Science // Ecosystems & Habitats

Variation in Species

Explain variation within and between species, distinguishing between continuous variation (e.g. height) and discontinuous variation (e.g. blood group), and between genetic and environmental causes

Ages
12 to 13
Type
Concept
Builds on
2 topics
Leads to
1 topic
Standards
2 codes

Quick check

If the learner looked around a classroom and noticed that everyone has slightly different heights, hair colours, and eye colours, could they explain why these differences exist — and which ones are inherited, which are down to the environment, and which are both?

Ask this to see whether the topic is secure.

Topic // Mastery

What mastery looks like

What a learner can do once this topic is secure.
  • Gives examples of continuous and discontinuous variation in humans
  • Distinguishes between genetic causes of variation (inherited differences) and environmental causes (diet, sunlight, etc.)
  • Explains why sexual reproduction produces greater variation than asexual reproduction
  • Draws or interprets a frequency graph showing continuous variation

Topic // Map

Where this fits

What to learn first and what this topic opens up, with the reason for each link. Required links come first.
  • RequiredMust come first
  • HelpfulHelps, but not essential

Before this

2 topics
  • Chromosomes, Genes & DNARequired

    Ecosystems & Habitats // Ages 12 to 13

    Explaining genetic vs environmental causes of variation requires first knowing what genes and chromosomes are

  • Grouping Living ThingsHelpful

    Ecosystems & Habitats // Ages 8 to 9

    KS3 variation within species builds on KS2 recognition that living things can be grouped by observable features

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Variation in Species

Ages 12 to 13 // Concept

Leads to

1 topic
  • How Natural Selection WorksRequired

    Ecosystems & Habitats // Ages 12 to 14

    Natural selection requires heritable variation to act on — understanding the nature of variation is a prerequisite

Topic // Research-informed

Teaching notes

From the Marble Skill Taxonomy. These notes have not been validated in classrooms, so treat them as a starting point for your own judgement.

The approach

Pupils aged 12–13 learn variation within and between species by measuring real traits, plotting them, and reading the graph shape to classify variation as continuous or discontinuous, then reasoning about whether differences are genetic, environmental or both, and why sexual reproduction generates more variation than asexual reproduction. The central working-scientifically skill — measure, plot, classify — is named by the curriculum and should be experienced through data collection, not only recognised. Alternatives (classification/interpretation practice, a reference gallery, and a Socratic dialogue) are retained for their distinct pedagogical purposes.

Common misconceptions

  • Confusing continuous and discontinuous — e.g. treating shoe size (really discontinuous/categorical due to sizing) or eye colour as continuous, without applying the 'in-between value?' test.
  • Thinking a trait must be EITHER genetic OR environmental, missing that many (height, mass) are both.
  • Believing all variation is inherited/genetic, overlooking environmental variation (scars, tan, learned language, diet effects).
  • Assuming environmentally-caused (acquired) variation can be passed to offspring — a Lamarckian slip that must be pre-empted before natural selection is taught.
  • Reading a bell-shaped graph as 'discontinuous' or a bar chart as 'continuous' — not connecting graph shape to variation type.
  • Thinking asexual and sexual reproduction produce similar variation, missing that asexual gives clones (identical) while sexual shuffles alleles.

Useful representations

  • A histogram/frequency curve of a measured continuous trait (class heights) showing a smooth bell over a range.
  • A bar chart of a discontinuous trait (blood groups, tongue-rolling) showing separate categories with gaps.
  • A side-by-side of the two graph shapes as the visual signature of continuous vs discontinuous.
  • A cause table sorting traits into genetic / environmental / both, with height flagged as 'both'; plus a simple allele-shuffling diagram showing sexual reproduction combining two parents' variation.

Teaching practices

  • Make it hands-on and data-driven: have pupils MEASURE a real trait across the class (height, hand span) and a categorical one (blood group, tongue-rolling), then plot both — the curriculum names measurement and graphing as part of the objective.
  • Teach the graph SHAPE as the diagnostic: continuous variation gives a smooth spread/bell over a range; discontinuous gives separate bars with gaps — pupils should read 'shape of distribution' back to 'type of variation'.
  • Give pupils the reliable test for the continuous/discontinuous split — 'can there be an in-between value?' — which resolves most borderline cases better than memorising examples.
  • Handle genetic vs environmental as often-BOTH, using height (genes set potential, nutrition realises it) as the anchor, so pupils don't force every trait into one bin.
  • Link greater variation under sexual reproduction back to the reproduction/inheritance work — two parents' alleles are shuffled — rather than presenting it as an isolated fact (MS-LS3-2).

Activities to try

  1. 1

    Variation lab: measure, plot, classify

    Why it works: The shape of a distribution is the visible signature of the type of variation: continuous traits spread across a range with no gaps (approaching a smooth curve with large samples), while discontinuous traits fall into discrete columns. Environment can shift a continuous distribution (height) while leaving a purely inherited categorical trait (blood group) unchanged, and sexual reproduction's mixing of alleles from two parents is the source of greater genetic variety among offspring.

    1. Choose a continuous trait (e.g. height or hand span) and record it person-by-person across the class (or a virtual population), adding each value to a building histogram/frequency graph.
    2. Watch the values spread across a continuous range with no natural gaps as more data arrive; with a large sample this often approaches a smooth, roughly bell-shaped curve, though a single class's histogram may be lumpy or uneven.
    3. Switch to a categorical trait such as blood group (A/B/AB/O) and record again, watching the graph stack into separate bars with no values between them. (Tongue-rolling or earlobe type are often used too, but real earlobes show intermediate forms and tongue-rolling is not purely genetic, so blood group is the clearest example.)
    4. Compare the two graph shapes side by side and state the rule in your own words: 'if there can be an in-between value, it's continuous; if it falls into distinct separate categories, it's discontinuous'.
    5. Apply the 'can there be an in-between value?' test to borderline cases (e.g. shoe size, eye colour) and justify your classification.
  2. 2

    Predict-then-plot the class height graph

    Why it works: Many learners expect height to come in discrete categories; confronting that prediction with real measured heights spread continuously across a range makes the continuous-variation idea stick and exposes the reasoning behind any misclassification.

    1. Before any measuring, predict the shape of a graph of everyone's height: will people fall into a few distinct groups (short/medium/tall) or spread smoothly across a range?
    2. Commit to your prediction in writing and state your REASON first.
    3. Measure and plot the real class heights.
    4. Compare the plotted heights with your prediction: look for values spread across a range with no natural gaps between 'groups' (a single class may give an uneven histogram rather than a perfect bell), and revise your reasoning if you expected distinct types.
  3. 3

    Sort traits by type and by cause

    Why it works: Classification has two independent axes — continuous/discontinuous (graph shape) and genetic/environmental/both (cause) — and traits like height occupy 'both' rather than one bin.

    1. Sort a set of trait cards into continuous vs discontinuous, applying the in-between test to justify each.
    2. Sort the same or additional traits into genetic / environmental / both, placing height deliberately in 'both' (genes set a potential, nutrition determines how much is realised).
    3. Match a given bell curve or bar chart to the correct variation type.
    4. Read a supplied frequency graph (histogram) and state what it shows about the trait.

Topic // Standards

Curriculum standards

Codes for the curriculum statements this topic maps to. Each publisher owns its standard text.
Next Generation Science Standards (NGSS) Middle School
MS-LS3-2
The national curriculum in England: Key stages 1 and 2 framework document
KS3.Sci.Bio.Adaptation.1

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