Notice that the plots end when one allele becomes extinct. Each of the plots is just another running of the simulation program and the differences between then are the result of random chance. Sometimes one allele becomes extinct and sometimes the other. Sometimes extinction takes a short time and sometimes a long time. Darwin and Wallace described the process of natural selection .
Natural selection is a bit more complicated to describe than the previous processes.
The following link discusses the Charles Darwin's voyage, Darwin's Finches, the Peppered Moth, and other examples.