B37: Variation
Causes of variation within and between species
Key Definitions
Variation: The differences between individuals within a species. Variation can be caused by genetic factors, environmental factors, or a combination of both.
Mutation: A change in the DNA sequence of a gene or chromosome. Most mutations have no effect, some are harmful, and a very small number are beneficial. Mutations are the ultimate source of new alleles.
Genetic Variation
Genetic variation is caused by differences in the alleles inherited from parents. The three sources of genetic variation are: mutations, meiosis (independent assortment and crossing over), and random fertilisation.
| Source | How It Causes Variation |
|---|---|
| Mutations | Changes in the DNA sequence can create new alleles, producing new characteristics |
| Meiosis | During meiosis, chromosomes are shuffled (independent assortment) and sections of DNA are exchanged (crossing over), producing genetically unique gametes |
| Random fertilisation | Any sperm can fertilise any egg — the combination of alleles is random, producing genetically unique zygotes |
Example 1: Genetic Variation from Meiosis
Humans have 23 pairs of chromosomes. During meiosis, the way each pair lines up and separates is random (independent assortment). The number of possible combinations from meiosis alone is 223 = over 8 million different gametes. When you consider random fertilisation, the number of possible zygotes is 8 million × 8 million = over 70 trillion — which is why siblings can look so different from each other.
Environmental Variation
Environmental variation is caused by differences in the environment in which an organism lives and develops. Environmental factors include diet, exercise, climate, light, scars, and injuries.
Example 2: Environmental Variation
- Diet: Two people with the same genes for height will grow to different heights if one has poor nutrition
- Exercise: Identical twins have the same genes, but one who exercises will have larger muscles
- Climate: Arctic hares grow white fur in winter and brown fur in summer (same genes, different environment)
- Scars: A scar is caused by the environment (injury), not genes
- Light: Plants grown in the dark become tall and pale (etiolation) — same genes, different light conditions
Continuous vs Discontinuous Variation
| Feature | Continuous Variation | Discontinuous Variation |
|---|---|---|
| Definition | A range of values with no distinct categories — any value is possible within a range | Distinct categories with no intermediate values |
| Distribution | Normal distribution curve (bell-shaped) | Bar chart |
| Caused by | Both genetic and environmental factors (polygenic — many genes) | Genetic factors only (often controlled by a single gene) |
| Examples | Height, weight, foot size, skin colour | Blood group, eye colour, ear lobe type, sex |
Example 3: Continuous Variation — Height
Human height is an example of continuous variation. People do not fall into “tall” or “short” categories — there is a smooth range of heights from very short to very tall. When plotted on a graph, height forms a normal distribution curve (bell-shaped), with most people near the middle and fewer at the extremes. Height is influenced by genes (many genes contribute) AND the environment (nutrition during childhood).
Example 4: Discontinuous Variation — Blood Group
Human blood group is an example of discontinuous variation. You can only have blood group A, B, AB, or O — there are no intermediate values. Blood group is controlled by a single gene with three alleles. The environment has no effect on blood group — it is entirely genetically determined.
Mutations
A mutation is a change in the DNA sequence of a gene or chromosome. Most mutations have no effect on the organism. Some mutations are harmful (e.g. causing cancer or genetic disorders). A very small number are beneficial (e.g. producing a new allele that gives a survival advantage).
Mutations can be caused by mutagens — agents that increase the rate of mutation:
- Ionising radiation — X-rays, UV light, gamma rays
- Chemical mutagens — certain chemicals in tobacco, asbestos, some industrial chemicals
Mutations occur spontaneously but are more likely when organisms are exposed to mutagens. Mutations are the ultimate source of new alleles and are essential for evolution, even though most are neutral or harmful.
Comparison: Types of Variation
| Feature | Genetic Variation | Environmental Variation |
|---|---|---|
| Cause | Differences in alleles (DNA) | Differences in the environment |
| Inherited? | Yes — passed from parent to offspring | No — not passed on |
| Examples | Eye colour, blood group, natural hair colour | Muscle size, scars, dyed hair, calluses |
| Can it change? | No (except by mutation) | Yes — can change throughout life |
| Feature | Continuous | Discontinuous |
| Values | Any value within a range | Distinct categories only |
| Graph shape | Bell curve (normal distribution) | Bar chart |
| Causes | Genes + environment (polygenic) | Genes only (often single gene) |
| Examples | Height, weight, hand span | Blood group, eye colour, sex |
Practice Questions
Q1: Foundation State three causes of genetic variation.
Q2: Foundation Explain the difference between continuous and discontinuous variation. Give one example of each.
Q3: Foundation Give two examples of environmental variation and explain how the environment causes the difference.
Q4: Higher Explain why height shows continuous variation but blood group shows discontinuous variation.
Q5: Higher What is a mutation? Name two mutagens and explain why most mutations have no effect.
Answers
- 1. Mutations — changes in DNA that create new alleles. 2. Meiosis — produces genetically unique gametes through independent assortment and crossing over. 3. Random fertilisation — any sperm can fertilise any egg, creating unique combinations of alleles.
- Continuous variation has a range of values with no distinct categories (e.g. height — any value between a range is possible). It produces a normal distribution curve and is influenced by both genes and environment. Discontinuous variation has distinct categories with no intermediate values (e.g. blood group — only A, B, AB, or O). It is controlled by genes alone and is not affected by the environment.
- 1. Muscle size: A person who exercises regularly will develop larger muscles than someone with the same genes who does not exercise — the environment (physical activity) causes the difference. 2. Plant growth in different light: A plant grown in bright light will be short and green, while a genetically identical plant grown in the dark will be tall and pale (etiolation) — the light environment causes the difference.
- Height is controlled by many genes (polygenic) and is also influenced by the environment (e.g. nutrition). The combination of many genes with different alleles plus environmental effects creates a smooth range of values — continuous variation. Blood group is controlled by a single gene with three alleles. The environment has no effect. Only four possible phenotypes exist (A, B, AB, O) with no intermediate values — discontinuous variation.
- A mutation is a change in the DNA sequence of a gene or chromosome. Two mutagens are: ionising radiation (e.g. UV light, X-rays) and chemical mutagens (e.g. chemicals in tobacco). Most mutations have no effect because: the genetic code is degenerate (multiple codons code for the same amino acid), so a change may not alter the protein produced; or the mutation may occur in non-coding DNA that does not affect protein production; or the change may not significantly affect the protein’s function.
Exam Tips
- Remember: continuous variation = range of values + normal distribution + both genetic AND environmental. Discontinuous variation = distinct categories + genetic only. This is a very common exam question
- When asked about the sources of genetic variation, mention all three: mutations, meiosis, and random fertilisation
- Most characteristics are influenced by both genes and environment — only a few (like blood group) are purely genetic
- Mutations are random — they are not caused by the environment. Mutagens increase the rate of mutations but don’t direct what the mutation will be
🔢 Maths Skills
Mathematical Skills
Mean, median, mode and range: given data on heights of 10 students (cm): 152, 148, 155, 160, 147, 155, 163, 150, 155, 157. Mean = (152+148+155+160+147+155+163+150+155+157) ÷ 10 = 1542 ÷ 10 = 154.2 cm. Median = 155 cm (middle value when ordered). Mode = 155 cm (most frequent). Range = 163 − 147 = 16 cm.
Interpreting normal distribution curves: the mean is at the peak; approximately 68% of data falls within one standard deviation of the mean; a tall narrow curve shows less variation, a wide flat curve shows more.
⚠️ Common Misconceptions
Watch Out!
Students often think all variation is genetic. Wrong: All variation is caused by genes Correct: Variation can be genetic, environmental, or a combination of both (e.g. height is influenced by genes AND nutrition)
Students often think discontinuous variation is always purely genetic. Wrong: Discontinuous variation is always genetic only Correct: While usually genetic, some characteristics like eye colour can have minor environmental influence, and the distinction is not always absolute
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the causes of variation, distinguishing between genetic and environmental factors. Use examples in your answer.
Genetic variation is caused by differences in alleles inherited from parents. The three sources are: mutations (changes in DNA creating new alleles); meiosis (independent assortment and crossing over produce genetically unique gametes); and random fertilisation (any sperm can fertilise any egg). Examples include eye colour and blood group, which are entirely genetic. Environmental variation is caused by differences in the environment: diet (affecting height and weight), exercise (affecting muscle size), light (affecting plant growth), and injuries (scars). Many characteristics are influenced by both: for example, height is polygenic (many genes) but also requires adequate nutrition to reach genetic potential. Identical twins have the same genes but differ due to environment.
Mark scheme: 1 mark for each genetic source (up to 3), 1 mark for genetic examples, 1 mark for environmental factors with examples, 1 mark for combined influence with example
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A study measured the body mass of 50 adult males: the mean was 82 kg with a range of 58 to 120 kg. A second study of 50 marathon runners had a mean of 68 kg with a range of 55 to 78 kg. Interpret the data: what does the difference in range suggest about the causes of variation in each group? Explain your reasoning.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Biology exams?
Get the best revision books and guides to boost your grades.