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89 Modelling Natural Selection
Key Idea: Natural selection acts on phenotypes. Those individuals better suited to an environment will leave more offspring in the next generation. This can be illustrated using a simple model.
Natural selection can be modelled in a simple activity based on predation. You can carry out the following activity by yourself, or work with a partner to increase the size of the population. The black, grey, and white squares opposite represent the phenotypes of a population. Cut them out and follow the instructions below to model natural selection. You will also need a sheet of white paper and a sheet of black paper.
1. Cut out the squares on the following pages and record the number of black, grey, and white squares. Work out the proportion of each phenotype in the population (e.g. 0.33 black 0.34 grey, 0.33 white) and place these values in the table below. This represents your starting population (you can combine populations with a partner to increase the population size for more reliable results). 2. For the first half of the activity you will also need a black sheet of paper or material that will act as the environment (A3 is a good size). For the second half of the activity you will need a white sheet of paper. 3. Place 14 each of the black, grey, and white squares in a bag and shake them up to mix them. Keep the others for making up population proportions later. 4. Now take the squares out of the bag and randomly distribute them over the sheet of black paper (this works best if your partner does this while you aren't looking). 5. For 20 seconds, pick up the squares that stand out (are obvious) on the black paper. These squares represent snails in the population that have been preyed on and killed (you are acting the part of a predator). Place them to one side and pick up the remaining squares. These represent the population that survive to reproduce. 6. Count the remaining phenotype colours and calculate the proportions of each phenotype. Record them in the table below in the proportions row of generation 2.
Use the formula: Proportion = number of coloured squares ÷ total number of squares remaining.
For example: for one student doing this activity: proportion of white after predation = 10/30 = 0.33. 7. Before the next round of selection, the population must be rebuilt to its original total number using the newly calculated proportions of colours and the second half of the squares from step 3. Use the following formula to calculate the number of each colour: number of coloured squares required = proportion x number of squares in original population (42 if you are by yourself, 84 with a partner). For example: for one student doing this activity: 0.33 x 42 = 13.9 = 14 (you can't have a fraction of a phenotype). Therefore in generation 2 there should be 14 white squares. Do this for all phenotypes using the spare colours to make up the numbers if needed. Record the numbers in the numbers row of generation 2. Place generation 2 into the bag. 8. Repeat steps 4 to 7 for generation 2, and 3 more generations (5 generations in total or more if you wish). 9. On separate graph paper, draw a line graph of the proportions of each colour over the five generations. Which colours have increased, which have decreased? 10. Now repeat the whole activity using a white sheet background instead of the black sheet. What do you notice about the proportions this time?
Generation
1 Number Proportion 2 Number Proportion 3 Number Proportion 4 Number Proportion 5 Number Proportion
