Module 4: Causes of Variation
These free OCR A Level Biology Causes of Variation revision notes have been written for specification point 4.2.2(f).
Causes of Variation
Variation in organisms is caused by differences in their genetic information and environmental conditions.
- Environmental: Caused by differences in external conditions or experiences that affect an organism’s characteristics.
- Genetic: Caused by differences or changes in an organism’s DNA, including the expression of genes in its genome.
- Combined: Caused by the interaction between genetic factors and environmental influences.
Environmental Causes of Variation
Environmental variation is caused by the conditions in an organism’s surroundings influencing how genes are expressed.
The table below outlines some environmental factors and their effect on an organism:
| Environmental Factor | Effect on Organism |
|---|---|
| Diet | Affects nutrient availability; a poor childhood diet can limit growth (e.g. human height). |
| Predators or Disease Exposure | Triggers stress and immune responses. |
| Human Activity | Introduces pollutants that can disrupt normal gene function. |
| Light | Influences hormones that control development; causes tanning in human skin. |
| Temperature | Alters enzyme activity; e.g. hawthorn trees grow tall in the wild but stay bushy when trimmed. |
Mutation
A mutation is a permanent change in the base sequence of DNA.
Mutations can be neutral, harmful, or (rarely) beneficial.
A mutation may:
- Change the amino acid sequence of a protein, and its structure or function.
- Affect gene regulation and expression.
- Have no effect at all
Causes of Mutation
Mutations may occur due to cellular mishap or be induced by environmental factors (mutagens).
Genetic Causes of Mutation
Genetic mutations occur due to random errors by enzymes (e.g. DNA polymerase) or structural instability (e.g. spindle fibres failing to separate chromosomes).
Environmental Causes of Mutation
Mutations can occur due to environmental factors known as mutagens.
Mutagens increase the rate of mutation by damaging DNA or interfering with replication.
The table below outlines some mutagenic factors and the damage they cause:
| Cause | Type of Damage | Examples |
|---|---|---|
| Ionising radiation | Breaks DNA strands | X-rays, gamma rays |
| UV light | Causes thymine dimers | Sunlight |
| Chemicals | Modify bases or interfere with DNA replication | Cigarette smoke, asbestos, benzene |
| Viruses | Insert viral DNA into the host genome | HPV, retroviruses |