The four forces of evolution

There are four forces of evolution: gene flow, genetic drift, natural selection, and mutation.

Please discuss the possible ways in which one or more of these evolutionary forces might operate to produce a huge amount of anatomical and behavioral diversity seen in the various primate species, today.

In other words, with all the rich diversity of the monkeys, apes, lemurs, lorises we see in our world, today, how do YOU think evolution has shaped these differences? (hint: think mating strategies, diet, geological/climate, etc).