Question

After Darwin proposed his ideas about natural selection as a mechanism for evolution, it took time before evolution was generally considered a theory. Why do we now refer to it as a theory?

Answer 1

Answer:

It is a fact and not a theory

Explanation:

A scientific theory is some natural phenomenon that has been tried and tested several times and backed by logical and testable observations and hypotheses in a way that  in future anyone can test it. It is not a guess or hunch but a concept explained with logic and backed by solid unvarying proofs.

In case of biological evolution, observations have been made and tested several times so that no more further testing or trying is required. Hence, by this logic evolution can be considered as a fact but it can not be considered as a theory.

Answer 2

Answer:

A scientific theory refers to the possible explanation of the natural event. It can be tested repeatedly and can be verified by anyone with the help of predefined sets of protocols and scientific experimentation.  

Similarly, Darwin proposed the idea of natural selection as the mechanism of evolution.  

Various experiments in labs, as well as observations made under natural conditions, proved that natural selection plays a major role in the evolution.

Examples may include industrial melanism of peppered moths, long neck of Giraffe, different shapes and sizes of beaks of Darwin finches et cetera.

This is why natural selection is considered a theory now.