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Modelling Natural Selection

Modelling natural selection

Imagine a world where you find frogs a tasty treat. In this activity, you will be acting as a selective pressure on a frog species to see how this changes the population over time.


  • Frog lollies (red, green) - alternatively, red and green counters, paddlepop sticks, toothpicks, paper squares, etc
  • Graph paper

Natural Selection is the process where environmental pressures ‘select’ the most adapted individuals to survive and reproduce, whilst organisms that do not have favourable traits will not survive.

In this species of frog, we have a red variant and green variant. We start with equal numbers of each, and after two generations we shall see how natural selection favours different individuals.


  1. There are 30 red and 30 green frogs, living in the lawn of your school.
  2. You have two minutes to ‘prey’ upon (catch and eat) as many frogs as possible. These two minutes represent one year.
  3. Record the number of frogs ‘eaten’ (caught) and how many survived. 
  Survived Eaten
Green frogs    
Red frogs    
  1. Calculate the survival rate for each colour frog (number survived / 20):
  Red frogs Green frogs
Survival rate    
  1. This species reproduces yearly. Allow the surviving frogs to reproduce with the following offspring numbers:
  • Every pair of red frogs produces 4 red frogs
  • Every pair of green frogs produces 4 green frogs
  • Any remaining red frog pairs with a green frog. This pair produces 2 red frogs and 2 green frogs.
  • Single frogs do not reproduce.
  1. Calculate the number of offspring produced and add offspring to the surviving adults to determine the total number of frogs in the population.

Generation One

  Surviving adults Offspring Total individuals
Red frogs      
Green frogs      
  1. Use your survival rates on this population to determine how many frogs survive in the second year (survival rate x Generation One population).
  Total individuals Survival rate Surviving adults
Red frogs      
Green frogs      
  1. Allow these survivors to reproduce again.
Generation Two
  Surviving adults Offspring Total individuals
Red frogs      
Green frogs      

Discussion questions

  1. Create a line graph showing the change in the number of red frogs and green frogs over two generations. Don’t forget to include the starting population of frogs (30 red, 30 green)
  2. Describe how the population changed over time. Explain your results.
  3. Identify the cause of different colouring in the frogs (Hint: It isn’t food dye!).
  4. During spring, many red flowers come into bloom around this lawn. How might this change survival rate during spring? Justify your response.
  5. Predict how the population would change through time if red frogs were poisonous to predators?