Individuals in a population vary. Measuring one characteristic across a population gives a spread of values, often most near the middle. Some variation is inherited (genes) and some comes from the environment; inherited variation is what natural selection acts on.
Look closely at a patch of grass beetles, a school of fish or a class of Year 10s. No two are quite the same. That spread is the raw material of evolution.
Meiosis and fertilisation make every offspring different. This lesson: what that variation looks like across a whole population.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: all the same?
ExploreTry it on the board and compare what happens with your prediction.
Explore: measure a population
Click beetles to measure their colour (0 is palest, 1 darkest). Watch the bar chart under the board as it fills.
Try it yourselfNothing is saved: start again as often as you like.
Measure the colour of 20 beetles.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: kinds of variation
In your book: sketch your bar chart and describe its shape in one sentence.
Continuous (any value in a range) or discontinuous (a few distinct groups)?
Height
Blood group (A, B, AB, O)
The beetles' colour
Pea flower colour (purple or white)
Hand span
Mainly inherited, mainly environmental, or both?
Blood group
A scar
HeightGenes set a range; food and health affect where you end up.
Natural eye colour
Speaking French
Elaborate TypicalUse the same idea in a new context.
Elaborate: where variation comes from
Elaborate WiderUse the same idea in a new context.
Elaborate: sampling
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Individuals in a population vary; measuring a characteristic gives a spread.
Continuous variation takes any value (height); discontinuous falls into groups (blood group).
Variation can be inherited, environmental or both; only inherited variation is passed on, so only it matters for evolution.
New inherited variation comes from mutations and from meiosis and fertilisation.
For teachers
Students measure variation in a population, distinguish continuous from discontinuous and inherited from environmental variation, and explain why inherited variation matters for evolution (AC9S10U02, node 1, a hinge node). About 20 minutes.
I can describe the variation in a measured population.
I can classify variation as continuous or discontinuous, inherited or environmental.
I can explain why only inherited variation is passed on.
Before class
A concept tutorial for introducing or revising the idea; it pairs well with measuring hand spans or heights in class. Set the width with ?width=minimum, typical (default) or wider.
The hinge misconception is that a species is uniform. The board's population is spread around a middle colour.
Board notes: beetle colour runs from 0 (palest) to 1 (darkest); the starting population is spread around 0.5. Measuring doesn't change the population. The next lesson uses the same beetles under selection.
Running the lesson
Engage: count how many chose 'all the same'.
Explore: compare bar charts between students: similar shape, different details.
Explain: the 'why only inherited' question is the checkpoint.
The hard part (on purpose)
Telling inherited from environmental variation. When a student is unsure, ask whether the difference could be passed on in a sperm or an egg.
When students get stuck
A student says the variation is just 'mistakes'. Ask: Would a population where every beetle was identical be better or worse off if things changed? Fix: Worse: variation is normal and useful.
A student thinks a scar can be inherited. Ask: Does a scar change the DNA in sperm or eggs? Fix: No: only changes to the genes in gametes are passed on.
Stretch and support
Stretch: Why does a characteristic controlled by many genes give a smooth spread, but one gene gives a few groups?
Support: Give a partly drawn bar chart and a word bank (continuous, discontinuous, inherited, environmental).
Where it leads
Next: natural selection (node 2) with the same beetles.
In class: measure and graph hand spans; compare with blood groups or earlobes.
What each step is for
Engage: all the same?: Students who think a species is uniform ('beetles are brown') don't expect a spread; measuring produces one.
Explore: measure a population: Measuring twenty individuals and watching the bar chart grow turns 'individuals vary' into a distribution.
Explain: kinds of variation: Separating continuous from discontinuous and inherited from environmental gives students the language for selection.
Elaborate: where variation comes from: Naming the sources of new inherited variation links the genetics unit to this one.
Elaborate: sampling: Questioning a sample's size and fairness uses the board's measurements as data.
Now you can
Describe the variation in a measured population.
Classify variation as continuous or discontinuous, inherited or environmental.
Explain why only inherited variation is passed on.