Causes of variation

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In short

Variation is the differences between individuals of the same species. It is caused by genes, the environment, or often a combination of both. Genetic variation comes from mutation and sexual reproduction, and environmental variation from conditions such as diet. Continuous variation, such as height, takes any value in a range; discontinuous variation, such as sex, has distinct categories.

Differences between individuals of the same species are called variation. Variation in phenotype has two causes: genes and the environment. Often it is a combination of both.

Genetic and environmental variation
	Genetic variation	Environmental variation
What it is	Different characteristics as a result of mutation and sexual reproduction	Different characteristics caused by an organism's environment (acquired characteristics)
Source	Differences in the alleles inherited	Conditions during life, such as diet, climate, injury or activity
Examples	Eye colour, sex	A scar, body mass affected by diet, plant height affected by light

Continuous and discontinuous variation

Continuous variation: the feature can take any value in a range, such as height or mass. The results are shown on a histogram.
Discontinuous variation: the feature falls into distinct categories with nothing in between, such as sex or purple versus white flowers. The results are shown on a bar chart.

Exam tip:

Practical to know: investigate variation within a species, for example the length of leaves from one type of plant, or the height of students in a class. Measure a large sample, then draw a graph to see whether the variation is continuous or discontinuous.

Calculating an arithmetic mean

The heights of five seedlings grown in the same conditions were 12 cm, 15 cm, 11 cm, 14 cm and 13 cm. Calculate the mean height.

Add the values: 12 + 15 + 11 + 14 + 13 = 65 cm.
Divide by the number of values: 65 ÷ 5 = 13 cm.

Answer: Mean height = 13 cm.

Maths skill:

The mean is the total of all the values divided by the number of values. Give the unit in your answer.

Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026

Frequently asked questions

What causes variation in a species?

Variation is caused by genes, the environment, or a combination of both. Genetic variation comes from mutation and sexual reproduction, which give individuals different alleles, as with eye colour. Environmental variation is caused by conditions during life, such as diet, climate or injury; for example, plant height can be affected by light.

What is the difference between continuous and discontinuous variation?

In continuous variation, a feature can take any value within a range, such as height or mass, and the results are shown on a histogram. In discontinuous variation, a feature falls into distinct categories with nothing in between, such as sex or flower colour, and the results are shown on a bar chart.

Are all mutations harmful?

No, most mutations are not harmful. A mutation is a change in the base sequence of DNA, and most mutations have no effect on the phenotype. Some have a small effect, and only rarely does a single mutation significantly affect the phenotype. Mutations are the source of genetic variation within a population.

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