Continuous and discontinuous variation

Variation and selection · Variation · note 1 of 2

Spec 18.1.1, 18.1.2, 18.1.3, 18.1.4, 18.1.5Download all Variation notes (PDF)Download all notes (PDF)4 pages

Continuous and discontinuous variationSpec 18.1.1, 18.1.2, 18.1.3, 18.1.4, 18.1.5

In short

Variation is the differences between individuals of the same species. Continuous variation gives a range of phenotypes between two extremes, such as body length, and is caused by both genes and the environment. Discontinuous variation gives a limited number of phenotypes with no intermediates, such as ABO blood groups, and is usually caused by genes only.

Variation is the differences between individuals of the same species. Two humans, two dogs or two pea plants are never exactly the same.

There are two types of variation: continuous variation and discontinuous variation.

Continuous and discontinuous variation
	Continuous variation	Discontinuous variation
Phenotypes	A range of phenotypes between two extremes	A limited number of phenotypes with no intermediates
Examples	Body length, body mass	ABO blood groups, seed shape in peas, seed colour in peas
Caused by	Both genes and the environment	Usually genes only
Graph	Histogram (bars touching), often bell-shaped	Bar chart (bars separate)

Think of height. People can be any height between very short and very tall, so there is a whole range. Blood group is different. You are A, B, AB or O. Nobody is half way between A and B.

The environment affects continuous variation. A plant with the genes to grow tall will still be short if it has too little light or water. Your blood group cannot be changed by diet or exercise.

Left: a histogram of body length with touching bars in a bell shape, showing continuous variation. Right: a bar chart of ABO blood groups A, B, AB and O with separate bars, showing discontinuous variation. — Continuous variation gives a range between two extremes (histogram); discontinuous variation gives a few distinct groups (bar chart).

Practical skill:

Measure or observe one feature in a large sample (at least 30 individuals of the same species and similar age), for example leaf length to the nearest mm with a ruler, or seed colour. Record the results in a tally table, then plot a histogram for a measured (continuous) feature or a bar chart for categories (discontinuous).

Exam tip:

Do not just say 'genes and environment'. Say discontinuous variation is usually caused by genes only, and continuous variation is caused by both genes and the environment.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is the difference between continuous and discontinuous variation?

Continuous variation gives a range of phenotypes between two extremes, such as body length or body mass, and is caused by both genes and the environment. Discontinuous variation gives a limited number of phenotypes with no intermediates, such as ABO blood groups or seed shape in peas, and is usually caused by genes only.

What causes genetic variation?

Supplement Genetic variation in a population is caused by four processes. Mutation forms new alleles. Meiosis makes genetically different gametes. Random mating brings different combinations of alleles together, and random fertilisation means any male gamete can fuse with any female gamete. The environment affects phenotype but is not a source of genetic variation.

What increases the rate of mutation?

The rate of mutation is increased by ionising radiation, such as X-rays and gamma rays, and by some chemicals, such as the chemicals in tobacco smoke. Mutation is genetic change and is the way new alleles are formed. Most mutations have no effect or are harmful, but a few can be useful if the environment changes.

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