Continuous variation shows a smooth range of values between two extremes, such as length or mass. Discontinuous variation shows a few clear categories with nothing in between, such as flower colour. You are asked to classify examples, read graphs of each type, and state the usual causes.
This skill opens the module on variation and selection and comes back whenever a question shows a data table or a graph.
How do you tell the two types apart?
Ask one question: can a new individual fall anywhere between the existing values? If yes, the variation is continuous. If individuals must fit into separate groups, it is discontinuous.
| Feature | Continuous | Discontinuous |
|---|---|---|
| Values | Any value in a range | Distinct categories |
| Measured or counted | Measured on a scale | Counted in groups |
| Typical graph | Histogram or smooth curve | Bar chart with gaps |
| Usual cause | Genes and environment | Genes only |
| Invented example | Leaf length | Seed shape, round or wrinkled |
Notice that “numbers in the data” does not decide it. Counting whole petals gives numbers, but the values are separate steps, so the variation is discontinuous.
Worked example
A student measures the leaf length of 50 leaves from a fictional shrub. The data are invented for practice.
| Leaf length (mm) | Frequency |
|---|---|
| 30 to under 40 | 3 |
| 40 to under 50 | 9 |
| 50 to under 60 | 19 |
| 60 to under 70 | 13 |
| 70 to under 80 | 6 |
Step 1, check the total: 3 + 9 + 19 + 13 + 6 = 50 leaves. The table accounts for every leaf.
Step 2, name the type: length is measured and a leaf can be any length in a range, so this is continuous variation.
Step 3, describe the shape: most leaves are in the middle class, 50 to under 60 mm, and the frequencies fall away on both sides. This bell shape is typical of continuous variation.
Step 4, use the data: leaves in the two classes from 50 to under 70 mm number 19 + 13 = 32. As a fraction of the sample that is 32/50 = 64%.
Step 5, give a cause: leaf length depends on genes, but also on light, water and soil. The same shrub grown in one uniform condition would give a narrower graph, which shows the environment is involved.
Now compare a discontinuous case. In a second invented sample of 50 plants, 34 have purple flowers and 16 have white flowers.
There is no pale purple group, so the plot is a bar chart with two separate bars. One gene with two alleles can cause this pattern.
The mistake to watch for
Mistaken answer: “Number of petals is continuous because it is a number.”
A number can come from counting or measuring. Counting whole items gives steps with nothing in between, so the variation is discontinuous. The correction is to use the test above: a flower cannot have 4.6 petals.
A second slip is drawing a bar chart with gaps for leaf length. Continuous data need touching bars or a smooth line, because the classes join on to each other.
Check yourself
All data are invented.
1. Classify each feature as continuous or discontinuous: (a) mass of seeds in grams; (b) presence or absence of spots on a fictional beetle; (c) height of bean plants in centimetres.
Show answer
(a) Continuous, measured on a scale. (b) Discontinuous, two categories only. (c) Continuous, measured on a scale.
2. A table shows ten classes of pod length with frequencies 2, 5, 9, 14, 18, 15, 10, 6, 3 and 1. Which type is this likely to be, and how many pods were measured?
Show answer
Pod length is measured, so it is likely continuous. The total is 2 + 5 + 9 + 14 + 18 + 15 + 10 + 6 + 3 + 1 = 83 pods.
3. Two seedlings with the same genes are grown, one in sun and one in shade. The shaded one is shorter. What does this show about continuous variation?
Show answer
The genes were the same, so the difference comes from the environment. This supports the idea that continuous variation is affected by both genes and environment.
Where this leads next
Next, explain a source of genetic variation to see where differences between individuals come from in the first place. The inheritance model board shows how a single gene can make a discontinuous pattern, and the module practice set mixes all five skills.
Some students know the table by heart and still misclassify a new example in an exam. A teacher in online one-to-one Biology tuition can ask you why you chose each label and tighten that reasoning.