Key Learning Objectives
- Outline Darwin's theory of natural selection: genetic mutation, phenotypic variation, differential survival, and allele inheritance.
- Explain antibiotic resistance in bacteria as an observable example of rapid modern natural selection.
- Structure full-mark explanations for natural selection questions.
Topic Summary & Core Notes
Walk through Darwin's theory of natural selection using an interactive PhET simulation. Master the 4-step exam response: Mutation → Variation → Survival Advantage → Reproductive Inheritance.
Key Rules, Formulas & Definitions
4-Step Natural Selection Framework
1. Random Mutation → 2. Phenotypic Variation → 3. Selection Pressure / Survival Advantage → 4. Breeding & Allele Inheritance
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Evolution of Antibiotic Resistance in Bacteria
Question: Explain how a population of bacteria can become resistant to an antibiotic over time.
Step-by-Step Solution:
- Step 1: Mutation: A random genetic mutation arises during bacterial DNA replication.
- Step 2: Selection pressure: When the antibiotic is applied, non-resistant bacteria are killed.
- Step 3: Differential survival: The mutant bacterium survives the antibiotic treatment.
- Step 4: Reproduction: The surviving bacterium reproduces rapidly via binary fission, passing the resistance allele to all offspring.
Final Answer: Random mutation provides resistance; exposure to antibiotics creates selection pressure where resistant bacteria survive, reproduce, and pass the allele to descendants.
Senior Examiner Insight: Never write 'bacteria become immune' (bacteria have no immune systems); write 'resistant bacteria survive and reproduce'.
Worked Example 2
Worked Example 2: Peppered Moth Camouflage
Question: Explain why the frequency of dark-coloured peppered moths increased dramatically during the Industrial Revolution.
Step-by-Step Solution:
- Step 1: Environmental change: Soot from coal factories coated birch tree trunks dark.
- Step 2: Selection advantage: Dark mutant moths were camouflaged against dark bark; pale moths were easily spotted and predated by birds.
- Step 3: Allele propagation: Dark moths survived longer, bred, and passed on the dominant allele for dark melanic coloration.
Final Answer: Soot-darkened trees camouflaged dark moths from predators, allowing them to survive, reproduce, and increase the dark allele frequency.
Senior Examiner Insight: Explicitly mention predator evasion leading to increased reproductive success.
Common Pitfalls & Examiner Warnings
Common Mistake: Lamarckian wording ('the animal adapted in order to...')
Organisms do not adapt by conscious desire; random mutations pre-exist and are selected by the environment.
Common Mistake: Saying bacteria become 'immune'
Bacteria do not possess antibodies or immune systems; they become RESISTANT.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What is the ultimate primary source of novel alleles that create genetic variation in populations?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Random DNA mutations
Detailed Explanation: Spontaneous random mutations in DNA nucleotide sequences generate new alleles.
Mark Scheme & Scoring Points: 1 mark for Random DNA mutations.
Question 2 of 3
Why is it dangerous when patients fail to complete their prescribed course of antibiotics?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Less sensitive bacteria survive, reproduce, and form an antibiotic-resistant strain
Detailed Explanation: Stopping early leaves moderately resistant bacteria alive to multiply, spreading resistance alleles.
Mark Scheme & Scoring Points: 1 mark for allowing partially resistant bacteria to survive and multiply.
Question 3 of 3
Which scientist independently conceived the theory of evolution by natural selection alongside Charles Darwin?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Alfred Russel Wallace
Detailed Explanation: Wallace jointly published the theory of evolution by natural selection with Darwin in 1858.
Mark Scheme & Scoring Points: 1 mark for Alfred Russel Wallace.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
Accelerate Your Academic Progress
Join tailored 1-to-1 or masterclass revision programmes with senior examiner insights.
Book a Free Consultation