Types and Factors of Variation

What variation means

Variation refers to the differences that exist among individuals of the same species. No two individuals, other than identical twins, are exactly alike, and biologists classify these differences into two broad types based on how the characteristic is distributed across a population.

Continuous variation

• Shows a full, unbroken range of values between two extremes, with no distinct categories.
• Usually controlled by many genes acting together (polygenic inheritance), and strongly influenced by the environment.
• When plotted, it typically produces a bell-shaped (normal) distribution curve.
• Examples: height, body mass, and skin colour in humans.

Discontinuous variation

• Shows distinct, separate categories with no intermediate forms.
• Usually controlled by one gene or a small number of genes, with little or no environmental influence.
• When plotted, individuals fall into a small number of clear-cut groups rather than a smooth curve.
• Examples: ABO blood group and the ability to roll the tongue in humans.

Factors causing variation and how this is examined

Variation arises from two broad sources: genetic factors, which are inherited from parents and include the mixing of alleles during sexual reproduction as well as mutation, and environmental factors, such as nutrition, climate, and lifestyle, which can affect how a characteristic is expressed. Continuous characteristics are typically shaped by both genetic and environmental factors together, while discontinuous characteristics are shaped mainly by genetic factors alone.

SPM questions commonly ask you to classify a described characteristic as continuous or discontinuous, to sketch or interpret the type of distribution graph each produces, or to state whether a given factor affecting a characteristic is genetic or environmental.

Worked exam-style question

Question. A student measures the height of 100 Form 5 students and also records their ABO blood group. The height data form a bell-shaped distribution when plotted, while the blood group data fall into four distinct, separate categories.

(a) Classify height and blood group as continuous or discontinuous variation. (b) State one reason why height shows a wider range of intermediate values than blood group.

(c) A student who has always had a poor diet is shorter than her genetic potential would predict. State whether this observation is evidence of a change in her genotype or her phenotype, and explain your answer.

Model answer. (a) Height is continuous variation; blood group is discontinuous variation. (b) Height is controlled by many genes acting together (polygenic inheritance) and is also strongly influenced by environmental factors such as nutrition, producing a full range of values, whereas blood group is controlled by a single gene with a limited number of alleles, producing only a few distinct categories.

(c) It is evidence of a change in her phenotype, not her genotype. Poor nutrition, an environmental factor, can prevent a person from reaching their full genetically determined height, but it does not alter the alleles inherited from her parents, which remain unchanged in her genotype.

Practice question

Try this. Tongue-rolling ability in a class of students is recorded as either 'can roll' or 'cannot roll', with no student showing a partial ability. (a) Classify this characteristic as continuous or discontinuous variation.

(b) State one feature of the data that supports your classification. (c) Name the two broad sources of variation in a population.

Exam tip

Key terms

• Variation, the differences in characteristics between individuals of the same species.
• Continuous variation, variation with a full range of intermediate values, controlled by many genes and the environment together.
• Discontinuous variation, variation with distinct, separate categories and no intermediates, usually controlled by one or a few genes.
• Mutation, a random change in an organism's DNA or chromosomes, which introduces new variation into a population.

Source:SRC-DSKP-EN

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