This is when there is change due to chance. There are two pink monkeys in the world, a male and a female, the female dies so pink monkeys are now extinct.
This is when there is a gain or loss of alleles from a population from a migration. Different kinds of monkeys mate and when they do and they pass along the gene for orange fur along to its offspring.
Mutations cause change in an organism and the DNA then creates a new allele and phenotype. This is what creates different colored and sized monkeys.
Non-random mating is the selection of mates other than by chance. For example male gorillas will fight one another to impress the female monkeys. The females are attracted to those who can show that they are strong and tough.