After watching TedED's "Five Fingers of Evolution", my understanding is that the possible mechanisms of evolution are:

• Environmental changes; in the video, I believe this mechanisms is called adaption. Examples include, change in availability of food (including water), shelter, climate, ecological zones, predators, etc.
• Mating preferences. Cultural, genetic, etc.
• Population Immigration. Genetic different populaces immigrating into existing populaces.
• Genetic Mutation.
• Population shift. Random applied growth or decline in population not based on above mechanisms. That said, it's given that population shifts within a existing populaces are complete random, it's hard to see why the would make a difference.

Misc:

• As for the meaning of evolution, "Evolution is any change across successive generations in the inherited characteristics of biological populations."
• Re: "availability of reproductive partners", I believe that if the availability reproductive partners was stable, it would not be a factor.