What are the basic driving forces behind evolution?

the driving forces behind evolution are Mutation, Natural Selection, Gene flow, and Genetic drift

So lets start with Natural selection, all this means is that that individuals best suited for their environment are more likely to survive and reproduce. That's all it is, Individuals with certain inherited traits leave more off spring than others.

an example of natural selection would be anti-biotic resistance in bacteria.

Now mutations, Mutations are random and can be beneficial, neutral, or harmful to an organism, however not all mutations matter in evolution. only those that can be passed on to the next generation matter. Those that occur in somatic cells do not matter to evolution.

Genetic drift - change in the gene pool of a small population due to chance. an example. Disasters such as the BP oil spill occur which kills off a large number of individuals, this results in a drastic reduction of the population size which in turn affects the genetic variation.

and finally Gene flow which is when individuals migrate between populations, this brings in more genetic variation.