There is such strong quantitative support for the second that scientists regard common descent as being as factual as the understanding that in the Solar System the Earth orbits the Sun, although the examination of the fundamentals of these processes is still in progress.
There are several theories about the mechanisms of evolution, and there are still active debates about specific mechanisms.
In biology, the meaning is more specific: heritable changes which accumulate over generations of a population.Scientists continue to argue about particular explanations or mechanisms at work in specific instances of evolution – but the fact that evolution has occurred, and is still occurring, is undisputed.A common misconception is that evolution cannot be reliably observed because it all happened millions of years ago and the science therefore is not dependent on facts (in the initial sense above).To a scientist, fact can describe a repeatable observation that all can agree on; it can refer to something that is so well established that nobody in a community disagrees with it; and it can also refer to the truth or falsity of a proposition.
To the public, theory can mean an opinion or conjecture (e.g., "it's only a theory"), but among scientists it has a much stronger connotation of "well-substantiated explanation".
But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples.