Scientists rarely say theory is proven . they only only say whether the evidence

Scientists rarely say that a theory is proven. They will say that whether their the evidence presented supports the theory or not. This is because a scientific theory is subjected to change. Any new evidence found in contradiction to the previous theory will lead to the formation of another theory. Hence, a theory is no proved, it is supported through evidences or experiments.

In contrast, when an evidence is proved it will be subjected to become a scientific law. A scientific law cannot be changed.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".