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Natural Selection Lesson

Shodor > CSERD > Resources > Activities > Natural Selection Lesson

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Lesson Plan - Natural Selection


The key misconceptions faced by students in understanding evolution generally result from a world view that evolution is a "one-stage" process, whereby an individual animal, by exercise or nonuse of it's bodily parts, adapts to its environment, and passes acquired traits to it's offspring.

Students often explain evolution by citing the "need for", "use and disuse of", and "adaption of" traits, often coupled with a misuse of the phrase "the strongest survive".

It is important to impress on students the modern view of evolution as a two stage process, where random mutation is offset by nonrandom selection.

Also, many students have a perception of evolution occurring in a smooth, gradual process, rather than a series of sudden dramatic changes.


Bishop, Beth A., and Anderson, Charles W., Student conceptions of natural selection and its role in evolution. Journal of Research in Science Teaching, 27:415-427, 1990.

Weiner, Jonathan, The Beak of the Finch: A Story of Evolution in Our Time.


* Note: as the model is based on random occurrences, students may see different things each time they run the model. They should run the model multiple times and look for patterns that emerge, and should not assume that what happens the first time they run a model will always happen.

  1. Using the Spotted Fish applet with the default parameters, notice what happens to a population of 100 fish that are introduced into a closed environment with ample food and a constant number of predators.
    Generally one of two things will happen. The spot size of the fish will increase, and the predators will eat all of the fish, or the spot size of the fish will decrease, and the population will first drop and then rise dramatically.
  2. What happens to the population and spot size with 0, 1, 2, 3, 4, and 5 predators?
    The fewer predators are in the environment, the more likely the population is to thrive. In addition, with fewer predators the spot size for which the population can thrive increases.
  3. If you run the model a second time, do you get the same result?
    Some characteristics of the model stay the same, but in general each run is unique.
  4. Which of the following categories would each of your models fall into?
    1. Population was always thriving
      no predators
    2. Population became extinct
      many predators
    3. Population was endangered, then became extinct
      few predators
    4. Population was endangered, then began to thrive
      few predators
  5. For each of the above categories, how does the way in which the spot size changes over time differ?
    The more predators there are, the more likely the fish are to have smaller spots after many generations.
  6. For each of the above categories, rerun the models using a larger and a smaller value of the mutation factor. Since models are random, be sure to run each test case more than once. How do the model results differ for differing mutation factors?
    A larger mutation factor decreases the number of generations needed for a significant change in the spot size of the fish to occur, but as that change can be towards larger or smaller spots, it may make the fish more or less likely to survive. Not all mutations are positive.
  7. As each model depends on random events, two models with the same input parameters can have different results. How many models did you have to run in order to determine what the "typical" results for a given set of input parameters was?
    For the default input parameters, the two possible patterns of either extinction or survival can typically be seen with between 6 and 20 model runs. Once the overall behavior can be categorized (extinction or survival) other differences between runs are minor. What may be needed for some cases is to run many trials to see if survival is possible at all. For example, with 6 predators, one might run 20 models and never see a population survive, but that does not mean that there is no chance that a population in that environment could not survive.
  8. For the default parameters, with the exception of using a value of 5 for the number of predators, what is the observed probability of a population of 100 fish with an initial spot size of 0.1 surviving? (hint: run the model 10 times, how many times out of 10 did the population of fish survive?)
    Results will vary. Typical results will be 1 or 2 out of 10, or a 10 to 20% chance that the population would be able to adapt to the environment.
  9. Answer the following questions true/false
    1. For one set of initial conditions, the spotted fish got smaller spots because they needed them to survive.
      False. For a given set of environmental conditions, the offspring with smaller spots where more likely to survive to adulthood and reproduce.
    2. In the absence of any predators, the spotted fish population will most likely have an increasing spot size over time.
      True. The model used here assumes two primary influences. Predators see larger spots more clearly and are more likely to eat the fish. Potential mates see larger spots more clearly and are more likely to choose a large spotted fish for a mate. In the absence of predators, the large spotted fish reproduce more often.
    3. For any given birth, the chance of the spot size being smaller than it's parents is equal to the chance of being larger than it's parents.
    4. For a birth in a pond with predators, the offspring's spot size is more likely to be smaller than it's parents than larger.
      False. Offspring with a smaller spot size are more likely to avoid being eaten.
    5. An individual fish's spot size can change as it gets older.
      False. The fishes appearance is determined by its genetic makeup.
    6. In a pond with predators, a fish will choose a mate with a smaller spot size, in order to have offspring that survive.
      False. Fish do not have the mental capacity for logical thought.
    7. The average spot size of the fish in the pond will be determined by the balance of large spotted fish being eaten more often and small spotted fish having a more difficult time finding a mate.

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