Topic 18.1 · Core
Continuous and Discontinuous Variation
Variation is simply the differences between individuals of the same species — but Cambridge wants you to sort that variation into two distinct patterns, and the sorting rule is not whether a characteristic is genetic. Both patterns can involve genes. The rule is whether the phenotypes form a smooth range or fall into separate boxes with nothing in between.
What counts as variation
Individuals belonging to one species are never identical. A difference that can be observed or measured between them is variation, and it can come from genetic differences, from environmental effects, or from both acting together. It is worth being precise about the scope of the word: variation describes differences within a species. Comparing a human to a chimpanzee is not an example of variation in the sense this chapter uses it — the comparison has to stay inside one species.
Continuous variation
Continuous variation produces a range of phenotypes between two extremes, with genuine intermediate values all along that range rather than a handful of separate groups. If you measured the body mass of a large group of people, you would not find them clustering into three or four fixed categories — you would find values scattered right across the range from lowest to highest, with plenty of people sitting between any two figures you picked. Cambridge’s named examples are body length and body mass, and both are usually influenced by genes and the environment together. Someone’s genes may set a potential range for their size, but environmental factors such as nutrition affect where within that range the actual phenotype ends up — which is exactly why two genetically similar individuals can still look noticeably different if they grow up in different conditions.
Discontinuous variation
Discontinuous variation produces a limited number of distinct phenotypes, with no intermediate forms between them. Cambridge’s named examples are ABO blood groups, seed shape in peas and seed colour in peas. Nobody has a blood group that sits halfway between A and B, and a pea seed is not partway between round and wrinkled — each individual falls cleanly into one category or another. Discontinuous variation is usually caused by genes only, which is the one place this topic invites a shortcut students should resist taking: “usually” is doing real work in that sentence, and it is not the same claim as “always caused by genes, never by environment, in every discontinuous case.”
Continuous vs discontinuous variation
| Feature | Continuous variation | Discontinuous variation |
|---|---|---|
| Pattern | A range between two extremes | A limited number of distinct categories |
| Intermediates | Present | Absent |
| Typical data | Measurements | Category counts |
| Main causes | Genes and environment | Usually genes only |
| Cambridge examples | Body length, body mass | ABO blood groups, pea seed shape, pea seed colour |
The distinction that actually matters for marks is the pattern of the phenotypes, not simply whether genes are involved somewhere in the story — both types can involve genes, and treating “genetic” as the dividing line is one of the more common ways this topic gets muddled.
Investigating both types
Cambridge explicitly expects you to be able to investigate examples of both patterns, and the two procedures are genuinely different because the underlying data is different in kind.
For a continuous characteristic: define exactly what you are measuring, measure it the same way for every individual, record a numerical value with units each time, collect enough individuals to show the spread properly, then organise the values into intervals with frequencies so the range becomes visible. A frequency distribution — essentially a histogram of measured values — is the natural way to display this kind of data.
For a discontinuous characteristic: define the categories clearly and unambiguously first, then place each individual into the correct category, count how many fall into each one, and compare the category frequencies. A simple bar chart with one bar per distinct category is the natural way to display this kind of data, and the gaps between the bars are meaningful — they represent the absence of intermediate forms, not just a stylistic choice.
It is worth noticing that how you display the data supports the classification but does not decide it. Dividing a set of continuous measurements into tidy intervals for a graph does not turn body mass into a discontinuous characteristic — the underlying biology is still a genuine range, just grouped for readability. And a very small sample can easily fail to show every value or every category that actually exists in the wider population, so classify a characteristic by its biological nature, not by what a handful of measurements happened to produce.
What examiners actually check for
A weak answer states that a characteristic is “continuous because it’s genetic” or “discontinuous because you can count it” — neither reason is the one Cambridge is testing. A strong answer identifies whether intermediate phenotypes exist between the extremes, names the correct cause (genes and environment for continuous; usually genes only for discontinuous), and uses one of the four named Cambridge examples correctly rather than inventing a new one. If a question gives you an unfamiliar characteristic and asks you to classify it, look for whether a value between two individuals is biologically possible — if it is, you are looking at continuous variation; if every individual has to belong to one of a small, fixed set of categories, you are looking at discontinuous variation.
Both types describe differences that already exist in a population — variation on its own is not yet adaptation or selection. Where that existing variation comes from is covered on the mutation and genetic variation page, and what the environment does with it once it exists is covered under natural selection.