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Variation and Natural Selection: Question 1

Syllabus 18.1

Multiple choice Core 1 mark

A biology class studied a population of garden snails living in a hedge. For each snail, they recorded two features:

  • shell length, measured in millimetres, which ranged continuously from the smallest to the largest snail with no natural gaps between values
  • shell colour, which fell into only three distinct categories (yellow, pink or brown), with no snails showing an in-between colour

Which statement correctly classifies these two features and identifies what causes each type of variation?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the definitions of continuous and discontinuous variation

Continuous variation produces a range of phenotypes between two extremes, with no natural gaps between one value and the next. It is caused by both genes and the environment. Discontinuous variation produces a small number of distinct categories, with no intermediates between them. It is usually caused by genes only.

Step 2: Classify shell length

Shell length is described as ranging continuously from the smallest to the largest snail, with no natural gaps between values. This matches the definition of continuous variation. A measurement like length can also be affected by non-genetic factors, such as how much food was available to the snail as it grew, so continuous variation is caused by both genes and the environment.

Step 3: Classify shell colour

Shell colour falls into only three distinct categories, with no snails showing an in-between colour. This matches the definition of discontinuous variation. Because there are no intermediate shades, and colour categories like this are not affected by diet or other environmental conditions, discontinuous variation of this kind is usually caused by genes only.

Why the other options are wrong

  • B: this swaps the two classifications around. Shell length (a continuous range of measurements) is continuous variation, not discontinuous, and shell colour (a small number of fixed categories) is discontinuous variation, not continuous.
  • C: this correctly identifies both causes as possible in general, but wrongly classifies shell colour as continuous, when its three sharply distinct categories with no intermediates are the hallmark of discontinuous variation.
  • D: this correctly classifies shell colour, but wrongly classifies shell length as discontinuous, when a range of values with no natural gaps is continuous variation, not discontinuous. It also wrongly says continuous variation (if it existed here) would be caused by genes only.

Final answer

Shell length is continuous variation (genes and environment); shell colour is discontinuous variation (genes only), option A.