9.11 - Investigating Variation Within a Species
The technique
Aim
To measure a characteristic that varies continuously within one species (for example the length of leaves from one type of tree), across a large sample, then group the readings into classes, plot a histogram, and describe the pattern.
Background
- Variation is the differences between individuals of the same species.
- In continuous variation a characteristic can take any value across a range, like height, hand span, mass, or leaf length.
- In discontinuous variation a characteristic falls into a few distinct categories, like blood group or being able to roll your tongue.
- You can't see the pattern of continuous variation by looking at a few individuals, but you can measure many of them and count how many fall into each range of values.
- So in this practical you measure one characteristic across a large sample, group the readings into classes, and plot a histogram to show how it varies.
Method
- Choose a characteristic - Pick one that varies continuously, for example the length of leaves from one type of tree.
- Collect a large sample - At least 20 to 30 individuals.
- Measure each one the same way - Use the same instrument and the same units every time, for example leaf length in millimetres with a ruler, and record every reading in a table.
- Group the readings - Sort them into equal-sized classes, for example 40 to 49 mm and 50 to 59 mm, and count how many fall into each class. This count is the frequency.
- Plot a histogram - Plot the frequency of each class, with the bars touching, then describe the shape.
Safety
- Measuring people - They must agree to be measured, and the readings must be kept anonymous.
- Collecting leaves - Take them only from plants you can identify, and wash your hands afterwards.
Worked example - Leaf length variation
A student measured the length of 20 leaves picked from a single oak tree, recording each length in millimetres.
| Leaf lengths measured (mm) |
|---|
| 45, 52, 58, 61, 55, 63, 67, 59, 72, 54, 66, 60, 57, 64, 69, 62, 58, 75, 51, 68 |
Work out the mean length, group the readings into a frequency table, draw a histogram, and describe its shape.
Step 1 - Work out the mean length
Add every reading together and divide by the number of leaves. The 20 lengths add up to 1,216 mm.
Step 2 - Group the readings into classes
Sort the 20 readings into equal 10 mm classes and count how many fall in each. This count is the frequency.
| Leaf length class (mm) | Frequency (number of leaves) |
|---|---|
| 40-49 | 1 |
| 50-59 | 8 |
| 60-69 | 9 |
| 70-79 | 2 |
Check that the frequencies add up to the sample size. 1 + 8 + 9 + 2 = 20, which matches the 20 leaves measured.
Step 3 - Draw the histogram
Plot leaf length class along the x-axis and frequency up the y-axis. Draw one bar per class, with the bars touching, because leaf length is continuous. The tallest bars are the middle classes, and the bars at each end are short.
Step 4 - Describe the shape
Most leaves fall in the middle classes, around the mean of 60.8 mm, and only a few are much shorter or much longer. This bell-shaped pattern, high in the middle and tailing off at both ends, is a normal distribution. It's what continuous variation looks like when a large enough sample is plotted.
Common exam questions
Question 1
Why are the results plotted as a histogram rather than a bar chart?
The data is continuous, so it's grouped into classes and drawn with the bars touching. A bar chart with gaps is used for discontinuous data, which falls into separate categories.
Question 2
Why does the student measure a large sample rather than just a few leaves?
A larger sample makes the pattern clearer and is less affected by chance, so the normal distribution shows up and the mean is more reliable.
Question 3
What causes continuous variation within a species?
It's controlled by several genes acting together, usually combined with an effect from the environment.