Force Evolution is natural selection the force behind human evolution why are there so many races?
Strictly speaking races do not exist among humans, we are one race. It was not alway the case. A few thousands of years ago, in Europe, the Homo Sapiens Sapiens (us) was living alongside the Homo Neanderthalis (another species of Humans). This situation lasted about 10 thousand years and the Homo Neanderthalis vanished. It is still dabated whether modern Europeans carry some of the genes of this other type of humans. So far it has not been proved and if we do carry these genes they represent a tiny proportion of our genome.
To go back to you initial question:"is natural selection the force behind human evolution?"
Of course it is, or at least it was for most of the human history. For instance, in Europe during the great plagues in the middle ages, a lot of the survivors had a certain immunity to the disease and therefore Europeans nowadays are much more immune to the plague than their ancestors, say in Roman antiquity, because us modern Europeans are the descendants of those who survived the plague and we have inherited the genes of immunity that helped our ancestors to survive.
is human selection the dominant force for evolution?
No. Human selection typically only affects humans and domesticated animals. Human selection is a powerful force in the eradication of food, pest and disease-causing species (e.g. passenger pigeons, dodos).
You might be interested in reading about "sexual selection" as it does relate to your question.
