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Detailed notes on Inheritance for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Variation within populations is the raw material for natural selection. Cambridge tests continuous vs discontinuous variation, the causes of variation, and the mechanism of natural selection — including antibiotic resistance as the key applied example.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Continuous variation shows a range of values; discontinuous shows distinct groups. Both have different causes and genetic bases.
Variation: the differences between individual organisms of the same species.
Continuous variation:
Discontinuous variation:
| Feature | Continuous | Discontinuous |
|---|---|---|
| Categories | Range of values | Distinct groups |
| Graph | Normal distribution (bell curve) | Bar chart |
| Cause | Multiple genes + environment | Usually one or few genes |
| Example | Height, mass | Blood group, sex |
Variation has genetic causes (mutation, meiosis, fertilisation) and environmental causes (diet, exercise, climate).
Genetic causes (heritable):
Environmental causes (not heritable):
Most variation is BOTH genetic AND environmental: Identical twins (same DNA) still differ slightly — due to environmental factors. Height has a genetic component (tall parents → taller children on average) AND environmental (nutrition, illness). This interaction is key to understanding complex traits.
Natural selection is the process by which organisms with advantageous heritable variation survive and reproduce more, causing allele frequencies to change over generations.
Natural selection is the mechanism proposed by Darwin for evolution. It requires:
'Fitness' in biology: An organism's ability to survive and reproduce in its current environment — NOT about being physically strong or fast.
Selective pressure: Any environmental factor that causes differential survival — predators, disease, climate, food availability.
Antibiotic resistance evolves by natural selection — a small number of bacteria with resistance mutations survive treatment and reproduce to create a resistant population.
Antibiotic resistance is the best-understood real-world example of natural selection.
How resistance develops (step by step):
Why this is natural selection:
Key point: The antibiotic does NOT cause the mutation. The mutation occurred randomly BEFORE the antibiotic was applied — the antibiotic is the selective pressure that kills non-resistant bacteria.
Why antibiotic resistance is a public health crisis:
How to slow resistance:
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'Explain how a population of bacteria could become resistant to an antibiotic over many generations' (5 marks — must include: variation/mutation, antibiotic kills non-resistant, resistant survive, reproduce, pass on allele, resistant population increases). 'Describe the difference between continuous and discontinuous variation, with one example of each' (4 marks). MCQ tests causes of variation and identifying which type of variation a given trait shows.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B23); 0654/42 May/Jun 2023 — Q11 (natural selection); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on variation and selection , written exactly the way a tutor would explain them at the board.
Question
Explain how antibiotic resistance arises in a population of bacteria.
Step-by-step solution
Step 1
Within a population of bacteria, individuals show variation in their sensitivity to antibiotics, due to random mutations in their DNA.
Step 2
When an antibiotic is used, it kills non-resistant bacteria (they cannot survive). Bacteria with resistance mutations survive and reproduce.
Step 3
Resistant bacteria pass the resistance allele (or mutation) to their offspring by binary fission (asexual reproduction) — rapid population growth of resistant strain.
Step 4
Over many generations, the resistant bacteria outcompete non-resistant ones. The proportion of resistant bacteria in the population increases — the population evolves resistance.
Answer
Random mutations give some bacteria resistance. Antibiotic treatment kills non-resistant bacteria; resistant bacteria survive and reproduce. Resistance is inherited by offspring. Over generations, the resistant strain increases in frequency — this is natural selection (survival of the fittest).
Examiner tip
Never say bacteria 'adapt' to antibiotics deliberately. Resistance arises by random mutation and is selected for by antibiotic exposure.
Question
Distinguish between continuous and discontinuous variation, using one example of each.
Step-by-step solution
Step 1
Continuous variation: shows a range of values with no distinct categories; affected by both genes and environment; plotted as a normal distribution curve. Example: height (ranges from about 150 cm to over 200 cm in a population).
Step 2
Discontinuous variation: shows distinct separate categories with no intermediates; determined mainly by genes; plotted as a bar chart. Example: ABO blood group (four distinct groups: A, B, AB, O).
Answer
Continuous variation: a range of values (e.g. height); influenced by genes AND environment; forms a normal distribution. Discontinuous variation: distinct categories (e.g. blood group A, B, AB, O); mainly genetic; shown as separate bars.
Question
Describe how natural selection could lead to a population of dark-coloured moths in a polluted environment.
Step-by-step solution
Step 1
The moth population shows variation in colour — some pale, some dark — due to genetic differences (mutations).
Step 2
In a polluted environment, tree bark is dark (coated in soot). Pale moths are visible to predators (birds) on dark bark → pale moths are predated at a higher rate.
Step 3
Dark moths are camouflaged on dark bark → less predation → dark moths survive and reproduce more successfully → pass dark colour alleles to offspring.
Step 4
Over many generations, the proportion of dark moths increases in the population. Natural selection has favoured the dark phenotype in this environment.
Answer
Variation in moth colour exists. Pale moths are visible on dark (soot-covered) bark and are predated more. Dark moths are camouflaged, survive more, and reproduce, passing dark colour alleles to offspring. Over generations, the proportion of dark moths increases — natural selection in action.
Question
Explain why selective breeding reduces genetic variation.
Step-by-step solution
Step 1
In selective breeding, only individuals with desired traits are chosen as parents for the next generation.
Step 2
Individuals without the desired traits do not breed — their alleles are not passed on to offspring.
Step 3
The gene pool (total variety of alleles in the population) becomes smaller with each generation. The population becomes more genetically uniform.
Step 4
Reduced variation makes the population more vulnerable to new diseases or environmental changes (all individuals share similar genetic susceptibilities).
Answer
Selective breeding allows only individuals with desired traits to reproduce; alleles from individuals without desired traits are not passed on; the gene pool shrinks; the population becomes less genetically diverse and more vulnerable to disease.
Definitions to memorise and the exact keywords mark schemes credit for variation and selection answers — sharpened from recent examiner reports for the 2026 0654 sitting.
Variation in a characteristic that shows a range of values with no distinct categories. Influenced by both genetic and environmental factors. E.g. height, body mass.
Variation in a characteristic that shows distinct, separate categories with no intermediate forms. Mainly determined by genes. E.g. ABO blood group, sex.
The process by which organisms better adapted to their environment survive, reproduce, and pass on advantageous alleles to offspring, causing populations to change over time.
The process in which humans choose parents with desired characteristics to breed together over many generations, producing offspring with those traits.
An inherited feature that makes an organism better suited to survive and reproduce in its environment.
The total collection of alleles present in a population at a given time. A large, diverse gene pool provides more variation for natural selection to act upon.
The traps other students keep falling into on variation and selection questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2023
Why it happens
Students describe evolution as a deliberate adaptation by organisms.
How to avoid it
Resistance arises by random mutation before exposure to antibiotics. Antibiotics select for pre-existing resistant bacteria — they do not cause the mutation. Evolution is not directed or deliberate.
Why it happens
Students associate evolution with 'survival of the fittest' and interpret 'fittest' as physically strongest.
How to avoid it
'Fittest' means best adapted to the current environment, not physically strongest. A camouflaged slow moth may survive better than a fast but conspicuous one.
Why it happens
Students know genes control characteristics and overlook environmental influence.
How to avoid it
Continuous variation is influenced by both genes AND environment. For example, height is partly genetic but also influenced by diet and health during childhood.
Why it happens
Students focus on mutation as the origin of new alleles.
How to avoid it
Genetic variation also arises from sexual reproduction: crossing over (during meiosis) and independent assortment create new allele combinations. Random fertilisation adds further variation.
The things students keep getting wrong in this sub-topic, answered.