I used the expression "strong theory" in the technically correct sense, meaning that it is "demonstrated" i.e. real, has no alternatives, and is extremely well tested. A "strong theory" is indistinguishable from fact, in other words, it is provable to any degree of certainty that a particular situation demands.
One of the best introductory books on evolution (as opposed to introductory biology) is that by Douglas J. Futuyma, and he makes the following comment:
A few words need to be said about the "theory of evolution," which most people take to mean the proposition that organisms have evolved from common ancestors. In everyday speech, "theory" often means a hypothesis or even a mere speculation. But in science, "theory" means "a statement of what are held to be the general laws, principles, or causes of something known or observed", as the Oxford English Dictionary defines it. The theory of evolution is a body of interconnected statements about natural selection and the other processes that are thought to cause evolution, just as the atomic theory of chemistry and the Newtonian theory of mechanics are bodies of statements that describe causes of chemical and physical phenomena. In contrast, the statement that organisms have descended with modifications from common ancestors - the historical reality of evolution - is not a theory. It is a fact, as fully as the fact of the earth's revolution about the sun. Like the heliocentric solar system, evolution began as a hypothesis, and achieved "facthood" as the evidence in its favor became so strong that no knowledgeable and unbiased person could deny its reality. No biologist today would think of submitting a paper entitled "New evidence for evolution"; it simply has not been an issue for a century. - Douglas J. Futuyma, op. cit., p.15
There are people who reject evolution for religious reasons. In general
these readers oppose both the fact of evolution and theories of
mechanisms, although some anti-evolutionists have come to realize that
there is a difference between the two concepts. That is why we see some
leading anti-evolutionists admitting to the fact of "microevolution" -
they know that evolution can be demonstrated. These readers will not be
convinced of the "facthood" of (macro)evolution by any logical argument
and it is a waste of time to make the attempt. The best that we can hope
for is that they understand the argument that they oppose. Even this
simple hope is rarely fulfilled.
There are some readers who are not anti-evolutionist but still claim that evolution is "only" a theory which can't be proven. This group needs to distinguish between the fact that evolution occurs and the theory of the mechanism of evolution.
We also need to distinguish between facts that are easy to demonstrate and those that are more circumstantial. Examples of evolution that are readily apparent include the fact that modern populations are evolving and the fact that two closely related species share a common ancestor. The evidence that Homo sapiens and chimpanzees share a recent common ancestor falls into this category. There is so much evidence in support of this aspect of primate evolution that it qualifies as a fact by any common definition of the word "fact".
In other cases the available evidence is less strong. For example, the relationships of some of the major phyla are still being worked out. Also, the statement that all organisms have descended from a single common ancestor is strongly supported by the available evidence, and there is no opposing evidence. However, it is not yet appropriate to call this a "fact" since there are reasonable alternatives.
Finally, there is an epistemological argument against evolution as fact. Some people point out that nothing in science can ever be "proven" and this includes evolution. According to this argument, the probability that evolution is the correct explanation of life as we know it may approach 99.9999...9% but it will never be 100%. Thus evolution cannot be a fact. This kind of argument might be appropriate in a philosophy class (it is essentially correct) but it won't do in the real world. A "fact", as Stephen J. Gould pointed out (see above), means something that is so highly probable that it would be silly not to accept it. This point has also been made by others who contest the nit-picking epistemologists.
The honest scientist, like the philosopher, will tell you that nothing
whatever can be or has been proved with fully 100% certainty, not even
that you or I exist, nor anyone except himself, since he might be
dreaming the whole thing. Thus there is no sharp line between
speculation, hypothesis, theory, principle, and fact, but only a
difference along a sliding scale, in the degree of probability of the
idea. When we say a thing is a fact, then, we only mean that its
probability is an extremely high one: so high that we are not bothered
by doubt about it and are ready to act accordingly. Now in this use of
the term fact, the only proper one, evolution is a fact. For the
evidence in favor of it is as voluminous, diverse, and convincing as in
the case of any other well established fact of science concerning the
existence of things that cannot be directly seen, such as atoms,
neutrons, or solar gravitation ....
So enormous, ramifying, and consistent has the evidence for evolution become that if anyone could now disprove it, I should have my conception of the orderliness of the universe so shaken as to lead me to doubt even my own existence. If you like, then, I will grant you that in an absolute sense evolution is not a fact, or rather, that it is no more a fact than that you are hearing or reading these words.
In any meaningful sense evolution is a fact, but there are various theories concerning the mechanism of evolution.
From: firstname.lastname@example.org [mailto:email@example.com]On Behalf Of Wade T.Smith
Sent: Thursday, June 03, 1999 5:41
To: Church of Virus
Subject: RE: virus: About Evolution
>Evolution is a strong theory,
>with an immense amount of evidence and support.
I am far from alone in saying evolution is a fact. The mechanisms involved are being explored on theoretical levels, but evolution is a fact. It's time everybody got over it and dealt with it.
Why people want their gods in machines to be anything but a dramatic device (and a cheap one at that- in fact, 'deus ex machina' is usually used pejoratively in criticism) is beyond me, when the machinery is obviously too beautiful to hold an actor....