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Natural Selection Lesson

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Lesson - Natural Selection

Charles Darwin's theory of natural selection was the first plausible mechanism to explain the change of species over time, however, in it's original form it did not explain how new traits could form, or how traits that had formed could be passed on to successive generations. The rise in modern genetics helped to modify biologists understanding of evolution by attributing the origin of new traits in a species to random genetic processes of mutation and sexual recombination, with the survivability of species with the new traits subject to natural selection. This combination of random mutation and natural selection is often referred to as Neodarwinism.

Modern models of evolution stress the relative importance of competition and survival, competition within a species for limited resources and the ability to mate, and survival from predators. As random changes develop within a species, the ability for the change to continue on to successive generations depends upon the ability of the mutated subject to survive long enough to produce viable offspring which are also capable of reproducing. Adaptation is the process of random mutations, subject to selection, accumulating over time to make a species better suited to survive and reproduce in their environment.

One criticism of early evolutionary theories is their reliance on gradual change. While this notion is popularly held, it does not agree with the evidence. Observations of the fossil record show that organism traits stay more or less constant over time, punctuated by sudden dramatic changes, as opposed to a steady gradual change. The Spotted Fish model can help to understand how the interplay of random mutation and nonrandom selection supports the evidence found in the fossil record of short bursts of rapid change. Use the Spotted Fish model to help answer the following questions.


  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.
  2. What happens to the population and spot size with 0, 1, 2, 3, 4, and 5 predators?
  3. If you run the model a second time, do you get the same result?
  4. Which of the following categories would each of your models fall into?
    1. Population was always thriving
    2. Population became extinct
    3. Population was endangered, then became extinct
    4. Population was endangered, then began to thrive
  5. For each of the above categories, how does the way in which the spot size changes over time differ?
  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?
  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?
  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?)
  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.
    2. In the absence of any predators, the spotted fish population will most likely have an increasing spot size over time.
    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.
    5. An individual fish's spot size can change as it gets older.
    6. In a pond with predators, a fish will choose a mate with a smaller spot size, in order to have offspring that survive.
    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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