compare Lamarck and Darwin, do not only say one is "wrong" and one is "right". Explain exactly what each one claims about inheritance and where the heritable variation comes from. Natural selection explains why the evidence for evolution exists by linking heritable variation to changes in reproductive success across generations.
Pause, copy the highlighted Lamarck vs Darwin comparison into your book before the check below.
What is the key difference between Lamarckian inheritance and Darwinian natural selection?
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Last learn step
Apply the mechanism, then record it
Work two scenarios yourself, then copy the historical context and definitions into your books.
Activities
Activity 1
ApplyBand 3–4
Run the Four Conditions
Use the example of a beetle population with green and brown body colour living on dark bark. Explain how the four conditions of natural selection could lead to the brown form becoming more common over generations. Work through the conditions in order instead of jumping straight to the final outcome.
Activity 2
EvaluateBand 4–5
Fix the Wrong Explanation
A student writes, "Bacteria became resistant because the antibiotic forced them to adapt." Rewrite this explanation so it correctly uses natural selection, mutation and allele frequency. Aim for a compact but fully causal explanation.
Which statement best describes the outcome of natural selection?
Historical Context
Darwin and Wallace independently developed natural selection.
The theory was presented jointly in 1858 and expanded by Darwin in 1859.
Four Conditions
Variation, heritability, differential survival/reproduction, and selection pressure.
If all four occur, allele frequencies can change over generations.
MRSA Example
Resistance already existed in some bacteria due to mutation.
Antibiotics selected resistant bacteria, increasing their frequency.
Lamarck vs Darwin
Lamarck proposed inheritance of acquired characteristics.
Natural selection acts on existing heritable variation in populations.
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Independent practice
Show what you can do without prompts
Apply the lesson to exam-style questions, then compare your reasoning with the model answers.
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Multiple Choice
+5 XP
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
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Short Answer, 10 marks
+5 XP
UnderstandBand 3(4 marks) 1. State the four conditions required for natural selection and explain why each matters.
AnalyseBand 3–4(3 marks) 2. Explain how antibiotic resistance in MRSA provides evidence for natural selection.
EvaluateBand 4–5(3 marks) 3. Assess the statement: "Natural selection means organisms change because they need to."
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1, Run the Four Conditions
Variation exists because the beetle population has individuals with both green and brown body colour, a heritable difference. Heritability applies because offspring can inherit the same colour from their parents. Differential survival and reproduction: on dark bark, green beetles are more visible to predators and are eaten more often, while brown beetles blend in, survive more, and leave more offspring. The selection pressure is predation, which consistently favours brown colouration. Over generations, the brown allele increases in frequency and the green form becomes less common.
Activity 2, Fix the Wrong Explanation
The better explanation is: before antibiotic treatment, some bacteria in the population already carried a resistance mutation from random genetic variation. The antibiotic acted as a selection pressure by killing susceptible bacteria while resistant bacteria survived and reproduced. Because resistance is heritable, the descendants of resistant bacteria also carried the resistance allele. Over successive generations, the frequency of the resistance allele increased throughout the population, the population evolved resistance through natural selection, not through individual bacteria adapting on demand.
Short Answer Model Responses
SA1 (4 marks): The four conditions are variation, heritability, differential survival and reproduction, and selection pressure. Variation matters because individuals must differ for selection to act [1]. Heritability matters because those differences must be passed to offspring [1]. Differential survival and reproduction matter because some variants must leave more descendants than others [1]. Selection pressure matters because the environment must favour some variants over others, causing allele frequencies to change over generations [1].
SA2 (3 marks): MRSA provides evidence for natural selection because resistant bacteria already existed in the population due to mutation before antibiotic treatment [1]. The antibiotic acted as a selection pressure by killing more susceptible bacteria, while resistant bacteria survived and reproduced [1]. Over time, the frequency of resistance increased in the population, showing selection acting on existing heritable variation [1].
SA3 (3 marks): This statement is incorrect because natural selection does not mean organisms change because they need to [1]. Selection acts on heritable variation that already exists in a population. Individuals with favourable variants survive or reproduce more successfully, so those variants become more common over generations [1]. Therefore populations evolve; individual organisms do not intentionally adapt their genes in response to need [1].
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Final step
Retrieve, reflect and finish
Check what actually stuck, revisit your opening answer, and save the completed lesson.
Check what actually stuck
RAPID REVIEW
The big ideas in four tiles
Four conditions
Variation, heritability, differential reproduction, selection pressure, all four must operate for natural selection to work.
MRSA = pre-existing variation
Resistant variants already existed. Antibiotics selected them, bacteria did not "learn" resistance after exposure.
Populations evolve
Selection shifts allele frequencies across generations. Individual organisms do not change their genotype by effort.
Most common exam trap
Saying organisms "adapted" or "developed" new traits in response to need, this is the Lamarckian error. Use pre-existing variation instead.
Test yourself against the clock
boss
Rapid-fire questions on the four conditions, MRSA, Lamarck vs Darwin and how natural selection acts on populations. Beat the boss to bank a tier.
Revisit Your Thinking
Natural selection is about which heritable variants become more common, not about organisms changing because they want to. Peter and Rosemary Grant's 1977–2012 study of Geospiza fortis on Daphne Major, Galápagos, measured this directly: after the 1977 drought killed 85% of the population, mean beak depth increased by 0.5 mm in a single generation, with heritability of 0.79. The birds didn't grow deeper beaks because they needed them, the deeper-beaked individuals already existed in the population, and the drought's removal of soft seeds meant those birds survived and reproduced while shallow-beaked birds died. Allele frequencies shifted across one generation, not because individuals changed, but because differential survival filtered the existing variation.
The same mechanism explains antibiotic resistance: bacteria did not "learn" to resist the drug. Resistance mutations already existed in some individuals, and the antibiotic acted as a selection pressure, exactly as Daphne Major's drought did for finch beaks.
