By this point we have built a case that evolution happened. Natural selection is the mechanism that explains how those fossil, anatomical, biogeographical and molecular patterns arise. This lesson turns evidence into process.
IQ2~50 minLesson 9 of 185 MC + 3 short answer
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Use digital mode if you want to work through the MRSA example and selection steps directly on-screen. Switch to book mode if you want to sketch the four conditions and allele-frequency flow by hand before checking your explanation against the page.
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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?
Write your first answer now. We will revisit it after the natural-selection mechanism and Lamarck-vs-Darwin comparison.
Write your initial answer in your book, then return later to compare it with your final explanation.
Write this in your book, then revisit it later.
Saved locally
📚 Know
Key facts and definitions for Darwin, Wallace and Natural Selection
Relevant terminology and conventions
🔗 Understand
The concepts and principles underlying Darwin, Wallace and Natural Selection
How to explain the reasoning behind key ideas
✅ Can Do
Apply concepts from Darwin, Wallace and Natural Selection to exam-style questions
Justify answers using appropriate biological reasoning
Key Terms
Natural selectionthe mechanism that explains how those fossil, anatomical, biogeographical and molecular patterns arise
idea that acquired characteristicspassed to offspring
Homeostasisthe body stays exactly the same all the time
power of the theorythat it is not just a slogan
all four conditionsin place, allele frequencies can shift over generations and the population changes
Some differencespassed to offspring
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 Lessons 05-08.
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.
Key Terms — scan these before reading
Definition relevant to Darwin, Wallace and Natural Selection.
Definition relevant to Darwin, Wallace and Natural Selection.
Definition relevant to Darwin, Wallace and Natural Selection.
Definition relevant to Darwin, Wallace and Natural Selection.
Definition relevant to Darwin, Wallace and Natural Selection.
Definition relevant to Darwin, Wallace and Natural Selection.
Misconceptions to Fix
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Wrong: Homeostasis means the body stays exactly the same all the time.
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Right: Homeostasis involves dynamic equilibrium — constant small adjustments around a set point.
Core Content
01
Darwin, Wallace and the Four Conditions
The historical idea and the logic it depends on
Charles Darwin and Alfred Russel Wallace independently developed the idea of natural selection, then presented it jointly to the Linnean Society in 1858. Darwin expanded the theory in On the Origin of Species in 1859.
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.
VariationIndividuals differ in their traits.
HeritabilitySome differences are passed to offspring.
Differential SuccessSome variants leave more offspring.
Selection PressureThe environment favours some variants over others.
Population ChangeFavoured alleles increase in frequency.
Not InstantThis happens across generations, not inside one organism.
Natural selection is a process with conditions, not a vague idea that organisms simply "adapt because they need to".
Exam tip: if a question asks for natural selection, name the conditions explicitly. Examiners usually want mechanism, not just the phrase "survival of the fittest".
02
Antibiotic Resistance as a Worked Example
Why MRSA is selection, not intentional change
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
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.
Working diagram prompt: draw a bacterial population before and after antibiotic use. Show a few resistant cells at the start, massive death of susceptible cells after treatment, and resistant descendants dominating later generations.
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"
03
Lamarck vs Darwin
Why acquired characteristics are not the mechanism
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.
LamarckAcquired characteristics passed to offspring.
DarwinSelection acts on existing heritable variation.
Key DistinctionNeed or effort does not create adaptive genetic change in individuals.
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.
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
Core idea: natural selection explains why the evidence for evolution exists by linking heritable variation to changes in reproductive success across generations.
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.
Activities
ApplyBand 3-4
Activity 01
Run the Four Conditions
Pattern B - Apply mechanism
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.
Sketch the sequence in your book first, then summarise the explanation here.
Map the four conditions in your book, then record your explanation here.
EvaluateBand 4-5
Activity 02
Fix the Wrong Explanation
Pattern B - Diagnose and correct
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.
Write the corrected paragraph in your book, then record your clean version here.
Write the corrected explanation in your book, then condense it here.
Revisit Your Thinking
Natural selection is about which heritable variants become more common, not about organisms changing because they want to. Once that shift in thinking clicks, the earlier evidence lessons line up more clearly: the evidence shows the pattern, and natural selection explains the mechanism.
If your original answer assumed exposure creates the useful trait, the key correction is this: selection does not invent the variation. It filters the variation that already exists.
Assessment
MC
Check Your Understanding
Answer first, then read the explanation
1. Which pair correctly identifies two of the required conditions for natural selection?
2. What is the correct explanation for antibiotic resistance increasing in a bacterial population?
What is NOT the correct explanation for antibiotic resistance increasing in a bacterial population?
3. Which statement best describes the outcome of natural selection?
4. What is the key difference between Lamarckian inheritance and Darwinian natural selection?
What is NOT the key difference between Lamarckian inheritance and Darwinian natural selection?
5. Which statement best captures the role of Wallace in the history of natural selection?
Short Answer - 10 marks
1. State the four conditions required for natural selection and explain why each matters. (4 marks)
1 mark each for condition plus valid significance/explanation
2. Explain how antibiotic resistance in MRSA provides evidence for natural selection. (3 marks)
1 mark: resistant variants pre-exist | 1 mark: antibiotic is selection pressure | 1 mark: resistance frequency increases
3. Assess the statement: "Natural selection means organisms change because they need to." (3 marks)
Q1 - A: Heritable variation and differential reproductive success are essential conditions for natural selection.
Q2 - B: Resistance already existed in some bacteria and antibiotic use selected for it.
Q3 - B: Natural selection changes allele frequencies in populations over generations.
Q4 - B: Lamarck proposed acquired-trait inheritance, while Darwinian selection acts on existing heritable variation.
Q5 - B: Wallace independently developed natural selection and presented it jointly with Darwin.
SA1: The four conditions are variation, heritability, differential survival and reproduction, and selection pressure. Variation matters because individuals must differ for selection to act. Heritability matters because those differences must be passed to offspring. Differential survival and reproduction matter because some variants must leave more descendants than others. Selection pressure matters because the environment must favour some variants over others, causing allele frequencies to change over generations.
SA2: MRSA provides evidence for natural selection because resistant bacteria already existed in the population due to mutation before antibiotic treatment. The antibiotic acted as a selection pressure by killing more susceptible bacteria, while resistant bacteria survived and reproduced. Over time, the frequency of resistance increased in the population, showing selection acting on existing heritable variation.
SA3: This statement is incorrect because natural selection does not mean organisms change because they need to. 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. Therefore populations evolve; individual organisms do not intentionally adapt their genes in response to need.
AR
Rapid Recall
Say each answer aloud before moving to the next prompt
What are the four conditions of natural selection?
Why does a population need heritable variation before natural selection can act?
How does antibiotic resistance increase in a bacterial population?
What is the difference between Lamarckian inheritance and Darwinian selection?
Why do populations evolve rather than individuals?
What was Wallace's role in the history of natural selection?