Enter a starting value for the allele frequency, the number of
organisms in the population, and the number of generations. Clicking
on the Simulate button will perform a simulation and graph the change
in allele frequency over time.
- Keep the values at their starting numbers (allele frequency =
.50, population size = 25, and number of generations = 50). Click
on Simulate repeatedly, pausing after each click to examine the
graph. Notice how the allele frequency changes over time. If you
do this enough times, you will eventually encounter the phenomenon
of fixation, i.e., when the allele frequency gets fixed at either
0 or 1. Here one of the alleles is lost so that the other allele
is the only one left in the population.
- Let us now examine the effect of population size on genetic
drift. Keep the starting allele frequency at .50 and the number of
generations at 50 but set the population size to 10. Click on
Simulate about 10 times and notice how much allele frequencies
bounce around from one generation to the next. Now change the
population size to 1000 and click on Simulate 10 times. What does
the line look like now? What accounts for the difference between
the plots when population size = 10 and when population size =
- Finally, let us examine the ultimate effect of drift in a
small population. That is, what will eventually happen when the
process of drift continues for a long time? Set the allele
frequency to .50 and the population size to 25, but now fix the
number of generations to 1000. Click on Simulate about 10 times.
What happens is every case?