On the validity of scientific theories

Hypotheses, sometimes colloquially defined as educated guesses, are testable and falsifiable ideas based on evidence which predict future outcomes. Many hypotheses can be phrased as if-then statement (e.g. If land-dwelling tetrapods evolved from fish, then we should be able to find transitional fossils in near shore environments just before the age of the first fossils of land-dwelling tetrapods.*) Scientists then perform experiments or make observations, which either support or refute the hypothesis. Unfortunately, common usage of the word "theory," and even its usage by the media in coverage of science, is nearly synonymous with a hypothesis.

Scientific theories coherently explain hundreds of thousands of confirmed or refuted hypotheses and millions of observations in a relatively simple way. Theories deal with mechanisms and underlying principles. In other words, theories address why all the observations and experiments came out the way they did. Because there is usually the possibility of some yet to be discovered alternative explanation or mechanism, many scientific theories can never be 100% proven. By definition, they may be falsified at any time.

For a scientific theory to gain acceptance and remain accepted in the scientific community, by definition, it is necessary that it explain all or nearly all of the available data. Anomalies are not counter examples, but if you can demonstrate a couple "anomalies" are not anomalous experiments or observations at all, but consistently yield the data which cannot be explained by the theory, then the theory in its current form begins to lose scientific acceptance. It will ultimately be revised or replaced.

Organisms evolve. We know this purely based upon experiment and observation. Evolution is a fact. The theory in question is evolution by natural selection, first proposed by Darwin and Wallace. Natural selection theory could be disproven at any time, by definition, if an alternative mechanism which better explained evolution were to be demonstrated. To disprove all of the observations of evolution, however, would be remarkably difficult.

If you really want to try to disprove evolution though, here are some silver bullets: - Demonstrate that rates of radioactive decay vary widely over time, and that when this new variable is accounted for, the world is extremely young - Demonstrate numerous out-of-sequence fossils of several phyla in place in rocks (e.g. rabbits in pre-Cambrian strata)

No one has yet done these or similar things to the satisfaction of the scientific community; therefore, by definition, there is no evidence against evolution.

*Note: This is obviously leads to an inductive argument, not a deductive one. Most hypotheses cannot be proved 100% for sure; in many cases, only the null hypothesis can be disproved. Rather, experiments and observations can lend support to, or reduce confidence in, a h