Evolution is a product of the environment. If a species lives in trees, then every time an infant is born with a mutation that benefits the animal living in trees, there is a high chance this animal will go on to mate, with the possibility that the infant of that next generation will carry the same mutation, as animals breed this mutation will spread through the species as the older generations before the mutations die out. Breeding race horse works in exactly the same way. The structure and proportions of the leg make a great deal of difference how fast the horse will run and how smooth it will be. As the small mutations are passed from generation to generation of race horse, they will become gradually more suited to running at faster and faster speeds, because those mutations are benefitting them, and making it more likely they will be used for breeding. In terms of people and monkeys, I guess ages ago at some point, the environment we were living in it became necessary to outsmart our prey/whatever, which meant we had to become more more intelligent, the intelligent would prosper and more than likely reproduce more than the less smart counterparts. If we were separate from another group of primates, of the smae species, and breeding did not reach the other group, then they would evolve separately, each evolving to suit thier environment, separating the species even more. If you see what I mean?