Modeling Genetic Drift

This is a fairly trivial exercise, but takes a bit of time, and does not involve everyone at the same time.  It works best to divide the class into groups of 5-7 students, and give each group a sheet like that below.  Then, it is necessary to walk through what to do:

Step 1:  Here is a population of individuals (plants, animals, whatever).  It has been split into subpopulations (as many subpopulations as there are student groups).  Within each group, do the following:

Step 2:  Imagine that one of these individuals was born with a  mutation.  Draw a modification of one of these ten individuals to indicate this mutation.  Illustrate this on an overhead transparency, by editing one of the lines--like one of these:

Step 3:  Pass the paper to another person in your group.  Now, imagine that another individual was born with a mutation.  Draw a modification of one of these individuals.

Step 4:  [repeat step 3]

Step 5:  Pass the paper to another person in your group.  This person's job is to allow some of these 10 individuals to have some number of offspring, and thus create the next generation.  You may choose any of the individuals to have zero, one, two, or more offspring--but the total population cannot exceed ten individuals.  [This is the carrying capacity of the environment.]

Remaining steps:  repeat this sequence for several generations.  Each generation, have three students imagine that one of the individuals was born with a mutation, and have another student create the next generation.

When you've finished as many generations as you can stand to wait for, compare the different sheets.  They tend to be rather different.  Some will be similar to the starting population (if students chose not to allow the mutants to have many offspring).  Some will be different from the starting population (if students allowed the mutants to out-compete the originals).  But, since the different groups could not "share" mutations (the populations were physically separated, and thus reproductively isolated), and since mutations occur at random (we had no rules about what mutations were allowed), each group tends to become different.  Note that there is no particular selection here, so this models genetic drift better than any other evolutionary process.

Constant carrying capacity, combined with random factors, resulted in differential reproduction.  As a result, we observe change in the three populations.  We donÕt even have to mention E_________ .  This is simply something that happens when a population is split into subgroups that canÕt interbreed with each other.  It is a natural consequence of the way genetics works.

1st generation               |    |    |    |    |    |    |    |    |    |

2nd generation

3rd generation

4th generation

5th generation