Focusing on the process:
* Evolution by natural selection
* Darwinian selection
* Survival of the fittest (though this can be misleading, as it focuses on individuals rather than populations)
Focusing on the outcome:
* Adaptive evolution
* Evolutionary adaptation
More formal terms:
* Darwinian evolution
* Neo-Darwinism (which incorporates modern genetics)
The best alternative depends on the context and what you want to emphasize.