Final answer:
Evolution by natural selection is considered a theory because it is well supported by evidence from different fields of science and has been extensively tested and verified. It is not a guess, but a reliable explanation for species changes over time.
Step-by-step explanation:
The theory of evolution by natural selection is considered a theory and not a hypothesis because it is well supported by a wide range of evidence from different fields of science. Scientists have extensively tested and verified the explanations provided by the theory of evolution, making it a reliable account of the natural world. Evolution is not a guess, but rather a scientifically accepted explanation for the changes in species over time.