Genetic drift is the process in which a random event occurs that changes the gene pool. In this case, a tornado wiped out a large amount of the green deer variant, changing the gene pool
Gene flow is the gain or loss of alleles caused by a migration in the population. Often times, this reduces genetic differences between populations. In this example, you can see the purple gene being spread.
Mutations are changes in an organism's DNA that lead to new alleles and can cause genetic variability. It is very rare, and often leads to death. In this example, a red fur allele is created.
Non-random mating is when mating does not happen simply by chance, but partners select each other based on traits. This can either happen with assortative mating, when they choose similar mates, or disassortative, when they choose different mates.