A student studying a population of ladybird beetles in a garden collected data on two features of 60 individuals. - Elytra (wing case) pattern: every beetle in the sample belonged to one of three sharply distinct forms (red with black spots, black with two red spots, or all blac…

Answer: ((a)) Body length shows continuous variation; elytra pattern shows discontinuous variation. ((b)) The bar chart forms a single smooth, unbroken range from the shortest to the longest beetle, with no natural gaps between one length and the next, and values cluster around a middle range with fewer beetles at each extreme. This unbroken, tapering spread of measured values, rath… ((c)) Discontinuous variation of this kind is usually caused by genes only. The larval feeding information shows that a beetle's body length still depends on how much it was fed as a larva, which is an environmental factor separate from its genes, so body length must be influenced by…. Part (a): Classifying the two features Body length was measured on a scale (millimetres) and forms a single unbroken range from the shortest to the longest beetle, which matches the definition of continuous variation. Elytra pattern, in co…

IGCSE Biology 0610: Interpreting Continuous and Discontinuous Variation Data

A student studying a population of ladybird beetles in a garden collected data on two features of 60 individuals.

Elytra (wing case) pattern: every beetle in the sample belonged to one of three sharply distinct forms (red with black spots, black with two red spots, or all black) with no beetles showing an in-between pattern.
Body length: the student measured body length, in millimetres, for all 60 beetles and plotted the results as a bar chart. The bar chart formed a single smooth, unbroken range from the shortest to the longest beetle, with the greatest number of beetles found in the middle of the range and progressively fewer beetles towards each end.

The student also read that beetle larvae that feed on more aphids during their larval stage grow into larger adult beetles, regardless of which elytra pattern they display as adults.

(a) Using the description above, state which of the two features shows continuous variation and which shows discontinuous variation. [2]

(b) Explain how the shape of the body-length bar chart supports your classification of body length in part (a). [2]

(c) State what usually causes discontinuous variation of the kind shown by elytra pattern, and explain how the information about larval feeding supports your classification of body length in part (a) as being caused by both genes and the environment. [3]

Part (a): Classifying the two features

Body length was measured on a scale (millimetres) and forms a single unbroken range from the shortest to the longest beetle, which matches the definition of continuous variation. Elytra pattern, in contrast, falls into only three sharply distinct forms with no beetles showing an in-between pattern, which matches the definition of discontinuous variation.

Part (b): Why the bar chart supports this classification

A feature showing continuous variation produces a smooth, unbroken spread of values between two extremes, typically with most individuals clustered around a middle value and progressively fewer individuals towards each end, rather than falling into a small number of separate, isolated groups. The body-length bar chart described here matches this pattern exactly: there is one continuous range from shortest to longest, with no gaps splitting the data into distinct clusters, and the highest bars sit in the middle of the range. This shape is what would be expected if body length is controlled by many genes acting together with environmental effects, rather than by a single gene producing a small number of fixed categories.

Part (c): Causes of the two types of variation

Discontinuous variation, such as the three elytra patterns, is usually caused by genes only, a beetle inherits one of a small number of possible forms, and no environmental factor blends these categories into intermediates. Body length, however, is shown to depend partly on the environment: the passage states that beetles fed more aphids as larvae grow into larger adults regardless of their elytra pattern, which means larval food supply (an environmental factor) affects the adult body length actually reached. Because beetles that inherit similar genes can still end up different lengths depending on how well fed they were as larvae, body length must be influenced by genes and by the environment together, which is consistent with classifying it as continuous variation.

Final answers

(a) Body length: continuous variation. Elytra pattern: discontinuous variation.
(b) An unbroken range of values, clustered around the middle with fewer beetles at each extreme, is the characteristic shape of continuous variation, unlike the separate, non-overlapping clusters expected for discontinuous variation.
(c) Discontinuous variation (elytra pattern) is usually caused by genes only. Body length is shown to depend on larval food supply as well as genes, confirming continuous variation is caused by both genes and the environment.