The evolutionary process is observable, and has been many times in populations of fruitflies microbes etc
An example experiment
Whether evolutionary process accounts for the diversity of living species on the planet it a theory, and perhaps an unprovable theory as it postulates what has already happened.
The original origin of life is a seperate matter. There are theories that would postulate for the spontanous occurance of self replicating organisms/chemicals but this is a seperate matter from the theory of species diversity through evolutionary process and each must be review seperatly on it's own merits