C. Evolution · Evolution

Investigation: Simulating Natural and Artificial Selection

C2.4 C3.1 A1.1 A1.5 A1.6 A1.8 A1.10 A1.11About 75 minutes

Learning goals

I can write a testable question and hypothesis for a selection simulation and identify its variables.
I can run a simulation of natural selection over several generations and record the data in a table and graph.
I can analyse my results, including sources of error, and draw a conclusion.
I can compare the mechanisms of natural selection and artificial selection.

Words to know

simulation
A model that imitates a real process so it can be studied safely, quickly or cheaply.

hypothesis
A testable prediction with a reason, often written as "If ..., then ..., because ...".

independent variable
The factor the investigator changes on purpose.

dependent variable
The factor that is measured to see the effect.

controlled variables
Factors kept the same so the test is fair.

random error
Unpredictable variation in measurements; reduced by repeating trials.

systematic error
An error that pushes all results in the same direction, such as an unfair rule or a faulty tool.

artificial selection
Selection in which humans choose which individuals breed, based on traits people want.

Evolution in your kitchen

You cannot watch finches evolve on the Galápagos from your kitchen table. But you can build a model that follows the same rules: variation, competition for limited food, and offspring that resemble their parents. Biologists use simulations like this, on paper and on computers, to test ideas about how populations change.

In this lesson you will plan, run and analyse a simulation of natural selection like a scientist, then compare it with artificial selection.

The model

In this simulation:

Birds have different "beaks": a spoon, a fork and a clothespin. Beak type is the inherited trait. Offspring get the same beak as their parent.
Food is scattered on a towel: dried beans, rice or small pasta. The food type is the environment.
Each bird has a short time to collect food. Food is limited, so there is competition.
The share of the next generation each beak type gets is equal to its share of the food collected. (Birds that eat more raise more young.)

Because it is a model, it simplifies: real beaks vary continuously, real birds learn, and real food varies in many ways. Part of a good analysis is recognizing these limits.

The simulation's variation (three kinds of "beaks") and environment (types of food). Changing the food partway through models an environmental change.

Planning the investigation

Testable question: How does the type of food available affect the number of birds with each beak type over several generations?

Hypothesis: If the only food is dried beans, then the number of spoon-beaked birds will increase over generations, because a spoon can scoop several beans at once and so collects more food than a fork or a clothespin.

Variables:

Independent variable: the type of food (the environment).
Dependent variable: the number of birds of each beak type in each generation.
Controlled variables: the same amount of food each round, the same time limit (for example 20 seconds), the same towel or tray, the same "stomach" (a cup) for each bird, the same person or people picking, and the same picking rule for every bird (food may be moved into the cup only with the beak).

Safety: use only dry, uncooked food items, keep small items away from young children, and do not eat anything that has been on the floor.

Worked example

Example 1: Let's calculate the next generation

Generation 1 has 12 birds: 4 spoon, 4 fork and 4 clothespin. In one round, spoon birds together collected 60 beans, fork birds 30 and clothespin birds 30. Using the rule each beak type's share of the next generation = its share of the food, find the number of each type in Generation 2 (still 12 birds).

Total food = 60 + 30 + 30 = 120 beans.
Spoon share = 60 ÷ 120 = 0.5 → 0.5 × 12 = 6 spoon birds.
Fork share = 30 ÷ 120 = 0.25 → 0.25 × 12 = 3 fork birds.
Clothespin share = 30 ÷ 120 = 0.25 → 3 clothespin birds.
Check: 6 + 3 + 3 = 12. ✔ If a share is not a whole number, round to the nearest whole bird and adjust so the total stays 12.

Generation 2: 6 spoon, 3 fork and 3 clothespin birds.

Organizing and analysing the data

Record each generation in a table, then graph number of birds (y-axis) against generation (x-axis), with one line per beak type and a key. A line graph shows trends over time clearly.

Here is a sample result from a run with dried beans as food.

Generation	Spoon	Fork	Clothespin	Total
1	4	4	4	12
2	5	4	3	12
3	7	3	2	12
4	9	2	1	12

Sample data (invented for illustration; your numbers will differ).

A line graph of the sample data. Spoon-beaked birds increase from 4 to 9 of 12, while the other types decline: the population is evolving toward the beak that suits the food.

Worked example

Example 2: Let's analyse the sample data

Use the sample table. (a) By how many percentage points did the share of spoon birds change from Generation 1 to Generation 4? (b) Does the result support the hypothesis? (c) Identify one source of error and one limitation of the model.

(a) Spoon share in Gen 1 = 4 ÷ 12 ≈ 33%. In Gen 4 = 9 ÷ 12 = 75%. The share rose by about 42 percentage points (from 33% to 75%).
(b) Yes. With beans as the only food, spoon birds increased every generation, as predicted.
(c) Source of error: the people "feeding" with different beaks may differ in skill or speed (a systematic error if one person always uses the same beak). Swapping roles each round, or doing several trials and averaging, reduces this.
Limitation: real beaks vary continuously and are coded by many genes; real offspring are not exact copies; and real birds can learn and switch foods. The model shows the principle of selection, not the exact rate in nature.

Spoon birds went from about 33% to 75%; the data support the hypothesis; error from unequal "feeders"; the model is simplified.

Natural selection vs artificial selection

C2.4 also asks you to analyse artificial selection. You can run the same model as artificial selection: instead of letting the food decide, you choose, say, that only clothespin birds may breed, because you like clothespins. The clothespin birds would increase even though they collect the least food.

That shows the key difference:

	Natural selection	Artificial selection
Who or what selects?	The environment (food, predators, climate, disease)	Humans (breeders, farmers)
Which traits are favoured?	Traits that improve survival and reproduction in that environment	Traits people want (more milk, bigger fruit, a certain look)
Is the trait always good for the organism?	Yes, in that environment	Not necessarily (e.g., some dog breeds have breathing problems)
Speed	Usually slower	Can be fast, because breeders choose strongly
Example	Peppered moths, antibiotic resistance	Dog breeds, Holstein dairy cows, canola, broccoli from wild cabbage

Both mechanisms need inherited variation and differential reproduction; they differ in what does the selecting.

Just for fun

What do you call a fork-beaked bird at a soup buffet?

Hungry. Very, very hungry.

Hands-on activity

Run the beak simulation

You need: 1 spoon, 1 fork and 1 clothespin (or tweezers), 3 small cups ("stomachs"), a towel, about 200 dried beans and about 2 cups of uncooked rice or small pasta, a timer, a helper or two, paper and a pencil.

Write your question, hypothesis and variables (use the planning section as a guide).
Start Generation 1 with 4 birds of each beak type (12 "birds"). Since you only have one of each utensil, each beak type feeds 4 times for 20 seconds each round (once per bird), and you add up its food.
Spread a fixed amount of food (for example 100 beans) evenly on the towel before each 20-second turn.
Use the share-of-food rule (Example 1) to find Generation 2. Repeat for 4 generations.
Environmental change: for Generations 5 and 6, switch the food to rice. Predict first!
Record all data in a table. Draw a line graph with a key.
Write a conclusion: Was your hypothesis supported? What happened after the environmental change? Give one source of error and how you would reduce it.

What to look for: a testable question; correctly identified variables; a neat table and labelled graph; a conclusion that uses the evidence and the words variation, selection, inherited and environment. Typically the best beak changes when the food changes (for example, clothespins or forks may do better with rice or pasta), showing that fitness depends on the environment.

Your turn

In the beak simulation, what represents (a) variation, (b) the environment, (c) inheritance?

Identify the independent and dependent variables in the beak simulation.

Name three controlled variables.

In a round, spoon birds collect 45 items, forks 30 and clothespins 15. There will be 12 birds in the next generation. How many of each?

Why is a line graph a good choice for this data?

What is the main difference between natural and artificial selection?

How could you reduce random error in this investigation?

More practice: warm-up

What is a simulation?

Write a hypothesis in the "If ..., then ..., because ..." form for rice as the only food.

Give one example of artificial selection.

What does the model's "share of the food" rule represent in real life?

Is a trait chosen by artificial selection always good for the organism?

More practice: on your own

In the sample data, what percentage of the population were fork birds in Generation 4? How does this compare with Generation 1?

A classmate's results show spoons winning in Generation 2 but forks winning in Generation 3, with the same food. Suggest a reason and a fix.

Explain why switching the food to rice in Generation 5 models an environmental change, and predict the result.

Name two ways the beak model is less realistic than real natural selection.

How would you change the simulation to model artificial selection? What result would you expect?

Challenge

In Generation 3, spoons collect 70 beans, forks 22 and clothespins 8. The next generation has 12 birds. Calculate the numbers, rounding to whole birds, and check the total.

Suppose every bird that collects fewer than 5 beans dies and cannot reproduce. Explain how this rule changes the model, and which real-world situation it resembles.

A computer simulation of selection gives the same results every time it is run. A biologist says it is missing something important. What could it be, and why does it matter?

Check your understanding

Quiz

In the beak simulation, the food type is the:

Spoons collect 50 items, forks 25 and clothespins 25. With 8 birds in the next generation, how many spoon birds?

Which change would best reduce random error?

What distinguishes artificial selection?

Which statement is a limitation of the beak model?

Which is the best hypothesis?

For parents and teachers

Big idea (C2.4, A1): students investigate natural and artificial selection through a simulation and practise the full inquiry cycle: question, hypothesis, variables, data table, graph, error analysis and conclusion (A1.1, A1.5, A1.6, A1.8, A1.10, A1.11).

How to help: Be the second "bird"! Time the 20-second rounds and keep score. Ask your teen to explain each generation's change before you calculate it.

Common difficulties:

Rounding so the total is wrong. Agree on a rounding rule before starting.
Writing a hypothesis without a reason. Use "If ..., then ..., because ...".
Ignoring error. Every investigation has sources of error; naming them and suggesting fixes is part of the mark.

Safety: use dry food only, supervise young children nearby (beans and rice are choking hazards), and discard food used in the simulation.

Sources: the beak simulation is a common classroom model; the sample data are illustrative.