There isn't a single perfect synonym for "theory of natural selection," but here are some alternatives that capture different aspects of the concept:

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.