Evolution must be a force because it has been proven that outside influences(such as cold etc) do not change a species but merely emphasize the variation within a species.
For instance if I cut off my index finger and all my progeny cut theirs off, my g, g, g, g, g, great grandchildren would still be born with index fingers. This has been known for many, many years but some evos still try to survival of the fittest drives evolution, it doesn't and has been proven.
Breed a species for instance for one characteristic and if you breed long enough you won't come up with a new species(as had been thought originally) but will wind up with the original species.
Evolution, to work as it is theorized, has to be a force, and unknown one, but still an invisible force. Try to warp it as you will, it is just as myterious and unknown as any religion.