NATURAL SELECTION SIMULATION

Burt C. Kessler (email burt)
Time: 2 class periods
Materials: 100 beans ( or reasonable facsimile)
32 cups or beakers
masking tape
graph paper

Overview: Students are told that they are simulating the changes in a natural population of bird or some other species. The birds eat beans. They obtain the beans one at a time and place them in their "stomach", i.e. a cup. When they've eaten 10 beans they are ready to reproduce. All members of a generation reproduce simultaneously and each has one offspring. Birds that do not obtain 10 beans are unable to reproduce and die during breeding season. The first class period establishes baseline data of population fluctuation for a closed population with a stable food supply. The data is graphed with a line graph. The second class period begins with the same population but deleterious and beneficial mutations are introduced. Competition and natural selection ensue. Eventually speciation occurs when a beneficial mutant type replaces the wild type. Wild type and mutant populations are graphed on a single set of axes.

Procedure: Day 1: Begin with 100 beans on a table and 2 student volunteers. They each pick up 10 beans, one bean at a time, placing each bean into their cup. The arm holding the cup must be held out from the table and that arm cannot bend. When they each have exactly 10 beans their generation is ready to reproduce.
To reproduce the students return the beans to the table, restoring the original 100. They each pick up an additional cup and going out into the class they select their "offspring", one each. The second generation thus has 4 individuals, the 2 survivors plus their offspring. They begin together, picking beans, and then reproduce. Generation 3 has 8 individuals. When they've finished eating is a good time to recount the beans to ensure there are still 100 (beans tend to get lost on the floor or break). Then they reproduce making generation 4 with 16 individuals.
Generation 4 has grown beyond the carrying capacity of the food supply and a population crash follows. Watch carefully for cheating perhaps appointing student referees. Generally 2-4 individuals survive with 10 beans. Those with less than 10 die, return their beans and sit down. Those with 10 beans reproduce. This pattern continues with geometric growth followed by a crash when the population exceeds the carrying capacity of 10.
Typical data for Day one looks like this:
Generation # of individuals
1                      2
2                      4
3                      8
4                      16
5                      4
6                      8
7                      16
8                      6
9                      12
10                    18
11                    2
12                    4
13                    8

The number of generations varies with the class. More cooperative classes may exceed 25 generations in a 55 minute period. Difficult classes may only reach 10-12 generations. A student records the data on the board. All students are responsible for copying the data and graphing it for homework. The sample data graph would look like this:

Rules: All individuals begin eating simultaneously
Arm with cup cannot bend
One bean at a time
Stop when you reach 10
Classmates may not refuse to be offspring (cannot say no)
Offspring are always the same "type" as parent

Day 2: Begin the same as day one. When you reach generation 3 with 8 individuals, select one (volunteer) to be a mutant. Generation 3 then has 7 wild type and one mutant. The mutant types include: No thumb (thumb is taped securely to palm), No fingers (4 fingers are taped together), Straight arm (neither arm can bend), Bent arm (both arms can bend), 5 beans (individuals reproduce after collecting only 5 beans instead of 10). I usually introduce mutants one at a time, during a generation that ensures reproduction. Deleterious mutations usually die out quickly. Eventually the 5 bean mutants will replace the wild type. Rarely can all 5 mutants be used in a 55 min. period, 3 or 4 is more realistic.
Note: Always draw a new mutant type from the new generation of wild types. This reduces the wild type generation by one. The new mutant appears in its own column in the data table at the row for that particular generation.
If both beneficial mutants are used in a class, they may both replace the wild type. This demonstrates adaptive radiation as well as speciation.

Typical data for Day 2:
Generation Wild Type No Thumb No Fingers 5 Beans
1                     2
2                     4
3                     7              1
4                     14            2
5                     10            2
6                     7              0                  1
7                     14            2
8                     5              0                  1
9                     10            2
10                     8            4
11                     16                              8
12                     4                                                  16
13                     0                                                  20

The graph for the sample data for day 2 would look like this:

Extensions:

Students can be asked to speculate about what other real world variables would alter the outcomes or data. These might include double mutants, changes in available food, drought, disease, immigration, …

How would the model change as a result of these new variables?

Describe how to model or act out your variation.

Predict what the graph and data would look like if this new variable were included.

Assessment:

The data tables and graphs produced by the students can be used to assess the activity.

Questions could be posed such as:
1. What makes a mutation harmful ? or beneficial?
2. Under what circumstances is having no thumb or no fingers not harmful?
3. Can you think of a mutation that would be harmful in one environment/condition yet beneficial in a different environment?