2.4.2Unit 2 — Multicellular Organisms

Practical: Variation in Organisms

5 min read 6 parts Lesson 16 of 39

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Part 1 of 6Variation Being Investigated

This suggested practical uses snails, which have a small number of easily recognised shell types.

Variation Being Investigated

Discrete variation

Individuals fall into clear separate categories with no in-between forms, such as the shell patterns of snails of one genus.

Continuous variation

Individuals show a smooth range of values, such as height, so the data are measured rather than sorted into categories.

Habitat link

Different habitats may favour different forms, so the frequency of each variant can differ between woodland and grassland.

Aim

Compare the frequency of each snail shell variant in two habitats and describe any differences that the data show.

Part 2 of 6Method

Choose two contrasting habitats, for example woodland and open grassland, where the same snail species can be found in reasonable numbers.

Place a quadrat at random positions in the first habitat, using random coordinates so that the sampler cannot pick convenient or easy spots.

Photograph or record each snail seen in the quadrat without moving it, repeating at new random positions until 50 snails have been recorded.

Sort each snail into one of four variants: pink or brown plain, yellow plain, pink or brown striped, and yellow striped.

Repeat the whole procedure in the second habitat, keeping quadrat size, sampling method and time of day the same to make it a fair test.

Part 3 of 6Recording and Presenting Results

Tally chart

Mark each snail as it is classified, then total the tallies to give the number of each variant in each habitat.

Percentage

Divide the number of one variant by the total sample and multiply by 100, so that the two habitats can be compared directly.

Bar chart

Use bars for categories with gaps between them, because discrete data have no in-between values to join.

Sample size

Using the same total of 50 snails in each habitat means that the counts are directly comparable.

Variant	Habitat A (number)	Habitat B (number)
Pink or brown plain	20	10
Yellow plain	5	18
Pink or brown striped	17	6
Yellow striped	8	16
Total	50	50

Part 4 of 6Drawing Conclusions

Drawing Conclusions

Describe trends

State which variants are commoner in each habitat and use figures from the table to support every comparison you make.

Fair test

Same quadrat size, sampling method and sample size are kept constant so only the habitat is different between the two samples.

Reliability

A larger sample, or repeating the survey on different days, would make the percentages more reliable.

Ethics

Snails are left in their natural environment and are not handled, so that wildlife is disturbed as little as possible.

Part 5 of 6Worked Example

Question

In habitat B, 16 of the 50 snails recorded were yellow striped.
Calculate the percentage that were yellow striped.

Write the fraction of the sample16 out of 50.
Divide 16 by 50 to give 0.32.
Multiply by 100 to convert to a percentage.

Answer 32% of the snails in habitat B were yellow striped.

Part 6 of 6Exam Checklist & Tips

2.4.2 – Key Points for the Exam

Discrete variation A characteristic with distinct categories and no intermediate forms, usually controlled by a single gene.
Random sample A sample taken at positions chosen by chance so that selection is not biased by the investigator.
Quadrat A square frame placed on the ground to mark out a sample area of fixed size.
Frequency The number of individuals of a particular type found in a sample.
Fair test An investigation in which only one variable is changed and all other variables are kept the same.

Common Mistakes

Choosing easy-to-see snails instead of using random quadrat positions, which makes the sample biased.
Drawing a line graph for discrete data;
- separate bars are needed because there are no values between categories.
Comparing raw counts from samples of different sizes instead of converting to percentages.
Describing a trend without quoting figures from the results to support it.