A student runs a simulation to examine allele frequencies in a population over 20 generations. In the initial trial, the student sets the population size to 200, and there is no natural or sexual selection, gene flow, or mutations. In this trial, there was not a significant change in the allele frequencies.
In the next trial, the student sets the population size to 50. All other parameters are the same as the first trial. In this trial, the student observes a significant change to the allele frequencies.
Which of the following is the cause of evolution in this population?