Variation is the differences in characteristics between individuals of the same species. At GCSE you need to sort those differences into two categories — continuous (a smooth range of values) and discontinuous (distinct, separate groups) — and know the correct graph type, causes, and examples for each.
What is variation?
Variation refers to the differences in observable characteristics (the phenotype) among individuals of the same species. It arises from:
- Genetic factors — differences in DNA sequences (alleles) inherited from parents
- Environmental factors — the effect of diet, temperature, sunlight, or lifestyle on how genes are expressed
- A combination of both — most real characteristics are influenced by both genetics and environment
What is continuous variation?
Continuous variation produces a range of values between two extremes, with no distinct groups — individuals can take any value within the range.
Examples:
- Height (humans range from under 1.5 m to over 2 m, with every measurement possible in between)
- Mass (body weight)
- Hand span
- Foot length
- Skin colour (a continuous range of melanin concentrations)
- Intelligence (as measured by IQ scores)
Graph type: A histogram (a bar chart with no gaps between bars, where the x-axis is a continuous numerical scale). When enough individuals are measured, continuous variation often produces a bell-shaped (normal) distribution, where most individuals cluster near the mean and fewer individuals appear at the extremes.
Causes: Continuous traits are typically controlled by many genes (polygenic inheritance) each with a small effect, plus significant environmental influence. Height, for example, is influenced by hundreds of genes AND by nutrition, health, and exercise during development.
What is discontinuous variation?
Discontinuous variation produces distinct, separate categories with no intermediate values.
Examples:
- Blood group (A, B, AB, or O — there is no "between A and B")
- Tongue rolling ability (can or cannot)
- Earlobes (attached or free)
- Dimples (present or absent)
- Sex (male or female in most animals)
- Flower colour in some plants (e.g. red or white in certain pea varieties)
Graph type: A bar chart (gaps between bars, because the categories are discrete). The x-axis shows category names rather than numbers.
Causes: Discontinuous traits are typically controlled by one gene (or a small number of genes) with a large effect. Environmental factors usually have little or no influence — your blood group does not change with diet.
How do continuous and discontinuous variation compare?
| Feature | Continuous variation | Discontinuous variation |
|---|---|---|
| Type of values | Any value within a range | Distinct, separate categories |
| Graph type | Histogram (no gaps) | Bar chart (gaps between bars) |
| Distribution shape | Often bell-shaped (normal) | Distinct peaks at each category |
| Number of genes controlling | Many (polygenic) | One or a few |
| Environmental influence | Often large | Usually small |
| Examples | Height, mass, skin colour | Blood group, tongue rolling, earlobe type |
What causes variation — genes or environment?
Most phenotypes are influenced by both genetics and environment, but in different proportions:
- Mostly genetic: Blood group, earlobe attachment, eye colour (though UV exposure affects skin tone)
- Strongly environmental: Scar location, language spoken, overall fitness level
- Both in roughly equal measure: Height (genes set the potential; nutrition and health realise it), BMI, exam achievement
The classic way to separate genetic from environmental effects is to study identical (monozygotic) twins raised apart. Because they share identical DNA, any differences between them must result from their different environments. Studies show that height is about 80% heritable (genetics) in well-nourished populations; intelligence scores are 50–80% heritable, with the remainder due to environmental factors.
Why does continuous variation produce a bell-shaped distribution?
When many genes each contribute a small amount to a trait, and each gene independently varies between individuals, the combined effect follows a normal distribution by the central limit theorem in statistics. Most people inherit a mixture of "tall" and "short" versions of the relevant genes, clustering near the average; only a few inherit mostly "tall" or mostly "short" versions, placing them at the extremes. This is the mathematical basis for the bell shape seen in histograms of height, mass, and similar traits.
Frequently asked questions
How do I decide whether to draw a histogram or a bar chart?
Ask: are the categories on the x-axis numbers on a scale (e.g. height in cm, or shoe size)? If yes, use a histogram with no gaps between bars. Are they named categories (e.g. blood group A, B, AB, O)? If yes, use a bar chart with gaps. A common exam error is drawing a bar chart for height data — since height is continuous, a histogram is always correct.
Is tongue rolling really determined by one gene?
Tongue rolling is often cited as an example of simple Mendelian discontinuous variation, but genetic studies have shown that identical twins sometimes differ in their ability, suggesting environmental or developmental factors play a role. It was misclassified as simple single-gene inheritance for decades. For GCSE exam purposes it is still listed as a discontinuous example, but it is a reminder that real biology is often more complex than classroom models suggest.
Can an environmental factor turn a discontinuous trait into a continuous one?
Not easily — if a trait is controlled by one gene with two alleles, it will remain discontinuous regardless of environment. However, environmental influence can sometimes make a trait that looks discontinuous appear more like a continuum. For instance, flower colour in some plants is discontinuous genetically, but the intensity of the colour can be influenced by soil pH, creating a range of shades. The underlying categories remain, but the expression varies.
Why does variation matter for evolution?
Natural selection can only act on heritable variation — differences between individuals that are caused by genetic differences (not purely environmental ones). If all individuals were genetically identical, no selection could occur. Variation provides the raw material: individuals with genetic variants that happen to be advantageous in a particular environment survive and reproduce more successfully, passing those variants to offspring. Over many generations, advantageous variants become more common — this is evolution by natural selection.
For Socratic GCSE biology with Professor Darwin — from gene variants to population distributions, Socratic questions every step — visit aitutors.me.