Reasoning - Your best bet and term to describe the evolution by natural selection is theory as to still scientific theory/research is still ongoing and changes can be made in a different direction.
A theory is a explanation which is tested by all different experiments by the independently. It is like a fact or well substantiated explanation. It is not a guess as it always supported by the evidences acquired by scientific method over course of the time.
The evolution by natural selection is a theory is it is not proven wrong or false and is supported by well substantiated explanations.
