Why is an evolution refered as a theory?

Answer 1

Because while we can trace the APPARENT path of evolution yielding the species we see around us, in most instances the transition via evolution is implied, not observed. No one was around to observe when most of the evolutionary splits are believed to have occurred so no one documented them to establish as a known fact that that was how the split occurred. In fact there are quite a few instances where scientists have changed their views on which ancestor a particular species branched off from (usually based on further discoveries).

Also - theories remain theories until proven via experiments. It's rather difficult to craft an experiment to test natural evolution over any human time scale, although a limited number of experiments have been conducted via forced evolution of some species with very short life cycles.

Answer 2

In science, a theory is a well-substantiated explanation based on evidence and observations. Evolution is referred to as a theory because it is supported by substantial evidence from various fields such as Paleontology, genetics, and comparative anatomy. It explains the diversity of life on Earth and how species change over time through natural selection.