Natural Selection

If mutation is random, how can evolution give rise to adaptation? Furthermore, observation of living things indicates that most mutations are “bad.” How can mutations give rise to improvements?

The following is a simulation that should help address these questions. You will follow a population through several generations, during which mutations will occur at random. If randomness cannot produce adaptations, and if mutations are always bad, then the populations should remain the same because the mutant individuals die out.

What you need:

A 15 x 15 grid, to represent 15 generations of a population that contains 15 individuals. The following two pages provide such grids.

Colored markers to indicate the traits of different individuals.

A pair of dice.

How the simulation works: You will start with “generation 1.” They will reproduce, to make “generation 2.” The individuals of “generation 2” will reproduce to make “generation 3,” and so on. Most of the individuals in generation 1 are “white,” (the color of the paper), but occasionally, mutations occur to make individuals a different color (you will roll the dice to determine which color). To represent these individuals, fill in the appropriate squares of the grid with the appropriate color.

To make it possible for the simulation to demonstrate the principle in only 15 generations, we will use a mutation frequency of 1 mutation per 7 individuals. To make this easier to see, the 15 x 15 grids indicate every 7^th individual by a dot in the upper corner of the square that represents that individual. When you come to a square with a dot in it, roll the dice to determine what color that individual should be. Otherwise, just follow the rules that the environment imposes on the population—some individuals have more offspring, others have fewer.

Individuals of different colors reproduce well, or poorly, depending on the environmental conditions. Each of the 15 x 15 grids describes how the particular environment affects reproduction of each individual.

For determining colors, use this table:

Roll-of-dice	2	3	4	5	6	7	8	9	10-12
Color	Black	Purple	Blue	Green	Yellow	Orange	Red	White	Roll again

Selection in a Cold Environment

In this environment, individuals that are better adapted to cold reproduce more efficiently than other individuals. Although all individuals can reproduce, those that are better adapted out-compete those that are less well adapted. In the first generation, all individuals are white—except for the two marked by dots. For these two, roll the dice to see what mutations they were born with. Color them according to the directions on the previous page. To make the next generation, allow these individuals to reproduce, according to the following rules:

Purple always increases by one in each generation, unless all other offspring are purple.

Blue and green increase by one in each generation, unless out-competed by purple.

Black and white, give only one offspring unless out-competed by purple, blue, or green.

Yellow, orange, and red give only one offspring unless out-competed by any other color.

Continue the reproduction of this population, generation by generation. For every square with a dot, roll the dice to determine what mutation that individual is born with. Otherwise, follow the rules outlined above.

Generation 1

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Generation 2

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Generation 3

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Generation 4

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Generation 5

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Generation 6

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Generation 7

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Generation 8

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Generation 9

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Generation 10

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Generation 11

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Generation 12

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Generation 13

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Generation 14

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Generation 15

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Selection in a Warm Environment

In this environment, individuals that are better adapted to heat reproduce more efficiently than other individuals. Although all individuals can reproduce, those that are better adapted out-compete those that are less well adapted. In the first generation, all individuals are white—except for the two marked by dots. For these two, roll the dice to see what mutations they were born with. Color them according to the directions on the previous page. To make the next generation, allow these individuals to reproduce, according to the following rules:

Orange always increases by one in each generation, unless all other offspring are orange.

Yellow and red increase by one in each generation, unless out-competed by orange.

Black and white, give only one offspring unless out-competed by orange, red, or yellow.

Blue, green, and purple give only one offspring unless out-competed by any other color.

Continue the reproduction of this population, generation by generation. For every square with a dot, roll the dice to determine what mutation that individual is born with. Otherwise, follow the rules outlined above.

Generation 1

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Generation 2

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Generation 3

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Generation 4

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Generation 5

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Generation 6

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Generation 7

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Generation 8

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Generation 9

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Generation 10

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Generation 11

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Generation 12

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Generation 13

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Generation 14

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Generation 15

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Questions for Discussion

1. What is the source of variation in these two scenarios?

2. Is the source of variation random or directed?

3. In what ways do the two scenarios give the same, or different results?

4. Do the results appear to be directed or random?

5. If the results appear to be directed, how is the direction achieved?

6. What happens if you do this with identical mutations in the two scenarios—that is, the same genetic diversity, with the same mutations occurring at the same times?

7. If you were to continue for another 10 generations, what would you be likely to observe

a. for traits (colors) that are well-adapted to the environment?

b. for traits (colors) that are poorly-adapted to the environment?

c. Would it seem as if mutations are “always bad,” and if so, why?

8. Under what environmental conditions would it be easiest to see that mutations can be adaptive?

9. Apply the results of this simulation to a real-world example.

Sample results:

If we go through the table, roll the dice, and fill in the appropriate boxes beforehand, we may get something like this.

We can use this same set of mutations in both warm and cold scenarios. If we do, then warm produces this result, while cold produces this result.